diff --git a/doc/site/method-calls.markdown b/doc/site/method-calls.markdown
index f13305b2..d9284b94 100644
--- a/doc/site/method-calls.markdown
+++ b/doc/site/method-calls.markdown
@@ -61,8 +61,8 @@ any "if I got two arguments do this..." logic.
 
 ## Getters
 
-Some methods exist to expose some stored or computed property of an object.
-These are *getters* and have no parentheses:
+Some methods exist to expose a stored or computed property of an object. These
+are *getters* and have no parentheses:
 
     :::wren
     "string".count    //> 6
@@ -70,8 +70,9 @@ These are *getters* and have no parentheses:
     1.23.sin          //> 0.9424888019317
     [1, 2, 3].isEmpty //> false
 
-Sometimes you have a method doesn't need any parameters, but modifies the object
-or has some other side effect. For those, it's better to use empty parentheses:
+Sometimes you have a method that doesn't need any parameters, but modifies the
+object or has some other side effect. For those, it's better to use empty
+parentheses:
 
     :::wren
     list.clear()
@@ -89,7 +90,7 @@ like a getter and a `()` method like a `()` method. These don't work:
 
     :::wren
     "string".count()
-    list.clear
+    [1, 2, 3].clear
 
 ## Setters
 
@@ -104,7 +105,7 @@ language's perspective, the above line is just a call to the `height=(_)`
 method, passing in `74`.
 
 Since the `=(_)` is in the setter's signature, an object can have both a getter
-and setter with the same name without any collision. This way, you can have
+and setter with the same name without a collision. This way, you can have
 read/write properties.
 
 ## Operators
diff --git a/doc/site/modules/core/system.markdown b/doc/site/modules/core/system.markdown
index 1104c34d..ac091193 100644
--- a/doc/site/modules/core/system.markdown
+++ b/doc/site/modules/core/system.markdown
@@ -21,7 +21,7 @@ Prints a single newline to the console.
 
 ### System.**print**(object)
 
-Prints [object] to the console followed by a newline. If not already a string,
+Prints `object` to the console followed by a newline. If not already a string,
 the object is converted to a string by calling `toString` on it.
 
     :::wren
@@ -29,7 +29,7 @@ the object is converted to a string by calling `toString` on it.
 
 ### System.**printAll**(sequence)
 
-Iterates over [sequence] and prints each element, then prints a single newline
+Iterates over `sequence` and prints each element, then prints a single newline
 at the end. Each element is converted to a string by calling `toString` on it.
 
     :::wren
diff --git a/doc/site/modules/io/directory.markdown b/doc/site/modules/io/directory.markdown
new file mode 100644
index 00000000..0e41a33b
--- /dev/null
+++ b/doc/site/modules/io/directory.markdown
@@ -0,0 +1,10 @@
+^title Directory Class
+
+A directory on the file system.
+
+## Static Methods
+
+### Directory.**list**(path)
+
+Lists the contents of the directory at `path`. Returns a sorted list of path
+strings for all of the contents of the directory.
diff --git a/doc/site/modules/io/file.markdown b/doc/site/modules/io/file.markdown
index b9b690cd..cb32454e 100644
--- a/doc/site/modules/io/file.markdown
+++ b/doc/site/modules/io/file.markdown
@@ -1,7 +1,88 @@
 ^title File Class
 
-**TODO**
+Lets you work with files on the file system. An instance of this class
+represents an open file with a file descriptor.
+
+When you are done with a file object, it's a good idea to explicitly close it.
+If you don't, the GC will close it when the file is no longer used and gets
+finalized, but that may take a while. In the meantime, leaving it open wastes
+a file descriptor.
+
+## Static Methods
+
+### File.**open**(path, fn)
+
+Opens the file at `path` and passes it to `fn`. After the function returns, the
+file is automatically closed.
+
+    :::wren
+    File.open("words.txt") {|file|
+      file.readBytes(5)
+    }
+
+### File.**read**(path)
+
+Reads the entire contents of the file at `path` and returns it as a string.
+
+    :::wren
+    File.read("words.txt")
+
+The encoding or decoding is done. If the file is UTF-8, then the resulting
+string will be a UTF-8 string. Otherwise, it will be a string of bytes in
+whatever encoding the file uses.
+
+### File.**size**(path)
+
+Returns the size in bytes of the contents of the file at `path`.
+
+### File.**stat**(path)
+
+"Stats" the file or directory at `path`. Returns a [Stat][] object describing
+the low-level details of the file system entry.
+
+[stat]: stat.html
+
+## Constructors
+
+### File.**open**(path)
+
+Opens the file at `path` for reading.
 
 ## Methods
 
-**TODO**
+### **descriptor**
+
+The numeric file descriptor used to access the file.
+
+### **isOpen**
+
+Whether the file is still open or has been closed.
+
+### **size**
+
+The size of the contents of the file in bytes.
+
+### **close**()
+
+Closes the file. After calling this, you can read or write from it.
+
+### **readBytes**(count)
+
+Reads up to `count` bytes starting from the beginning of the file.
+
+    :::wren
+    // Assume this file contains "I am a file!".
+    File.open("example.txt") {|file|
+      System.print(file.readBytes(6)) //> I am a
+    }
+
+### **readBytes**(count, offset)
+
+Reads up to `count` bytes starting at `offset` bytes from the beginning of
+the file.
+
+    :::wren
+    // Assume this file contains "I am a file!".
+    File.open("example.txt") {|file|
+      System.print(file.readBytes(6, 2)) //> am a f
+    }
diff --git a/doc/site/modules/io/index.markdown b/doc/site/modules/io/index.markdown
index ba41efbd..2b5a765c 100644
--- a/doc/site/modules/io/index.markdown
+++ b/doc/site/modules/io/index.markdown
@@ -1,6 +1,8 @@
 ^title Module "io"
 
-**TODO**
+Provides access to operating system streams and the file system.
 
+* [Directory](directory.html)
 * [File](file.html)
+* [Stat](stat.html)
 * [Stdin](stdin.html)
diff --git a/doc/site/modules/io/stat.markdown b/doc/site/modules/io/stat.markdown
new file mode 100644
index 00000000..0b69b219
--- /dev/null
+++ b/doc/site/modules/io/stat.markdown
@@ -0,0 +1,50 @@
+^title Stat Class
+
+Contains the data returned by [File.stat()][stat].
+
+[stat]: file.html#file.stat(path)
+
+## Methods
+
+### **device**
+
+The ID of the device containing the entry.
+
+### **inode**
+
+The [inode][] number of the entry.
+
+[inode]: https://en.wikipedia.org/wiki/Inode
+
+### **mode**
+
+A bit field describing the entry's type and protection flags.
+
+### **linkCount**
+
+The number of hard links to the entry.
+
+### **user**
+
+Numeric user ID of the file's owner.
+
+### **group**
+
+Numeric group ID of the file's owner.
+
+### **specialDevice**
+
+The device ID for the entry, if it's a special file.
+
+### **size**
+
+The size of the entry in bytes.
+
+### **blockSize**
+
+The preferred block size in bytes for interacting with the file. It may vary
+from file to file.
+
+### **blockCount**
+
+The number of system blocks allocated on disk for the file.
diff --git a/doc/site/modules/io/stdin.markdown b/doc/site/modules/io/stdin.markdown
index 399a9af5..66377f85 100644
--- a/doc/site/modules/io/stdin.markdown
+++ b/doc/site/modules/io/stdin.markdown
@@ -1,7 +1,12 @@
 ^title Stdin Class
 
-**TODO**
+The standard input stream.
 
-## Methods
+## Static Methods
 
-**TODO**
+### **readLine**()
+
+Reads one line of input from stdin. Blocks the current fiber until a full line
+of input has been received.
+
+Returns the string of input or `null` if stdin is closed.
diff --git a/doc/site/modules/io/template.html b/doc/site/modules/io/template.html
index ab36a796..ebce39a1 100644
--- a/doc/site/modules/io/template.html
+++ b/doc/site/modules/io/template.html
@@ -27,6 +27,7 @@
     <section>
       <h2>io classes</h2>
       <ul>
+        <li><a href="directory.html">Directory</a></li>
         <li><a href="file.html">File</a></li>
         <li><a href="stdin.html">Stdin</a></li>
       </ul>
diff --git a/src/cli/modules.c b/src/cli/modules.c
index 093844e6..024156e3 100644
--- a/src/cli/modules.c
+++ b/src/cli/modules.c
@@ -7,10 +7,12 @@
 #include "scheduler.wren.inc"
 #include "timer.wren.inc"
 
+extern void directoryList(WrenVM* vm);
 extern void fileAllocate(WrenVM* vm);
-extern void fileFinalize(WrenVM* vm);
+extern void fileFinalize(void* data);
 extern void fileOpen(WrenVM* vm);
 extern void fileSizePath(WrenVM* vm);
+extern void fileStatPath(WrenVM* vm);
 extern void fileClose(WrenVM* vm);
 extern void fileDescriptor(WrenVM* vm);
 extern void fileReadBytes(WrenVM* vm);
@@ -28,14 +30,14 @@ extern void timerStartTimer(WrenVM* vm);
 // If you add a new method to the longest class below, make sure to bump this.
 // Note that it also includes an extra slot for the sentinel value indicating
 // the end of the list.
-#define MAX_METHODS_PER_CLASS 9
+#define MAX_METHODS_PER_CLASS 10
 
 // The maximum number of foreign classes a single built-in module defines.
 //
 // If you add a new class to the largest module below, make sure to bump this.
 // Note that it also includes an extra slot for the sentinel value indicating
 // the end of the list.
-#define MAX_CLASSES_PER_MODULE 3
+#define MAX_CLASSES_PER_MODULE 4
 
 // Describes one foreign method in a class.
 typedef struct
@@ -82,19 +84,24 @@ typedef struct
 
 #define METHOD(signature, fn) { false, signature, fn },
 #define STATIC_METHOD(signature, fn) { true, signature, fn },
+#define FINALIZER(fn) { true, "<finalize>", (WrenForeignMethodFn)fn },
 
 // The array of built-in modules.
 static ModuleRegistry modules[] =
 {
   MODULE(io)
+    CLASS(Directory)
+      STATIC_METHOD("list_(_,_)", directoryList)
+    END_CLASS
     CLASS(File)
       STATIC_METHOD("<allocate>", fileAllocate)
-      STATIC_METHOD("<finalize>", fileFinalize)
+      FINALIZER(fileFinalize)
       STATIC_METHOD("open_(_,_)", fileOpen)
       STATIC_METHOD("sizePath_(_,_)", fileSizePath)
+      STATIC_METHOD("statPath_(_,_)", fileStatPath)
       METHOD("close_(_)", fileClose)
       METHOD("descriptor", fileDescriptor)
-      METHOD("readBytes_(_,_)", fileReadBytes)
+      METHOD("readBytes_(_,_,_)", fileReadBytes)
       METHOD("size_(_)", fileSize)
     END_CLASS
     CLASS(Stdin)
@@ -204,7 +211,7 @@ WrenForeignClassMethods bindBuiltInForeignClass(
   if (clas == NULL) return methods;
 
   methods.allocate = findMethod(clas, true, "<allocate>");
-  methods.finalize = findMethod(clas, true, "<finalize>");
+  methods.finalize = (WrenFinalizerFn)findMethod(clas, true, "<finalize>");
 
   return methods;
 }
diff --git a/src/include/wren.h b/src/include/wren.h
index 29645f83..de2b7874 100644
--- a/src/include/wren.h
+++ b/src/include/wren.h
@@ -38,6 +38,12 @@ typedef void* (*WrenReallocateFn)(void* memory, size_t newSize);
 // A function callable from Wren code, but implemented in C.
 typedef void (*WrenForeignMethodFn)(WrenVM* vm);
 
+// A finalizer function for freeing resources owned by an instance of a foreign
+// class. Unlike most foreign methods, finalizers do not have access to the VM
+// and should not interact with it since it's in the middle of a garbage
+// collection.
+typedef void (*WrenFinalizerFn)(void* data);
+
 // Loads and returns the source code for the module [name].
 typedef char* (*WrenLoadModuleFn)(WrenVM* vm, const char* name);
 
@@ -64,7 +70,7 @@ typedef struct
   // foreign object's memory.
   //
   // This may be `NULL` if the foreign class does not need to finalize.
-  WrenForeignMethodFn finalize;
+  WrenFinalizerFn finalize;
 } WrenForeignClassMethods;
 
 // Returns a pair of pointers to the foreign methods used to allocate and
@@ -120,7 +126,7 @@ typedef struct
   //
   // If this is `NULL`, Wren discards any printed text.
   WrenWriteFn writeFn;
-  
+
   // The number of bytes Wren will allocate before triggering the first garbage
   // collection.
   //
@@ -183,53 +189,71 @@ void wrenCollectGarbage(WrenVM* vm);
 // Runs [source], a string of Wren source code in a new fiber in [vm].
 WrenInterpretResult wrenInterpret(WrenVM* vm, const char* source);
 
-// Creates a handle that can be used to invoke a method with [signature] on the
-// object in [module] currently stored in top-level [variable].
+// Creates a handle that can be used to invoke a method with [signature] on
+// using a receiver and arguments that are set up on the stack.
 //
 // This handle can be used repeatedly to directly invoke that method from C
 // code using [wrenCall].
 //
-// When done with this handle, it must be released using [wrenReleaseValue].
-WrenValue* wrenGetMethod(WrenVM* vm, const char* module, const char* variable,
-                         const char* signature);
+// When you are done with this handle, it must be released using
+// [wrenReleaseValue].
+WrenValue* wrenMakeCallHandle(WrenVM* vm, const char* signature);
 
-// Calls [method], passing in a series of arguments whose types must match the
-// specifed [argTypes]. This is a string where each character identifies the
-// type of a single argument, in order. The allowed types are:
+// Calls [method], using the receiver and arguments previously set up on the
+// stack.
 //
-// - "b" - A C `int` converted to a Wren Bool.
-// - "d" - A C `double` converted to a Wren Num.
-// - "i" - A C `int` converted to a Wren Num.
-// - "s" - A C null-terminated `const char*` converted to a Wren String. Wren
-//         will allocate its own string and copy the characters from this, so
-//         you don't have to worry about the lifetime of the string you pass to
-//         Wren.
-// - "a" - An array of bytes converted to a Wren String. This requires two
-//         consecutive arguments in the argument list: `const char*` pointing
-//         to the array of bytes, followed by an `int` defining the length of
-//         the array. This is used when the passed string may contain null
-//         bytes, or just to avoid the implicit `strlen()` call of "s" if you
-//         happen to already know the length.
-// - "v" - A previously acquired WrenValue*. Passing this in does not implicitly
-//         release the value. If the passed argument is NULL, this becomes a
-//         Wren NULL.
+// [method] must have been created by a call to [wrenMakeCallHandle]. The
+// arguments to the method must be already on the stack. The receiver should be
+// in slot 0 with the remaining arguments following it, in order. It is an
+// error if the number of arguments provided does not match the method's
+// signature.
 //
-// [method] must have been created by a call to [wrenGetMethod]. If
-// [returnValue] is not `NULL`, the return value of the method will be stored
-// in a new [WrenValue] that [returnValue] will point to. Don't forget to
-// release it, when done with it.
-WrenInterpretResult wrenCall(WrenVM* vm, WrenValue* method,
-                             WrenValue** returnValue,
-                             const char* argTypes, ...);
-
-WrenInterpretResult wrenCallVarArgs(WrenVM* vm, WrenValue* method,
-                                    WrenValue** returnValue,
-                                    const char* argTypes, va_list args);
+// After this returns, you can access the return value from slot 0 on the stack.
+WrenInterpretResult wrenCall(WrenVM* vm, WrenValue* method);
 
 // Releases the reference stored in [value]. After calling this, [value] can no
 // longer be used.
 void wrenReleaseValue(WrenVM* vm, WrenValue* value);
 
+// The following functions are intended to be called from foreign methods or
+// finalizers. The interface Wren provides to a foreign method is like a
+// register machine: you are given a numbered array of slots that values can be
+// read from and written to. Values always live in a slot (unless explicitly
+// captured using wrenGetSlotValue(), which ensures the garbage collector can
+// find them.
+//
+// When your foreign function is called, you are given one slot for the receiver
+// and each argument to the method. The receiver is in slot 0 and the arguments
+// are in increasingly numbered slots after that. You are free to read and
+// write to those slots as you want. If you want more slots to use as scratch
+// space, you can call wrenEnsureSlots() to add more.
+//
+// When your function returns, every slot except slot zero is discarded and the
+// value in slot zero is used as the return value of the method. If you don't
+// store a return value in that slot yourself, it will retain its previous
+// value, the receiver.
+//
+// While Wren is dynamically typed, C is not. This means the C interface has to
+// support the various types of primitive values a Wren variable can hold: bool,
+// double, string, etc. If we supported this for every operation in the C API,
+// there would be a combinatorial explosion of functions, like "get a
+// double-valued element from a list", "insert a string key and double value
+// into a map", etc.
+//
+// To avoid that, the only way to convert to and from a raw C value is by going
+// into and out of a slot. All other functions work with values already in a
+// slot. So, to add an element to a list, you put the list in one slot, and the
+// element in another. Then there is a single API function wrenInsertInList()
+// that takes the element out of that slot and puts it into the list.
+//
+// The goal of this API is to be easy to use while not compromising performance.
+// The latter means it does not do type or bounds checking at runtime except
+// using assertions which are generally removed from release builds. C is an
+// unsafe language, so it's up to you to be careful to use it correctly. In
+// return, you get a very fast FFI.
+
+// TODO: Generalize this to look up a foreign class in any slot and place the
+// object in a desired slot.
 // This must be called once inside a foreign class's allocator function.
 //
 // It tells Wren how many bytes of raw data need to be stored in the foreign
@@ -237,55 +261,60 @@ void wrenReleaseValue(WrenVM* vm, WrenValue* value);
 // the foreign object's data.
 void* wrenAllocateForeign(WrenVM* vm, size_t size);
 
-// Returns the number of arguments available to the current foreign method.
-int wrenGetArgumentCount(WrenVM* vm);
+// Returns the number of slots available to the current foreign method.
+int wrenGetSlotCount(WrenVM* vm);
 
-// The following functions read one of the arguments passed to a foreign call.
-// They may only be called while within a function provided to
-// [wrenDefineMethod] or [wrenDefineStaticMethod] that Wren has invoked.
+// Ensures that the foreign method stack has at least [numSlots] available for
+// use, growing the stack if needed.
 //
-// They retreive the argument at a given index which ranges from 0 to the number
-// of parameters the method expects. The zeroth parameter is used for the
-// receiver of the method. For example, given a foreign method "foo" on String
-// invoked like:
+// Does not shrink the stack if it has more than enough slots.
 //
-//     "receiver".foo("one", "two", "three")
-//
-// The foreign function will be able to access the arguments like so:
-//
-//     0: "receiver"
-//     1: "one"
-//     2: "two"
-//     3: "three"
-//
-// It is an error to pass an invalid argument index.
+// It is an error to call this from a finalizer.
+void wrenEnsureSlots(WrenVM* vm, int numSlots);
 
-// Reads a boolean argument for a foreign call. Returns false if the argument
-// is not a boolean.
-bool wrenGetArgumentBool(WrenVM* vm, int index);
+// Reads a boolean value from [slot].
+//
+// It is an error to call this if the slot does not contain a boolean value.
+bool wrenGetSlotBool(WrenVM* vm, int slot);
 
-// Reads a numeric argument for a foreign call. Returns 0 if the argument is not
-// a number.
-double wrenGetArgumentDouble(WrenVM* vm, int index);
-
-// Reads a foreign object argument for a foreign call and returns a pointer to
-// the foreign data stored with it. Returns NULL if the argument is not a
-// foreign object.
-void* wrenGetArgumentForeign(WrenVM* vm, int index);
-
-// Reads an string argument for a foreign call. Returns NULL if the argument is
-// not a string.
+// Reads a byte array from [slot].
 //
 // The memory for the returned string is owned by Wren. You can inspect it
-// while in your foreign function, but cannot keep a pointer to it after the
+// while in your foreign method, but cannot keep a pointer to it after the
 // function returns, since the garbage collector may reclaim it.
-const char* wrenGetArgumentString(WrenVM* vm, int index);
+//
+// Returns a pointer to the first byte of the array and fill [length] with the
+// number of bytes in the array.
+//
+// It is an error to call this if the slot does not contain a string.
+const char* wrenGetSlotBytes(WrenVM* vm, int slot, int* length);
 
-// Creates a handle for the value passed as an argument to a foreign call.
+// Reads a number from [slot].
+//
+// It is an error to call this if the slot does not contain a number.
+double wrenGetSlotDouble(WrenVM* vm, int slot);
+
+// Reads a foreign object from [slot] and returns a pointer to the foreign data
+// stored with it.
+//
+// It is an error to call this if the slot does not contain an instance of a
+// foreign class.
+void* wrenGetSlotForeign(WrenVM* vm, int slot);
+
+// Reads a string from [slot].
+//
+// The memory for the returned string is owned by Wren. You can inspect it
+// while in your foreign method, but cannot keep a pointer to it after the
+// function returns, since the garbage collector may reclaim it.
+//
+// It is an error to call this if the slot does not contain a string.
+const char* wrenGetSlotString(WrenVM* vm, int slot);
+
+// Creates a handle for the value stored in [slot].
 //
 // This will prevent the object that is referred to from being garbage collected
 // until the handle is released by calling [wrenReleaseValue()].
-WrenValue* wrenGetArgumentValue(WrenVM* vm, int index);
+WrenValue* wrenGetSlotValue(WrenVM* vm, int slot);
 
 // The following functions provide the return value for a foreign method back
 // to Wren. Like above, they may only be called during a foreign call invoked
@@ -296,27 +325,47 @@ WrenValue* wrenGetArgumentValue(WrenVM* vm, int index);
 // call one of these once. It is an error to access any of the foreign calls
 // arguments after one of these has been called.
 
-// Provides a boolean return value for a foreign call.
-void wrenReturnBool(WrenVM* vm, bool value);
+// Stores the boolean [value] in [slot].
+void wrenSetSlotBool(WrenVM* vm, int slot, bool value);
 
-// Provides a numeric return value for a foreign call.
-void wrenReturnDouble(WrenVM* vm, double value);
+// Stores the array [length] of [bytes] in [slot].
+//
+// The bytes are copied to a new string within Wren's heap, so you can free
+// memory used by them after this is called.
+void wrenSetSlotBytes(WrenVM* vm, int slot, const char* bytes, size_t length);
 
-// Provides a string return value for a foreign call.
-//
-// The [text] will be copied to a new string within Wren's heap, so you can
-// free memory used by it after this is called.
-//
-// If [length] is non-zero, Wren copies that many bytes from [text], including
-// any null bytes. If it is -1, then the length of [text] is calculated using
-// `strlen()`. If the string may contain any null bytes in the middle, then you
-// must pass an explicit length.
-void wrenReturnString(WrenVM* vm, const char* text, int length);
+// Stores the numeric [value] in [slot].
+void wrenSetSlotDouble(WrenVM* vm, int slot, double value);
 
-// Provides the return value for a foreign call.
+// Stores a new empty list in [slot].
+void wrenSetSlotNewList(WrenVM* vm, int slot);
+
+// Stores null in [slot].
+void wrenSetSlotNull(WrenVM* vm, int slot);
+
+// Stores the string [text] in [slot].
 //
-// This uses the value referred to by the handle as the return value, but it
-// does not release the handle.
-void wrenReturnValue(WrenVM* vm, WrenValue* value);
+// The [text] is copied to a new string within Wren's heap, so you can free
+// memory used by it after this is called. The length is calculated using
+// [strlen()]. If the string may contain any null bytes in the middle, then you
+// should use [wrenSetSlotBytes()] instead.
+void wrenSetSlotString(WrenVM* vm, int slot, const char* text);
+
+// Stores the value captured in [value] in [slot].
+//
+// This does not release the handle for the value.
+void wrenSetSlotValue(WrenVM* vm, int slot, WrenValue* value);
+
+// Takes the value stored at [elementSlot] and inserts it into the list stored
+// at [listSlot] at [index].
+//
+// As in Wren, negative indexes can be used to insert from the end. To append
+// an element, use `-1` for the index.
+void wrenInsertInList(WrenVM* vm, int listSlot, int index, int elementSlot);
+
+// Looks up the top level variable with [name] in [module] and stores it in
+// [slot].
+void wrenGetVariable(WrenVM* vm, const char* module, const char* name,
+                     int slot);
 
 #endif
diff --git a/src/module/io.c b/src/module/io.c
index 9f3a836b..06371264 100644
--- a/src/module/io.c
+++ b/src/module/io.c
@@ -18,7 +18,10 @@ typedef struct sFileRequestData
 
 static const int stdinDescriptor = 0;
 
-// Handle to Stdin.onData_(). Called when libuv provides data on stdin.
+// Handle to the Stdin class object.
+static WrenValue* stdinClass = NULL;
+
+// Handle to an onData_() method call. Called when libuv provides data on stdin.
 static WrenValue* stdinOnData = NULL;
 
 // The stream used to read from stdin. Initialized on the first read.
@@ -34,6 +37,12 @@ static void shutdownStdin()
     stdinStream = NULL;
   }
   
+  if (stdinClass != NULL)
+  {
+    wrenReleaseValue(getVM(), stdinClass);
+    stdinClass = NULL;
+  }
+  
   if (stdinOnData != NULL)
   {
     wrenReleaseValue(getVM(), stdinOnData);
@@ -46,26 +55,6 @@ void ioShutdown()
   shutdownStdin();
 }
 
-void fileAllocate(WrenVM* vm)
-{
-  // Store the file descriptor in the foreign data, so that we can get to it
-  // in the finalizer.
-  int* fd = (int*)wrenAllocateForeign(vm, sizeof(int));
-  *fd = (int)wrenGetArgumentDouble(vm, 1);
-}
-
-void fileFinalize(WrenVM* vm)
-{
-  int fd = *(int*)wrenGetArgumentForeign(vm, 0);
-  
-  // Already closed.
-  if (fd == -1) return;
-  
-  uv_fs_t request;
-  uv_fs_close(getLoop(), &request, fd, NULL);
-  uv_fs_req_cleanup(&request);
-}
-
 // If [request] failed with an error, sends the runtime error to the VM and
 // frees the request.
 //
@@ -73,12 +62,12 @@ void fileFinalize(WrenVM* vm)
 static bool handleRequestError(uv_fs_t* request)
 {
   if (request->result >= 0) return false;
-  
+
   FileRequestData* data = (FileRequestData*)request->data;
   WrenValue* fiber = (WrenValue*)data->fiber;
-
+  
   schedulerResumeError(fiber, uv_strerror((int)request->result));
-
+  
   free(data);
   uv_fs_req_cleanup(request);
   free(request);
@@ -89,10 +78,10 @@ static bool handleRequestError(uv_fs_t* request)
 uv_fs_t* createRequest(WrenValue* fiber)
 {
   uv_fs_t* request = (uv_fs_t*)malloc(sizeof(uv_fs_t));
-
+  
   FileRequestData* data = (FileRequestData*)malloc(sizeof(FileRequestData));
   data->fiber = fiber;
-
+  
   request->data = data;
   return request;
 }
@@ -104,84 +93,189 @@ WrenValue* freeRequest(uv_fs_t* request)
 {
   FileRequestData* data = (FileRequestData*)request->data;
   WrenValue* fiber = data->fiber;
-
+  
   free(data);
   uv_fs_req_cleanup(request);
   free(request);
-
+  
   return fiber;
 }
 
-static void openCallback(uv_fs_t* request)
+static void directoryListCallback(uv_fs_t* request)
+{
+  if (handleRequestError(request)) return;
+
+  uv_dirent_t entry;
+
+  WrenVM* vm = getVM();
+  wrenEnsureSlots(vm, 3);
+  wrenSetSlotNewList(vm, 2);
+  
+  while (uv_fs_scandir_next(request, &entry) != UV_EOF)
+  {
+    wrenSetSlotString(vm, 1, entry.name);
+    wrenInsertInList(vm, 2, -1, 1);
+  }
+  
+  schedulerResume(freeRequest(request), true);
+  schedulerFinishResume();
+}
+
+void directoryList(WrenVM* vm)
+{
+  const char* path = wrenGetSlotString(vm, 1);
+  
+  uv_fs_t* request = createRequest(wrenGetSlotValue(vm, 2));
+  
+  // TODO: Check return.
+  uv_fs_scandir(getLoop(), request, path, 0, directoryListCallback);
+}
+
+void fileAllocate(WrenVM* vm)
+{
+  // Store the file descriptor in the foreign data, so that we can get to it
+  // in the finalizer.
+  int* fd = (int*)wrenAllocateForeign(vm, sizeof(int));
+  *fd = (int)wrenGetSlotDouble(vm, 1);
+}
+
+void fileFinalize(void* data)
+{
+  int fd = *(int*)data;
+  
+  // Already closed.
+  if (fd == -1) return;
+  
+  uv_fs_t request;
+  uv_fs_close(getLoop(), &request, fd, NULL);
+  uv_fs_req_cleanup(&request);
+}
+
+static void fileOpenCallback(uv_fs_t* request)
 {
   if (handleRequestError(request)) return;
   
   double fd = (double)request->result;
-  WrenValue* fiber = freeRequest(request);
-  
-  schedulerResumeDouble(fiber, fd);
+  schedulerResume(freeRequest(request), true);
+  wrenSetSlotDouble(getVM(), 2, fd);
+  schedulerFinishResume();
 }
 
 void fileOpen(WrenVM* vm)
 {
-  const char* path = wrenGetArgumentString(vm, 1);
-  uv_fs_t* request = createRequest(wrenGetArgumentValue(vm, 2));
-  
+  const char* path = wrenGetSlotString(vm, 1);
+  uv_fs_t* request = createRequest(wrenGetSlotValue(vm, 2));
+
   // TODO: Allow controlling flags and modes.
-  uv_fs_open(getLoop(), request, path, O_RDONLY, 0, openCallback);
+  uv_fs_open(getLoop(), request, path, O_RDONLY, 0, fileOpenCallback);
 }
 
 // Called by libuv when the stat call for size completes.
-static void sizeCallback(uv_fs_t* request)
+static void fileSizeCallback(uv_fs_t* request)
 {
   if (handleRequestError(request)) return;
-  
+
   double size = (double)request->statbuf.st_size;
-  WrenValue* fiber = freeRequest(request);
-  
-  schedulerResumeDouble(fiber, size);
+  schedulerResume(freeRequest(request), true);
+  wrenSetSlotDouble(getVM(), 2, size);
+  schedulerFinishResume();
 }
 
 void fileSizePath(WrenVM* vm)
 {
-  const char* path = wrenGetArgumentString(vm, 1);
-  uv_fs_t* request = createRequest(wrenGetArgumentValue(vm, 2));
-  uv_fs_stat(getLoop(), request, path, sizeCallback);
+  const char* path = wrenGetSlotString(vm, 1);
+  uv_fs_t* request = createRequest(wrenGetSlotValue(vm, 2));
+  uv_fs_stat(getLoop(), request, path, fileSizeCallback);
 }
 
-static void closeCallback(uv_fs_t* request)
+// Called by libuv when the stat call completes.
+static void fileStatPathCallback(uv_fs_t* request)
 {
   if (handleRequestError(request)) return;
   
-  WrenValue* fiber = freeRequest(request);
-  schedulerResume(fiber);
+  WrenVM* vm = getVM();
+  wrenEnsureSlots(vm, 4);
+  wrenSetSlotNewList(vm, 2);
+  
+  wrenSetSlotDouble(vm, 3, (double)request->statbuf.st_dev);
+  wrenInsertInList(vm, 2, -1, 3);
+  
+  wrenSetSlotDouble(vm, 3, (double)request->statbuf.st_ino);
+  wrenInsertInList(vm, 2, -1, 3);
+  
+  wrenSetSlotDouble(vm, 3, (double)request->statbuf.st_mode);
+  wrenInsertInList(vm, 2, -1, 3);
+  
+  wrenSetSlotDouble(vm, 3, (double)request->statbuf.st_nlink);
+  wrenInsertInList(vm, 2, -1, 3);
+  
+  wrenSetSlotDouble(vm, 3, (double)request->statbuf.st_uid);
+  wrenInsertInList(vm, 2, -1, 3);
+  
+  wrenSetSlotDouble(vm, 3, (double)request->statbuf.st_gid);
+  wrenInsertInList(vm, 2, -1, 3);
+
+  wrenSetSlotDouble(vm, 3, (double)request->statbuf.st_rdev);
+  wrenInsertInList(vm, 2, -1, 3);
+  
+  wrenSetSlotDouble(vm, 3, (double)request->statbuf.st_size);
+  wrenInsertInList(vm, 2, -1, 3);
+  
+  wrenSetSlotDouble(vm, 3, (double)request->statbuf.st_blksize);
+  wrenInsertInList(vm, 2, -1, 3);
+  
+  wrenSetSlotDouble(vm, 3, (double)request->statbuf.st_blocks);
+  wrenInsertInList(vm, 2, -1, 3);
+
+  // TODO: Include access, modification, and change times once we figure out
+  // how we want to represent it.
+  //  time_t    st_atime;   /* time of last access */
+  //  time_t    st_mtime;   /* time of last modification */
+  //  time_t    st_ctime;   /* time of last status change */
+
+  schedulerResume(freeRequest(request), true);
+  schedulerFinishResume();
+}
+
+void fileStatPath(WrenVM* vm)
+{
+  const char* path = wrenGetSlotString(vm, 1);
+  uv_fs_t* request = createRequest(wrenGetSlotValue(vm, 2));
+  uv_fs_stat(getLoop(), request, path, fileStatPathCallback);
+}
+
+static void fileCloseCallback(uv_fs_t* request)
+{
+  if (handleRequestError(request)) return;
+
+  schedulerResume(freeRequest(request), false);
 }
 
 void fileClose(WrenVM* vm)
 {
-  int* foreign = (int*)wrenGetArgumentForeign(vm, 0);
+  int* foreign = (int*)wrenGetSlotForeign(vm, 0);
   int fd = *foreign;
-  
+
   // If it's already closed, we're done.
   if (fd == -1)
   {
-    wrenReturnBool(vm, true);
+    wrenSetSlotBool(vm, 0, true);
     return;
   }
-  
+
   // Mark it closed immediately.
   *foreign = -1;
-  
-  uv_fs_t* request = createRequest(wrenGetArgumentValue(vm, 1));
-  uv_fs_close(getLoop(), request, fd, closeCallback);
-  wrenReturnBool(vm, false);
+
+  uv_fs_t* request = createRequest(wrenGetSlotValue(vm, 1));
+  uv_fs_close(getLoop(), request, fd, fileCloseCallback);
+  wrenSetSlotBool(vm, 0, false);
 }
 
 void fileDescriptor(WrenVM* vm)
 {
-  int* foreign = (int*)wrenGetArgumentForeign(vm, 0);
+  int* foreign = (int*)wrenGetSlotForeign(vm, 0);
   int fd = *foreign;
-  wrenReturnDouble(vm, fd);
+  wrenSetSlotDouble(vm, 0, fd);
 }
 
 static void fileReadBytesCallback(uv_fs_t* request)
@@ -190,43 +284,45 @@ static void fileReadBytesCallback(uv_fs_t* request)
 
   FileRequestData* data = (FileRequestData*)request->data;
   uv_buf_t buffer = data->buffer;
-
-  WrenValue* fiber = freeRequest(request);
+  size_t count = request->result;
 
   // TODO: Having to copy the bytes here is a drag. It would be good if Wren's
   // embedding API supported a way to *give* it bytes that were previously
   // allocated using Wren's own allocator.
-  schedulerResumeBytes(fiber, buffer.base, buffer.len);
-  
+  schedulerResume(freeRequest(request), true);
+  wrenSetSlotBytes(getVM(), 2, buffer.base, count);
+  schedulerFinishResume();
+
   // TODO: Likewise, freeing this after we resume is lame.
   free(buffer.base);
 }
 
 void fileReadBytes(WrenVM* vm)
 {
-  uv_fs_t* request = createRequest(wrenGetArgumentValue(vm, 2));
-  
-  int fd = *(int*)wrenGetArgumentForeign(vm, 0);
+  uv_fs_t* request = createRequest(wrenGetSlotValue(vm, 3));
+
+  int fd = *(int*)wrenGetSlotForeign(vm, 0);
   // TODO: Assert fd != -1.
 
   FileRequestData* data = (FileRequestData*)request->data;
-  size_t length = (size_t)wrenGetArgumentDouble(vm, 1);
+  size_t length = (size_t)wrenGetSlotDouble(vm, 1);
+  size_t offset = (size_t)wrenGetSlotDouble(vm, 2);
 
   data->buffer.len = length;
   data->buffer.base = (char*)malloc(length);
-  
-  // TODO: Allow passing in offset.
-  uv_fs_read(getLoop(), request, fd, &data->buffer, 1, 0, fileReadBytesCallback);
+
+  uv_fs_read(getLoop(), request, fd, &data->buffer, 1, offset,
+             fileReadBytesCallback);
 }
 
 void fileSize(WrenVM* vm)
 {
-  uv_fs_t* request = createRequest(wrenGetArgumentValue(vm, 1));
+  uv_fs_t* request = createRequest(wrenGetSlotValue(vm, 1));
 
-  int fd = *(int*)wrenGetArgumentForeign(vm, 0);
+  int fd = *(int*)wrenGetSlotForeign(vm, 0);
   // TODO: Assert fd != -1.
-  
-  uv_fs_fstat(getLoop(), request, fd, sizeCallback);
+
+  uv_fs_fstat(getLoop(), request, fd, fileSizeCallback);
 }
 
 static void allocCallback(uv_handle_t* handle, size_t suggestedSize,
@@ -240,26 +336,42 @@ static void allocCallback(uv_handle_t* handle, size_t suggestedSize,
 static void stdinReadCallback(uv_stream_t* stream, ssize_t numRead,
                                const uv_buf_t* buffer)
 {
-  // If stdin was closed, send null to let io.wren know.
-  if (numRead == UV_EOF)
-  {
-    wrenCall(getVM(), stdinOnData, NULL, "v", NULL);
-    shutdownStdin();
-    return;
-  }
+  WrenVM* vm = getVM();
   
-  // TODO: Handle other errors.
+  if (stdinClass == NULL)
+  {
+    wrenEnsureSlots(vm, 1);
+    wrenGetVariable(vm, "io", "Stdin", 0);
+    stdinClass = wrenGetSlotValue(vm, 0);
+  }
   
   if (stdinOnData == NULL)
   {
-    stdinOnData = wrenGetMethod(getVM(), "io", "Stdin", "onData_(_)");
+    stdinOnData = wrenMakeCallHandle(vm, "onData_(_)");
   }
+  
+  // If stdin was closed, send null to let io.wren know.
+  if (numRead == UV_EOF)
+  {
+    wrenEnsureSlots(vm, 2);
+    wrenSetSlotValue(vm, 0, stdinClass);
+    wrenSetSlotNull(vm, 1);
+    wrenCall(vm, stdinOnData);
+    
+    shutdownStdin();
+    return;
+  }
+
+  // TODO: Handle other errors.
 
   // TODO: Having to copy the bytes here is a drag. It would be good if Wren's
   // embedding API supported a way to *give* it bytes that were previously
   // allocated using Wren's own allocator.
-  wrenCall(getVM(), stdinOnData, NULL, "a", buffer->base, numRead);
-  
+  wrenEnsureSlots(vm, 2);
+  wrenSetSlotValue(vm, 0, stdinClass);
+  wrenSetSlotBytes(vm, 1, buffer->base, numRead);
+  wrenCall(vm, stdinOnData);
+
   // TODO: Likewise, freeing this after we resume is lame.
   free(buffer->base);
 }
@@ -284,7 +396,7 @@ void stdinReadStart(WrenVM* vm)
       stdinStream = (uv_stream_t*)handle;
     }
   }
-  
+
   uv_read_start(stdinStream, allocCallback, stdinReadCallback);
   // TODO: Check return.
 }
diff --git a/src/module/io.wren b/src/module/io.wren
index f2c5d570..2bc967bd 100644
--- a/src/module/io.wren
+++ b/src/module/io.wren
@@ -1,5 +1,16 @@
 import "scheduler" for Scheduler
 
+class Directory {
+  static def list(path) {
+    if (!(path is String)) Fiber.abort("Path must be a string.")
+
+    list_(path, Fiber.current)
+    return Scheduler.runNextScheduled_()
+  }
+
+  foreign static def list_(path, fiber)
+}
+
 foreign class File {
   static def open(path) {
     if (!(path is String)) Fiber.abort("Path must be a string.")
@@ -33,6 +44,13 @@ foreign class File {
     return Scheduler.runNextScheduled_()
   }
 
+  static def stat(path) {
+    if (!(path is String)) Fiber.abort("Path must be a string.")
+
+    statPath_(path, Fiber.current)
+    return Stat.new_(Scheduler.runNextScheduled_())
+  }
+
   construct new_(fd) {}
 
   def close() {
@@ -40,6 +58,8 @@ foreign class File {
     Scheduler.runNextScheduled_()
   }
 
+  foreign def descriptor
+
   def isOpen { descriptor != -1 }
 
   def size {
@@ -49,25 +69,48 @@ foreign class File {
     return Scheduler.runNextScheduled_()
   }
 
-  def readBytes(count) {
+  def readBytes(count) { readBytes(count, 0) }
+
+  def readBytes(count, offset) {
     if (!isOpen) Fiber.abort("File is not open.")
     if (!(count is Num)) Fiber.abort("Count must be an integer.")
     if (!count.isInteger) Fiber.abort("Count must be an integer.")
     if (count < 0) Fiber.abort("Count cannot be negative.")
 
-    readBytes_(count, Fiber.current)
+    if (!(offset is Num)) Fiber.abort("Offset must be an integer.")
+    if (!offset.isInteger) Fiber.abort("Offset must be an integer.")
+    if (offset < 0) Fiber.abort("Offset cannot be negative.")
+
+    readBytes_(count, offset, Fiber.current)
     return Scheduler.runNextScheduled_()
   }
 
   foreign static def open_(path, fiber)
   foreign static def sizePath_(path, fiber)
+  foreign static def statPath_(path, fiber)
 
   foreign def close_(fiber)
-  foreign def descriptor
-  foreign def readBytes_(count, fiber)
+  foreign def readBytes_(count, start, fiber)
   foreign def size_(fiber)
 }
 
+class Stat {
+  construct new_(fields) {
+    _fields = fields
+  }
+
+  def device { _fields[0] }
+  def inode { _fields[1] }
+  def mode { _fields[2] }
+  def linkCount { _fields[3] }
+  def user { _fields[4] }
+  def group { _fields[5] }
+  def specialDevice { _fields[6] }
+  def size { _fields[7] }
+  def blockSize { _fields[8] }
+  def blockCount { _fields[9] }
+}
+
 class Stdin {
   static def readLine() {
     if (__isClosed == true) {
@@ -90,6 +133,7 @@ class Stdin {
       readStop_()
 
       if (__line != null) {
+        // Emit the last line.
         var line = __line
         __line = null
         if (__waitingFiber != null) __waitingFiber.transfer(line)
diff --git a/src/module/io.wren.inc b/src/module/io.wren.inc
index b5e6981c..8353c5ce 100644
--- a/src/module/io.wren.inc
+++ b/src/module/io.wren.inc
@@ -2,6 +2,17 @@
 static const char* ioModuleSource =
 "import \"scheduler\" for Scheduler\n"
 "\n"
+"class Directory {\n"
+"  static def list(path) {\n"
+"    if (!(path is String)) Fiber.abort(\"Path must be a string.\")\n"
+"\n"
+"    list_(path, Fiber.current)\n"
+"    return Scheduler.runNextScheduled_()\n"
+"  }\n"
+"\n"
+"  foreign static def list_(path, fiber)\n"
+"}\n"
+"\n"
 "foreign class File {\n"
 "  static def open(path) {\n"
 "    if (!(path is String)) Fiber.abort(\"Path must be a string.\")\n"
@@ -35,6 +46,13 @@ static const char* ioModuleSource =
 "    return Scheduler.runNextScheduled_()\n"
 "  }\n"
 "\n"
+"  static def stat(path) {\n"
+"    if (!(path is String)) Fiber.abort(\"Path must be a string.\")\n"
+"\n"
+"    statPath_(path, Fiber.current)\n"
+"    return Stat.new_(Scheduler.runNextScheduled_())\n"
+"  }\n"
+"\n"
 "  construct new_(fd) {}\n"
 "\n"
 "  def close() {\n"
@@ -42,6 +60,8 @@ static const char* ioModuleSource =
 "    Scheduler.runNextScheduled_()\n"
 "  }\n"
 "\n"
+"  foreign def descriptor\n"
+"\n"
 "  def isOpen { descriptor != -1 }\n"
 "\n"
 "  def size {\n"
@@ -51,25 +71,48 @@ static const char* ioModuleSource =
 "    return Scheduler.runNextScheduled_()\n"
 "  }\n"
 "\n"
-"  def readBytes(count) {\n"
+"  def readBytes(count) { readBytes(count, 0) }\n"
+"\n"
+"  def readBytes(count, offset) {\n"
 "    if (!isOpen) Fiber.abort(\"File is not open.\")\n"
 "    if (!(count is Num)) Fiber.abort(\"Count must be an integer.\")\n"
 "    if (!count.isInteger) Fiber.abort(\"Count must be an integer.\")\n"
 "    if (count < 0) Fiber.abort(\"Count cannot be negative.\")\n"
 "\n"
-"    readBytes_(count, Fiber.current)\n"
+"    if (!(offset is Num)) Fiber.abort(\"Offset must be an integer.\")\n"
+"    if (!offset.isInteger) Fiber.abort(\"Offset must be an integer.\")\n"
+"    if (offset < 0) Fiber.abort(\"Offset cannot be negative.\")\n"
+"\n"
+"    readBytes_(count, offset, Fiber.current)\n"
 "    return Scheduler.runNextScheduled_()\n"
 "  }\n"
 "\n"
 "  foreign static def open_(path, fiber)\n"
 "  foreign static def sizePath_(path, fiber)\n"
+"  foreign static def statPath_(path, fiber)\n"
 "\n"
 "  foreign def close_(fiber)\n"
-"  foreign def descriptor\n"
-"  foreign def readBytes_(count, fiber)\n"
+"  foreign def readBytes_(count, start, fiber)\n"
 "  foreign def size_(fiber)\n"
 "}\n"
 "\n"
+"class Stat {\n"
+"  construct new_(fields) {\n"
+"    _fields = fields\n"
+"  }\n"
+"\n"
+"  def device { _fields[0] }\n"
+"  def inode { _fields[1] }\n"
+"  def mode { _fields[2] }\n"
+"  def linkCount { _fields[3] }\n"
+"  def user { _fields[4] }\n"
+"  def group { _fields[5] }\n"
+"  def specialDevice { _fields[6] }\n"
+"  def size { _fields[7] }\n"
+"  def blockSize { _fields[8] }\n"
+"  def blockCount { _fields[9] }\n"
+"}\n"
+"\n"
 "class Stdin {\n"
 "  static def readLine() {\n"
 "    if (__isClosed == true) {\n"
@@ -92,6 +135,7 @@ static const char* ioModuleSource =
 "      readStop_()\n"
 "\n"
 "      if (__line != null) {\n"
+"        // Emit the last line.\n"
 "        var line = __line\n"
 "        __line = null\n"
 "        if (__waitingFiber != null) __waitingFiber.transfer(line)\n"
diff --git a/src/module/scheduler.c b/src/module/scheduler.c
index 20347eb8..28f4989f 100644
--- a/src/module/scheduler.c
+++ b/src/module/scheduler.c
@@ -7,30 +7,19 @@
 #include "wren.h"
 #include "vm.h"
 
+// A handle to the "Scheduler" class object. Used to call static methods on it.
+static WrenValue* schedulerClass;
+
 // This method resumes a fiber that is suspended waiting on an asynchronous
 // operation. The first resumes it with zero arguments, and the second passes
 // one.
-static WrenValue* resume;
-static WrenValue* resumeWithArg;
+static WrenValue* resume1;
+static WrenValue* resume2;
 static WrenValue* resumeError;
 
-void schedulerCaptureMethods(WrenVM* vm)
+static void resume(WrenValue* method)
 {
-  resume = wrenGetMethod(vm, "scheduler", "Scheduler", "resume_(_)");
-  resumeWithArg = wrenGetMethod(vm, "scheduler", "Scheduler", "resume_(_,_)");
-  resumeError = wrenGetMethod(vm, "scheduler", "Scheduler", "resumeError_(_,_)");
-}
-
-static void callResume(WrenValue* resumeMethod, WrenValue* fiber,
-                       const char* argTypes, ...)
-{
-  va_list args;
-  va_start(args, argTypes);
-  WrenInterpretResult result = wrenCallVarArgs(getVM(), resumeMethod, NULL,
-                                               argTypes, args);
-  va_end(args);
-  
-  wrenReleaseValue(getVM(), fiber);
+  WrenInterpretResult result = wrenCall(getVM(), method);
   
   // If a runtime error occurs in response to an async operation and nothing
   // catches the error in the fiber, then exit the CLI.
@@ -41,34 +30,50 @@ static void callResume(WrenValue* resumeMethod, WrenValue* fiber,
   }
 }
 
-void schedulerResume(WrenValue* fiber)
+void schedulerCaptureMethods(WrenVM* vm)
 {
-  callResume(resume, fiber, "v", fiber);
+  wrenEnsureSlots(vm, 1);
+  wrenGetVariable(vm, "scheduler", "Scheduler", 0);
+  schedulerClass = wrenGetSlotValue(vm, 0);
+  
+  resume1 = wrenMakeCallHandle(vm, "resume_(_)");
+  resume2 = wrenMakeCallHandle(vm, "resume_(_,_)");
+  resumeError = wrenMakeCallHandle(vm, "resumeError_(_,_)");
 }
 
-void schedulerResumeBytes(WrenValue* fiber, const char* bytes, size_t length)
+void schedulerResume(WrenValue* fiber, bool hasArgument)
 {
-  callResume(resumeWithArg, fiber, "va", fiber, bytes, length);
+  WrenVM* vm = getVM();
+  wrenEnsureSlots(vm, 2 + (hasArgument ? 1 : 0));
+  wrenSetSlotValue(vm, 0, schedulerClass);
+  wrenSetSlotValue(vm, 1, fiber);
+  wrenReleaseValue(vm, fiber);
+  
+  // If we don't need to wait for an argument to be stored on the stack, resume
+  // it now.
+  if (!hasArgument) resume(resume1);
 }
 
-void schedulerResumeDouble(WrenValue* fiber, double value)
+void schedulerFinishResume()
 {
-  callResume(resumeWithArg, fiber, "vd", fiber, value);
-}
-
-void schedulerResumeString(WrenValue* fiber, const char* text)
-{
-  callResume(resumeWithArg, fiber, "vs", fiber, text);
+  resume(resume2);
 }
 
 void schedulerResumeError(WrenValue* fiber, const char* error)
 {
-  callResume(resumeError, fiber, "vs", fiber, error);
+  schedulerResume(fiber, true);
+  wrenSetSlotString(getVM(), 2, error);
+  resume(resumeError);
 }
 
 void schedulerShutdown()
 {
-  if (resume != NULL) wrenReleaseValue(getVM(), resume);
-  if (resumeWithArg != NULL) wrenReleaseValue(getVM(), resumeWithArg);
-  if (resumeError != NULL) wrenReleaseValue(getVM(), resumeError);
+  // If the module was never loaded, we don't have anything to release.
+  if (schedulerClass == NULL) return;
+  
+  WrenVM* vm = getVM();
+  wrenReleaseValue(vm, schedulerClass);
+  wrenReleaseValue(vm, resume1);
+  wrenReleaseValue(vm, resume2);
+  wrenReleaseValue(vm, resumeError);
 }
diff --git a/src/module/scheduler.h b/src/module/scheduler.h
index e90ce1e4..943d5780 100644
--- a/src/module/scheduler.h
+++ b/src/module/scheduler.h
@@ -3,10 +3,16 @@
 
 #include "wren.h"
 
-void schedulerResume(WrenValue* fiber);
-void schedulerResumeBytes(WrenValue* fiber, const char* bytes, size_t length);
-void schedulerResumeDouble(WrenValue* fiber, double value);
-void schedulerResumeString(WrenValue* fiber, const char* text);
+// Sets up the API stack to call one of the resume methods on Scheduler.
+//
+// If [hasArgument] is false, this just sets up the stack to have another
+// argument stored in slot 2 and returns. The module must store the argument
+// on the stack and then call [schedulerFinishResume] to complete the call.
+//
+// Otherwise, the call resumes immediately. Releases [fiber] when called.
+void schedulerResume(WrenValue* fiber, bool hasArgument);
+
+void schedulerFinishResume();
 void schedulerResumeError(WrenValue* fiber, const char* error);
 
 void schedulerShutdown();
diff --git a/src/module/timer.c b/src/module/timer.c
index 43b8d662..406635f0 100644
--- a/src/module/timer.c
+++ b/src/module/timer.c
@@ -22,13 +22,13 @@ static void timerCallback(uv_timer_t* handle)
   uv_close((uv_handle_t*)handle, timerCloseCallback);
 
   // Run the fiber that was sleeping.
-  schedulerResume(fiber);
+  schedulerResume(fiber, false);
 }
 
 void timerStartTimer(WrenVM* vm)
 {
-  int milliseconds = (int)wrenGetArgumentDouble(vm, 1);
-  WrenValue* fiber = wrenGetArgumentValue(vm, 2);
+  int milliseconds = (int)wrenGetSlotDouble(vm, 1);
+  WrenValue* fiber = wrenGetSlotValue(vm, 2);
 
   // Store the fiber to resume when the timer completes.
   uv_timer_t* handle = (uv_timer_t*)malloc(sizeof(uv_timer_t));
diff --git a/src/optional/wren_opt_meta.c b/src/optional/wren_opt_meta.c
index 435ab678..a4f922df 100644
--- a/src/optional/wren_opt_meta.c
+++ b/src/optional/wren_opt_meta.c
@@ -18,13 +18,11 @@ void metaCompile(WrenVM* vm)
       : AS_CLOSURE(callingFn)->fn->module;
 
   // Compile it.
-  ObjFn* fn = wrenCompile(vm, module, wrenGetArgumentString(vm, 1), false);
-  if (fn == NULL) return;
+  ObjFn* fn = wrenCompile(vm, module, wrenGetSlotString(vm, 1), false);
 
   // Return the result. We can't use the public API for this since we have a
   // bare ObjFn.
-  *vm->foreignStackStart = OBJ_VAL(fn);
-  vm->foreignStackStart = NULL;
+  vm->apiStack[0] = fn != NULL ? OBJ_VAL(fn) : NULL_VAL;
 }
 
 static WrenForeignMethodFn bindMetaForeignMethods(WrenVM* vm,
diff --git a/src/optional/wren_opt_random.c b/src/optional/wren_opt_random.c
index b02c7ca7..120f5850 100644
--- a/src/optional/wren_opt_random.c
+++ b/src/optional/wren_opt_random.c
@@ -45,7 +45,7 @@ static void randomAllocate(WrenVM* vm)
 
 static void randomSeed0(WrenVM* vm)
 {
-  Well512* well = (Well512*)wrenGetArgumentForeign(vm, 0);
+  Well512* well = (Well512*)wrenGetSlotForeign(vm, 0);
 
   srand((uint32_t)time(NULL));
   for (int i = 0; i < 16; i++)
@@ -56,9 +56,9 @@ static void randomSeed0(WrenVM* vm)
 
 static void randomSeed1(WrenVM* vm)
 {
-  Well512* well = (Well512*)wrenGetArgumentForeign(vm, 0);
+  Well512* well = (Well512*)wrenGetSlotForeign(vm, 0);
 
-  srand((uint32_t)wrenGetArgumentDouble(vm, 1));
+  srand((uint32_t)wrenGetSlotDouble(vm, 1));
   for (int i = 0; i < 16; i++)
   {
     well->state[i] = rand();
@@ -67,17 +67,17 @@ static void randomSeed1(WrenVM* vm)
 
 static void randomSeed16(WrenVM* vm)
 {
-  Well512* well = (Well512*)wrenGetArgumentForeign(vm, 0);
+  Well512* well = (Well512*)wrenGetSlotForeign(vm, 0);
 
   for (int i = 0; i < 16; i++)
   {
-    well->state[i] = (uint32_t)wrenGetArgumentDouble(vm, i + 1);
+    well->state[i] = (uint32_t)wrenGetSlotDouble(vm, i + 1);
   }
 }
 
 static void randomFloat(WrenVM* vm)
 {
-  Well512* well = (Well512*)wrenGetArgumentForeign(vm, 0);
+  Well512* well = (Well512*)wrenGetSlotForeign(vm, 0);
 
   // A double has 53 bits of precision in its mantissa, and we'd like to take
   // full advantage of that, so we need 53 bits of random source data.
@@ -92,14 +92,14 @@ static void randomFloat(WrenVM* vm)
   // from 0 to 1.0 (half-inclusive).
   result /= 9007199254740992.0;
 
-  wrenReturnDouble(vm, result);
+  wrenSetSlotDouble(vm, 0, result);
 }
 
 static void randomInt0(WrenVM* vm)
 {
-  Well512* well = (Well512*)wrenGetArgumentForeign(vm, 0);
+  Well512* well = (Well512*)wrenGetSlotForeign(vm, 0);
 
-  wrenReturnDouble(vm, (double)advanceState(well));
+  wrenSetSlotDouble(vm, 0, (double)advanceState(well));
 }
 
 // TODO: The way these are wired up is pretty verbose and tedious. Also, the
diff --git a/src/vm/wren_compiler.c b/src/vm/wren_compiler.c
index f905c916..8c8cc7d8 100644
--- a/src/vm/wren_compiler.c
+++ b/src/vm/wren_compiler.c
@@ -1331,7 +1331,7 @@ static int resolveLocal(Compiler* compiler, const char* name, int length)
 
 // Adds an upvalue to [compiler]'s function with the given properties. Does not
 // add one if an upvalue for that variable is already in the list. Returns the
-// index of the uvpalue.
+// index of the upvalue.
 static int addUpvalue(Compiler* compiler, bool isLocal, int index)
 {
   // Look for an existing one.
diff --git a/src/vm/wren_core.c b/src/vm/wren_core.c
index 79ef463a..a5b38dd3 100644
--- a/src/vm/wren_core.c
+++ b/src/vm/wren_core.c
@@ -150,6 +150,8 @@ DEF_PRIMITIVE(fiber_suspend)
 {
   // Switching to a null fiber tells the interpreter to stop and exit.
   vm->fiber = NULL;
+  vm->apiStack = NULL;
+  
   return false;
 }
 
diff --git a/src/vm/wren_debug.c b/src/vm/wren_debug.c
index c44522ad..2d5d7063 100644
--- a/src/vm/wren_debug.c
+++ b/src/vm/wren_debug.c
@@ -20,6 +20,9 @@ void wrenDebugPrintStackTrace(ObjFiber* fiber)
     CallFrame* frame = &fiber->frames[i];
     ObjFn* fn = wrenUpwrapClosure(frame->fn);
 
+    // Skip over stub functions for calling methods from the C API.
+    if (fn->module == NULL) continue;
+    
     // The built-in core module has no name. We explicitly omit it from stack
     // traces since we don't want to highlight to a user the implementation
     // detail of what part of the core module is written in C and what is Wren.
@@ -50,7 +53,7 @@ static void dumpObject(Obj* obj)
     case OBJ_RANGE: printf("[range %p]", obj); break;
     case OBJ_STRING: printf("%s", ((ObjString*)obj)->value); break;
     case OBJ_UPVALUE: printf("[upvalue %p]", obj); break;
-    default: printf("[unknown object]"); break;
+    default: printf("[unknown object %d]", obj->type); break;
   }
 }
 
diff --git a/src/vm/wren_value.c b/src/vm/wren_value.c
index 1dfbd4f8..8daae9c1 100644
--- a/src/vm/wren_value.c
+++ b/src/vm/wren_value.c
@@ -157,7 +157,11 @@ ObjFiber* wrenNewFiber(WrenVM* vm, Obj* fn)
   // Allocate the arrays before the fiber in case it triggers a GC.
   CallFrame* frames = ALLOCATE_ARRAY(vm, CallFrame, INITIAL_CALL_FRAMES);
   
-  int stackCapacity = wrenPowerOf2Ceil(wrenUpwrapClosure(fn)->maxSlots);
+  // Add one slot for the unused implicit receiver slot that the compiler
+  // assumes all functions have.
+  int stackCapacity = fn == NULL
+      ? 1
+      : wrenPowerOf2Ceil(wrenUpwrapClosure(fn)->maxSlots + 1);
   Value* stack = ALLOCATE_ARRAY(vm, Value, stackCapacity);
   
   ObjFiber* fiber = ALLOCATE(vm, ObjFiber);
@@ -179,10 +183,52 @@ void wrenResetFiber(WrenVM* vm, ObjFiber* fiber, Obj* fn)
   fiber->caller = NULL;
   fiber->error = NULL_VAL;
   fiber->callerIsTrying = false;
+  fiber->numFrames = 0;
 
   // Initialize the first call frame.
-  fiber->numFrames = 0;
-  wrenAppendCallFrame(vm, fiber, fn, fiber->stack);
+  if (fn != NULL) wrenAppendCallFrame(vm, fiber, fn, fiber->stack);
+}
+
+void wrenEnsureStack(WrenVM* vm, ObjFiber* fiber, int needed)
+{
+  if (fiber->stackCapacity >= needed) return;
+  
+  int capacity = wrenPowerOf2Ceil(needed);
+  
+  Value* oldStack = fiber->stack;
+  fiber->stack = (Value*)wrenReallocate(vm, fiber->stack,
+                                        sizeof(Value) * fiber->stackCapacity,
+                                        sizeof(Value) * capacity);
+  fiber->stackCapacity = capacity;
+  
+  // If the reallocation moves the stack, then we need to shift every pointer
+  // into the stack to point to its new location.
+  if (fiber->stack != oldStack)
+  {
+    // Top of the stack.
+    long offset = fiber->stack - oldStack;
+    
+    if (vm->apiStack >= oldStack && vm->apiStack <= fiber->stackTop)
+    {
+      vm->apiStack += offset;
+    }
+    
+    // Stack pointer for each call frame.
+    for (int i = 0; i < fiber->numFrames; i++)
+    {
+      fiber->frames[i].stackStart += offset;
+    }
+    
+    // Open upvalues.
+    for (ObjUpvalue* upvalue = fiber->openUpvalues;
+         upvalue != NULL;
+         upvalue = upvalue->next)
+    {
+      upvalue->value += offset;
+    }
+    
+    fiber->stackTop += offset;
+  }
 }
 
 ObjForeign* wrenNewForeign(WrenVM* vm, ObjClass* classObj, size_t size)
@@ -389,6 +435,8 @@ static uint32_t hashValue(Value value)
     case VAL_OBJ:   return hashObject(AS_OBJ(value));
     default:        UNREACHABLE();
   }
+  
+  return 0;
 #endif
 }
 
@@ -1163,10 +1211,10 @@ void wrenFreeObj(WrenVM* vm, Obj* obj)
       wrenValueBufferClear(vm, &((ObjModule*)obj)->variables);
       break;
 
-    case OBJ_STRING:
     case OBJ_CLOSURE:
     case OBJ_INSTANCE:
     case OBJ_RANGE:
+    case OBJ_STRING:
     case OBJ_UPVALUE:
       break;
   }
diff --git a/src/vm/wren_value.h b/src/vm/wren_value.h
index d901747c..322ba011 100644
--- a/src/vm/wren_value.h
+++ b/src/vm/wren_value.h
@@ -74,6 +74,7 @@
 #define IS_FN(value) (wrenIsObjType(value, OBJ_FN))             // ObjFn
 #define IS_FOREIGN(value) (wrenIsObjType(value, OBJ_FOREIGN))   // ObjForeign
 #define IS_INSTANCE(value) (wrenIsObjType(value, OBJ_INSTANCE)) // ObjInstance
+#define IS_LIST(value) (wrenIsObjType(value, OBJ_LIST))         // ObjList
 #define IS_RANGE(value) (wrenIsObjType(value, OBJ_RANGE))       // ObjRange
 #define IS_STRING(value) (wrenIsObjType(value, OBJ_STRING))     // ObjString
 
@@ -662,6 +663,9 @@ static inline void wrenAppendCallFrame(WrenVM* vm, ObjFiber* fiber,
   frame->ip = wrenUpwrapClosure(frame->fn)->bytecode;
 }
 
+// Ensures [fiber]'s stack has at least [needed] slots.
+void wrenEnsureStack(WrenVM* vm, ObjFiber* fiber, int needed);
+
 ObjForeign* wrenNewForeign(WrenVM* vm, ObjClass* classObj, size_t size);
 
 // TODO: The argument list here is getting a bit gratuitous.
diff --git a/src/vm/wren_vm.c b/src/vm/wren_vm.c
index 67fcc82f..ea93f4b3 100644
--- a/src/vm/wren_vm.c
+++ b/src/vm/wren_vm.c
@@ -65,7 +65,7 @@ WrenVM* wrenNewVM(WrenConfiguration* config)
   vm->modules = wrenNewMap(vm);
 
   wrenInitializeCore(vm);
-
+  
   // TODO: Lazy load these.
   #if WREN_OPT_META
     wrenLoadMetaModule(vm);
@@ -137,7 +137,7 @@ void wrenCollectGarbage(WrenVM* vm)
     wrenGrayObj(vm, vm->tempRoots[i]);
   }
 
-  // The current fiber.
+  // The rooted fiber.
   wrenGrayObj(vm, (Obj*)vm->fiber);
 
   // The value handles.
@@ -348,19 +348,14 @@ static void bindMethod(WrenVM* vm, int methodType, int symbol,
 static void callForeign(WrenVM* vm, ObjFiber* fiber,
                         WrenForeignMethodFn foreign, int numArgs)
 {
-  vm->foreignStackStart = fiber->stackTop - numArgs;
+  vm->apiStack = fiber->stackTop - numArgs;
+
   foreign(vm);
 
-  // Discard the stack slots for the arguments (but leave one for
-  // the result).
-  fiber->stackTop -= numArgs - 1;
-
-  // If nothing was returned, implicitly return null.
-  if (vm->foreignStackStart != NULL)
-  {
-    *vm->foreignStackStart = NULL_VAL;
-    vm->foreignStackStart = NULL;
-  }
+  // Discard the stack slots for the arguments and temporaries but leave one
+  // for the result.
+  fiber->stackTop = vm->apiStack + 1;
+  vm->apiStack = NULL;
 }
 
 // Handles the current fiber having aborted because of an error. Switches to
@@ -469,41 +464,8 @@ static inline void callFunction(
   // Grow the stack if needed.
   int stackSize = (int)(fiber->stackTop - fiber->stack);
   int needed = stackSize + wrenUpwrapClosure(function)->maxSlots;
+  wrenEnsureStack(vm, fiber, needed);
   
-  if (fiber->stackCapacity < needed)
-  {
-    int capacity = wrenPowerOf2Ceil(needed);
-    
-    Value* oldStack = fiber->stack;
-    fiber->stack = (Value*)wrenReallocate(vm, fiber->stack,
-        sizeof(Value) * fiber->stackCapacity,
-        sizeof(Value) * capacity);
-    fiber->stackCapacity = capacity;
-    
-    // If the reallocation moves the stack, then we need to shift every pointer
-    // into the stack to point to its new location.
-    if (fiber->stack != oldStack)
-    {
-      // Top of the stack.
-      long offset = fiber->stack - oldStack;
-      fiber->stackTop += offset;
-      
-      // Stack pointer for each call frame.
-      for (int i = 0; i < fiber->numFrames; i++)
-      {
-        fiber->frames[i].stackStart += offset;
-      }
-      
-      // Open upvalues.
-      for (ObjUpvalue* upvalue = fiber->openUpvalues;
-           upvalue != NULL;
-           upvalue = upvalue->next)
-      {
-        upvalue->value += offset;
-      }
-    }
-  }
-
   wrenAppendCallFrame(vm, fiber, function, fiber->stackTop - numArgs);
 }
 
@@ -689,7 +651,7 @@ static void bindForeignClass(WrenVM* vm, ObjClass* classObj, ObjModule* module)
 
   if (methods.finalize != NULL)
   {
-    method.fn.foreign = methods.finalize;
+    method.fn.foreign = (WrenForeignMethodFn)methods.finalize;
     wrenBindMethod(vm, classObj, symbol, method);
   }
 }
@@ -742,7 +704,7 @@ static void createForeign(WrenVM* vm, ObjFiber* fiber, Value* stack)
   ASSERT(method->type == METHOD_FOREIGN, "Allocator should be foreign.");
 
   // Pass the constructor arguments to the allocator as well.
-  vm->foreignStackStart = stack;
+  vm->apiStack = stack;
 
   method->fn.foreign(vm);
 
@@ -767,11 +729,8 @@ void wrenFinalizeForeign(WrenVM* vm, ObjForeign* foreign)
 
   ASSERT(method->type == METHOD_FOREIGN, "Finalizer should be foreign.");
 
-  // Pass the constructor arguments to the allocator as well.
-  Value slot = OBJ_VAL(foreign);
-  vm->foreignStackStart = &slot;
-
-  method->fn.foreign(vm);
+  WrenFinalizerFn finalizer = (WrenFinalizerFn)method->fn.foreign;
+  finalizer(foreign->data);
 }
 
 // The main bytecode interpreter loop. This is where the magic happens. It is
@@ -1179,18 +1138,23 @@ static WrenInterpretResult runInterpreter(WrenVM* vm, register ObjFiber* fiber)
       // If the fiber is complete, end it.
       if (fiber->numFrames == 0)
       {
-        // See if there's another fiber to return to.
-        ObjFiber* callingFiber = fiber->caller;
+        // See if there's another fiber to return to. If not, we're done.
+        if (fiber->caller == NULL)
+        {
+          // Store the final result value at the beginning of the stack so the
+          // C API can get it.
+          fiber->stack[0] = result;
+          fiber->stackTop = fiber->stack + 1;
+          return WREN_RESULT_SUCCESS;
+        }
+        
+        ObjFiber* resumingFiber = fiber->caller;
         fiber->caller = NULL;
+        fiber = resumingFiber;
+        vm->fiber = resumingFiber;
         
-        fiber = callingFiber;
-        vm->fiber = fiber;
-        
-        // If not, we're done.
-        if (fiber == NULL) return WREN_RESULT_SUCCESS;
-
         // Store the result in the resuming fiber.
-        *(fiber->stackTop - 1) = result;
+        fiber->stackTop[-1] = result;
       }
       else
       {
@@ -1202,7 +1166,7 @@ static WrenInterpretResult runInterpreter(WrenVM* vm, register ObjFiber* fiber)
         // result).
         fiber->stackTop = frame->stackStart + 1;
       }
-
+      
       LOAD_FRAME();
       DISPATCH();
     }
@@ -1330,11 +1294,10 @@ static WrenInterpretResult runInterpreter(WrenVM* vm, register ObjFiber* fiber)
   #undef READ_SHORT
 }
 
-// Creates an [ObjFn] that invokes a method with [signature] when called.
-static ObjFn* makeCallStub(WrenVM* vm, ObjModule* module, const char* signature)
+WrenValue* wrenMakeCallHandle(WrenVM* vm, const char* signature)
 {
   int signatureLength = (int)strlen(signature);
-
+  
   // Count the number parameters the method expects.
   int numParams = 0;
   if (signature[signatureLength - 1] == ')')
@@ -1345,167 +1308,73 @@ static ObjFn* makeCallStub(WrenVM* vm, ObjModule* module, const char* signature)
       if (*s == '_') numParams++;
     }
   }
-
+  
+  // Add the signatue to the method table.
   int method =  wrenSymbolTableEnsure(vm, &vm->methodNames,
                                       signature, signatureLength);
-
+  
+  // Create a little stub function that assumes the arguments are on the stack
+  // and calls the method.
   uint8_t* bytecode = ALLOCATE_ARRAY(vm, uint8_t, 5);
   bytecode[0] = (uint8_t)(CODE_CALL_0 + numParams);
   bytecode[1] = (method >> 8) & 0xff;
   bytecode[2] = method & 0xff;
   bytecode[3] = CODE_RETURN;
   bytecode[4] = CODE_END;
-
+  
   int* debugLines = ALLOCATE_ARRAY(vm, int, 5);
   memset(debugLines, 1, 5);
+  
+  ObjFn* fn = wrenNewFunction(vm, NULL, NULL, 0, 0, numParams + 1, 0, bytecode,
+                             5, signature, signatureLength, debugLines);
 
-  return wrenNewFunction(vm, module, NULL, 0, 0, numParams + 1, 0, bytecode, 5,
-                         signature, signatureLength, debugLines);
+  // Wrap the function in a handle.
+  return wrenCaptureValue(vm, OBJ_VAL(fn));
 }
 
-WrenValue* wrenGetMethod(WrenVM* vm, const char* module, const char* variable,
-                         const char* signature)
+WrenInterpretResult wrenCall(WrenVM* vm, WrenValue* method)
 {
-  Value moduleName = wrenStringFormat(vm, "$", module);
-  wrenPushRoot(vm, AS_OBJ(moduleName));
-
-  ObjModule* moduleObj = getModule(vm, moduleName);
-  // TODO: Handle module not being found.
-
-  int variableSlot = wrenSymbolTableFind(&moduleObj->variableNames,
-                                         variable, strlen(variable));
-  // TODO: Handle the variable not being found.
-
-  ObjFn* fn = makeCallStub(vm, moduleObj, signature);
-  wrenPushRoot(vm, (Obj*)fn);
-
-  // Create a single fiber that we can reuse each time the method is invoked.
-  ObjFiber* fiber = wrenNewFiber(vm, (Obj*)fn);
-  wrenPushRoot(vm, (Obj*)fiber);
-
-  // Create a handle that keeps track of the function that calls the method.
-  WrenValue* method = wrenCaptureValue(vm, OBJ_VAL(fiber));
-
-  // Store the receiver in the fiber's stack so we can use it later in the call.
-  *fiber->stackTop++ = moduleObj->variables.data[variableSlot];
-
-  wrenPopRoot(vm); // fiber.
-  wrenPopRoot(vm); // fn.
-  wrenPopRoot(vm); // moduleName.
-
-  return method;
-}
-
-WrenInterpretResult wrenCall(WrenVM* vm, WrenValue* method,
-                             WrenValue** returnValue,
-                             const char* argTypes, ...)
-{
-  va_list args;
-  va_start(args, argTypes);
-  WrenInterpretResult result = wrenCallVarArgs(vm, method, returnValue,
-                                               argTypes, args);
-  va_end(args);
-
-  return result;
-}
-
-WrenInterpretResult wrenCallVarArgs(WrenVM* vm, WrenValue* method,
-                                    WrenValue** returnValue,
-                                    const char* argTypes, va_list args)
-{
-  // TODO: Validate that the number of arguments matches what the method
-  // expects.
-
-  ASSERT(IS_FIBER(method->value), "Value must come from wrenGetMethod().");
-  ObjFiber* fiber = AS_FIBER(method->value);
-
-  // Push the arguments.
-  for (const char* argType = argTypes; *argType != '\0'; argType++)
-  {
-    Value value = NULL_VAL;
-    switch (*argType)
-    {
-      case 'a':
-      {
-        const char* bytes = va_arg(args, const char*);
-        int length = va_arg(args, int);
-        value = wrenNewString(vm, bytes, (size_t)length);
-        break;
-      }
-
-      case 'b': value = BOOL_VAL(va_arg(args, int)); break;
-      case 'd': value = NUM_VAL(va_arg(args, double)); break;
-      case 'i': value = NUM_VAL((double)va_arg(args, int)); break;
-      case 'n': value = NULL_VAL; va_arg(args, void*); break;
-      case 's':
-        value = wrenStringFormat(vm, "$", va_arg(args, const char*));
-        break;
-
-      case 'v':
-      {
-        // Allow a NULL value pointer for Wren null.
-        WrenValue* wrenValue = va_arg(args, WrenValue*);
-        if (wrenValue != NULL) value = wrenValue->value;
-        break;
-      }
-
-      default:
-        ASSERT(false, "Unknown argument type.");
-        break;
-    }
-
-    *fiber->stackTop++ = value;
-  }
-
-  Value receiver = fiber->stack[0];
-  Obj* fn = fiber->frames[0].fn;
-  wrenPushRoot(vm, (Obj*)fn);
-
-  WrenInterpretResult result = runInterpreter(vm, fiber);
-
-  if (result == WREN_RESULT_SUCCESS)
-  {
-    if (returnValue != NULL)
-    {
-      // Make sure the return value doesn't get collected while capturing it.
-      fiber->stackTop++;
-      *returnValue = wrenCaptureValue(vm, fiber->stack[0]);
-    }
-
-    // Reset the fiber to get ready for the next call.
-    wrenResetFiber(vm, fiber, fn);
-
-    // Push the receiver back on the stack.
-    *fiber->stackTop++ = receiver;
-  }
-  else
-  {
-    if (returnValue != NULL) *returnValue = NULL;
-  }
-
-  wrenPopRoot(vm);
-
-  return result;
+  ASSERT(method != NULL, "Method cannot be NULL.");
+  ASSERT(IS_FN(method->value), "Method must be a method handle.");
+  ASSERT(vm->fiber != NULL, "Must set up arguments for call first.");
+  ASSERT(vm->apiStack != NULL, "Must set up arguments for call first.");
+  ASSERT(vm->fiber->numFrames == 0, "Can not call from a foreign method.");
+  
+  ObjFn* fn = AS_FN(method->value);
+  
+  ASSERT(vm->fiber->stackTop - vm->fiber->stack >= fn->arity,
+         "Stack must have enough arguments for method.");
+  
+  // Discard any extra temporary slots. We take for granted that the stub
+  // function has exactly one slot for each arguments.
+  vm->fiber->stackTop = &vm->fiber->stack[fn->maxSlots];
+  
+  callFunction(vm, vm->fiber, (Obj*)fn, 0);
+  return runInterpreter(vm, vm->fiber);
 }
 
 WrenValue* wrenCaptureValue(WrenVM* vm, Value value)
 {
+  if (IS_OBJ(value)) wrenPushRoot(vm, AS_OBJ(value));
+  
   // Make a handle for it.
   WrenValue* wrappedValue = ALLOCATE(vm, WrenValue);
   wrappedValue->value = value;
 
+  if (IS_OBJ(value)) wrenPopRoot(vm);
+
   // Add it to the front of the linked list of handles.
   if (vm->valueHandles != NULL) vm->valueHandles->prev = wrappedValue;
   wrappedValue->prev = NULL;
   wrappedValue->next = vm->valueHandles;
   vm->valueHandles = wrappedValue;
-
+  
   return wrappedValue;
 }
 
 void wrenReleaseValue(WrenVM* vm, WrenValue* value)
 {
-  ASSERT(value != NULL, "NULL value.");
+  ASSERT(value != NULL, "Value cannot be NULL.");
 
   // Update the VM's head pointer if we're releasing the first handle.
   if (vm->valueHandles == value) vm->valueHandles = value->next;
@@ -1524,14 +1393,14 @@ void wrenReleaseValue(WrenVM* vm, WrenValue* value)
 
 void* wrenAllocateForeign(WrenVM* vm, size_t size)
 {
-  ASSERT(vm->foreignStackStart != NULL, "Must be in foreign call.");
+  ASSERT(vm->apiStack != NULL, "Must be in foreign call.");
 
   // TODO: Validate this. It can fail if the user calls this inside another
   // foreign method, or calls one of the return functions.
-  ObjClass* classObj = AS_CLASS(vm->foreignStackStart[0]);
+  ObjClass* classObj = AS_CLASS(vm->apiStack[0]);
 
   ObjForeign* foreign = wrenNewForeign(vm, classObj, size);
-  vm->foreignStackStart[0] = OBJ_VAL(foreign);
+  vm->apiStack[0] = OBJ_VAL(foreign);
 
   return (void*)foreign->data;
 }
@@ -1658,100 +1527,170 @@ void wrenPopRoot(WrenVM* vm)
   vm->numTempRoots--;
 }
 
-int wrenGetArgumentCount(WrenVM* vm)
+int wrenGetSlotCount(WrenVM* vm)
 {
-  ASSERT(vm->foreignStackStart != NULL, "Must be in foreign call.");
+  if (vm->apiStack == NULL) return 0;
   
-  // If no fiber is executing, we must be in a finalizer, in which case the
-  // "stack" just has one object, the object being finalized.
-  if (vm->fiber == NULL) return 1;
-
-  return (int)(vm->fiber->stackTop - vm->foreignStackStart);
+  return (int)(vm->fiber->stackTop - vm->apiStack);
 }
 
-static void validateForeignArgument(WrenVM* vm, int index)
+void wrenEnsureSlots(WrenVM* vm, int numSlots)
 {
-  ASSERT(vm->foreignStackStart != NULL, "Must be in foreign call.");
-  ASSERT(index >= 0, "index cannot be negative.");
-  ASSERT(index < wrenGetArgumentCount(vm), "Not that many arguments.");
+  // If we don't have a fiber accessible, create one for the API to use.
+  if (vm->apiStack == NULL)
+  {
+    vm->fiber = wrenNewFiber(vm, NULL);
+    vm->apiStack = vm->fiber->stack;
+  }
+  
+  int currentSize = (int)(vm->fiber->stackTop - vm->apiStack);
+  if (currentSize >= numSlots) return;
+  
+  // Grow the stack if needed.
+  int needed = (int)(vm->apiStack - vm->fiber->stack) + numSlots;
+  wrenEnsureStack(vm, vm->fiber, needed);
+  
+  vm->fiber->stackTop = vm->apiStack + numSlots;
 }
 
-bool wrenGetArgumentBool(WrenVM* vm, int index)
+// Ensures that [slot] is a valid index into the API's stack of slots.
+static void validateApiSlot(WrenVM* vm, int slot)
 {
-  validateForeignArgument(vm, index);
-
-  if (!IS_BOOL(vm->foreignStackStart[index])) return false;
-
-  return AS_BOOL(vm->foreignStackStart[index]);
+  ASSERT(slot >= 0, "Slot cannot be negative.");
+  ASSERT(slot < wrenGetSlotCount(vm), "Not that many slots.");
 }
 
-double wrenGetArgumentDouble(WrenVM* vm, int index)
+bool wrenGetSlotBool(WrenVM* vm, int slot)
 {
-  validateForeignArgument(vm, index);
+  validateApiSlot(vm, slot);
+  ASSERT(IS_BOOL(vm->apiStack[slot]), "Slot must hold a bool.");
 
-  if (!IS_NUM(vm->foreignStackStart[index])) return 0.0;
-
-  return AS_NUM(vm->foreignStackStart[index]);
+  return AS_BOOL(vm->apiStack[slot]);
 }
 
-void* wrenGetArgumentForeign(WrenVM* vm, int index)
+const char* wrenGetSlotBytes(WrenVM* vm, int slot, int* length)
 {
-  validateForeignArgument(vm, index);
-
-  if (!IS_FOREIGN(vm->foreignStackStart[index])) return NULL;
-
-  return AS_FOREIGN(vm->foreignStackStart[index])->data;
+  validateApiSlot(vm, slot);
+  ASSERT(IS_STRING(vm->apiStack[slot]), "Slot must hold a string.");
+  
+  ObjString* string = AS_STRING(vm->apiStack[slot]);
+  *length = string->length;
+  return string->value;
 }
 
-const char* wrenGetArgumentString(WrenVM* vm, int index)
+double wrenGetSlotDouble(WrenVM* vm, int slot)
 {
-  validateForeignArgument(vm, index);
+  validateApiSlot(vm, slot);
+  ASSERT(IS_NUM(vm->apiStack[slot]), "Slot must hold a number.");
 
-  if (!IS_STRING(vm->foreignStackStart[index])) return NULL;
-
-  return AS_CSTRING(vm->foreignStackStart[index]);
+  return AS_NUM(vm->apiStack[slot]);
 }
 
-WrenValue* wrenGetArgumentValue(WrenVM* vm, int index)
+void* wrenGetSlotForeign(WrenVM* vm, int slot)
 {
-  validateForeignArgument(vm, index);
+  validateApiSlot(vm, slot);
+  ASSERT(IS_FOREIGN(vm->apiStack[slot]),
+         "Slot must hold a foreign instance.");
 
-  return wrenCaptureValue(vm, vm->foreignStackStart[index]);
+  return AS_FOREIGN(vm->apiStack[slot])->data;
 }
 
-void wrenReturnBool(WrenVM* vm, bool value)
+const char* wrenGetSlotString(WrenVM* vm, int slot)
 {
-  ASSERT(vm->foreignStackStart != NULL, "Must be in foreign call.");
+  validateApiSlot(vm, slot);
+  ASSERT(IS_STRING(vm->apiStack[slot]), "Slot must hold a string.");
 
-  *vm->foreignStackStart = BOOL_VAL(value);
-  vm->foreignStackStart = NULL;
+  return AS_CSTRING(vm->apiStack[slot]);
 }
 
-void wrenReturnDouble(WrenVM* vm, double value)
+WrenValue* wrenGetSlotValue(WrenVM* vm, int slot)
 {
-  ASSERT(vm->foreignStackStart != NULL, "Must be in foreign call.");
-
-  *vm->foreignStackStart = NUM_VAL(value);
-  vm->foreignStackStart = NULL;
+  validateApiSlot(vm, slot);
+  return wrenCaptureValue(vm, vm->apiStack[slot]);
 }
 
-void wrenReturnString(WrenVM* vm, const char* text, int length)
+// Stores [value] in [slot] in the foreign call stack.
+static void setSlot(WrenVM* vm, int slot, Value value)
+{
+  validateApiSlot(vm, slot);
+  vm->apiStack[slot] = value;
+}
+
+void wrenSetSlotBool(WrenVM* vm, int slot, bool value)
+{
+  setSlot(vm, slot, BOOL_VAL(value));
+}
+
+void wrenSetSlotBytes(WrenVM* vm, int slot, const char* bytes, size_t length)
+{
+  ASSERT(bytes != NULL, "Byte array cannot be NULL.");
+  setSlot(vm, slot, wrenNewString(vm, bytes, length));
+}
+
+void wrenSetSlotDouble(WrenVM* vm, int slot, double value)
+{
+  setSlot(vm, slot, NUM_VAL(value));
+}
+
+void wrenSetSlotNewList(WrenVM* vm, int slot)
+{
+  setSlot(vm, slot, OBJ_VAL(wrenNewList(vm, 0)));
+}
+
+void wrenSetSlotNull(WrenVM* vm, int slot)
+{
+  setSlot(vm, slot, NULL_VAL);
+}
+
+void wrenSetSlotString(WrenVM* vm, int slot, const char* text)
 {
-  ASSERT(vm->foreignStackStart != NULL, "Must be in foreign call.");
   ASSERT(text != NULL, "String cannot be NULL.");
-
-  size_t size = length;
-  if (length == -1) size = strlen(text);
-
-  *vm->foreignStackStart = wrenNewString(vm, text, size);
-  vm->foreignStackStart = NULL;
+  setSlot(vm, slot, wrenNewString(vm, text, strlen(text)));
 }
 
-void wrenReturnValue(WrenVM* vm, WrenValue* value)
+void wrenSetSlotValue(WrenVM* vm, int slot, WrenValue* value)
 {
-  ASSERT(vm->foreignStackStart != NULL, "Must be in foreign call.");
   ASSERT(value != NULL, "Value cannot be NULL.");
 
-  *vm->foreignStackStart = value->value;
-  vm->foreignStackStart = NULL;
+  setSlot(vm, slot, value->value);
+}
+
+void wrenInsertInList(WrenVM* vm, int listSlot, int index, int elementSlot)
+{
+  validateApiSlot(vm, listSlot);
+  validateApiSlot(vm, elementSlot);
+  ASSERT(IS_LIST(vm->apiStack[listSlot]), "Must insert into a list.");
+  
+  ObjList* list = AS_LIST(vm->apiStack[listSlot]);
+  
+  // Negative indices count from the end.
+  if (index < 0) index = list->elements.count + 1 + index;
+  
+  ASSERT(index <= list->elements.count, "Index out of bounds.");
+  
+  wrenListInsert(vm, list, vm->apiStack[elementSlot], index);
+}
+
+// TODO: Maybe just have this always return a WrenValue* instead of having to
+// deal with slots?
+void wrenGetVariable(WrenVM* vm, const char* module, const char* name,
+                     int slot)
+{
+  ASSERT(module != NULL, "Module cannot be NULL.");
+  ASSERT(module != NULL, "Variable name cannot be NULL.");  
+  validateApiSlot(vm, slot);
+  
+  Value moduleName = wrenStringFormat(vm, "$", module);
+  wrenPushRoot(vm, AS_OBJ(moduleName));
+  
+  ObjModule* moduleObj = getModule(vm, moduleName);
+  ASSERT(moduleObj != NULL, "Could not find module.");
+  
+  wrenPopRoot(vm); // moduleName.
+
+  int variableSlot = wrenSymbolTableFind(&moduleObj->variableNames,
+                                         name, strlen(name));
+  ASSERT(variableSlot != -1, "Could not find variable.");
+  
+  setSlot(vm, slot, moduleObj->variables.data[variableSlot]);
 }
diff --git a/src/vm/wren_vm.h b/src/vm/wren_vm.h
index 1d0b0776..2b9d8d01 100644
--- a/src/vm/wren_vm.h
+++ b/src/vm/wren_vm.h
@@ -84,11 +84,14 @@ struct WrenVM
   // NULL if there are no handles.
   WrenValue* valueHandles;
   
-  // Foreign function data:
-
-  // During a foreign function call, this will point to the first argument (the
-  // receiver) of the call on the fiber's stack.
-  Value* foreignStackStart;
+  // Pointer to the bottom of the range of stack slots available for use from
+  // the C API. During a foreign method, this will be in the stack of the fiber
+  // that is executing a method.
+  //
+  // If not in a foreign method, this is initially NULL. If the user requests
+  // slots by calling wrenEnsureSlots(), a stack is created and this is
+  // initialized.
+  Value* apiStack;
 
   WrenConfiguration config;
   
diff --git a/test/api/benchmark.c b/test/api/benchmark.c
index 5c7f4ccb..672e97ba 100644
--- a/test/api/benchmark.c
+++ b/test/api/benchmark.c
@@ -1,21 +1,81 @@
 #include <string.h>
+#include <time.h>
 
 #include "benchmark.h"
 
 static void arguments(WrenVM* vm)
 {
   double result = 0;
-  result += wrenGetArgumentDouble(vm, 1);
-  result += wrenGetArgumentDouble(vm, 2);
-  result += wrenGetArgumentDouble(vm, 3);
-  result += wrenGetArgumentDouble(vm, 4);
+
+  result += wrenGetSlotDouble(vm, 1);
+  result += wrenGetSlotDouble(vm, 2);
+  result += wrenGetSlotDouble(vm, 3);
+  result += wrenGetSlotDouble(vm, 4);
+
+  wrenSetSlotDouble(vm, 0, result);
+}
+
+const char* testScript =
+"class Test {\n"
+"  static method(a, b, c, d) { a + b + c + d }\n"
+"}\n";
+
+static void call(WrenVM* vm)
+{
+  int iterations = (int)wrenGetSlotDouble(vm, 1);
   
-  wrenReturnDouble(vm, result);
+  // Since the VM is not re-entrant, we can't call from within this foreign
+  // method. Instead, make a new VM to run the call test in.
+  WrenConfiguration config;
+  wrenInitConfiguration(&config);
+  WrenVM* otherVM = wrenNewVM(&config);
+  
+  wrenInterpret(otherVM, testScript);
+  
+  WrenValue* method = wrenMakeCallHandle(otherVM, "method(_,_,_,_)");
+  
+  wrenEnsureSlots(otherVM, 1);
+  wrenGetVariable(otherVM, "main", "Test", 0);
+  WrenValue* testClass = wrenGetSlotValue(otherVM, 0);
+  
+  double startTime = (double)clock() / CLOCKS_PER_SEC;
+  
+  double result = 0;
+  for (int i = 0; i < iterations; i++)
+  {
+    wrenEnsureSlots(otherVM, 5);
+    wrenSetSlotValue(otherVM, 0, testClass);
+    wrenSetSlotDouble(otherVM, 1, 1.0);
+    wrenSetSlotDouble(otherVM, 2, 2.0);
+    wrenSetSlotDouble(otherVM, 3, 3.0);
+    wrenSetSlotDouble(otherVM, 4, 4.0);
+    
+    wrenCall(otherVM, method);
+    
+    result += wrenGetSlotDouble(otherVM, 0);
+  }
+  
+  double elapsed = (double)clock() / CLOCKS_PER_SEC - startTime;
+  
+  wrenReleaseValue(otherVM, testClass);
+  wrenReleaseValue(otherVM, method);
+  wrenFreeVM(otherVM);
+  
+  if (result == (1.0 + 2.0 + 3.0 + 4.0) * iterations)
+  {
+    wrenSetSlotDouble(vm, 0, elapsed);
+  }
+  else
+  {
+    // Got the wrong result.
+    wrenSetSlotBool(vm, 0, false);
+  }
 }
 
 WrenForeignMethodFn benchmarkBindMethod(const char* signature)
 {
   if (strcmp(signature, "static Benchmark.arguments(_,_,_,_)") == 0) return arguments;
-  
+  if (strcmp(signature, "static Benchmark.call(_)") == 0) return call;
+
   return NULL;
 }
diff --git a/test/api/call.c b/test/api/call.c
index 5207a456..d119dde9 100644
--- a/test/api/call.c
+++ b/test/api/call.c
@@ -5,33 +5,84 @@
 
 void callRunTests(WrenVM* vm)
 {
-  WrenValue* noParams = wrenGetMethod(vm, "main", "Call", "noParams");
-  WrenValue* zero = wrenGetMethod(vm, "main", "Call", "zero()");
-  WrenValue* one = wrenGetMethod(vm, "main", "Call", "one(_)");
-  WrenValue* two = wrenGetMethod(vm, "main", "Call", "two(_,_)");
+  wrenEnsureSlots(vm, 1);
+  wrenGetVariable(vm, "main", "Call", 0);
+  WrenValue* callClass = wrenGetSlotValue(vm, 0);
+  
+  WrenValue* noParams = wrenMakeCallHandle(vm, "noParams");
+  WrenValue* zero = wrenMakeCallHandle(vm, "zero()");
+  WrenValue* one = wrenMakeCallHandle(vm, "one(_)");
+  WrenValue* two = wrenMakeCallHandle(vm, "two(_,_)");
   
   // Different arity.
-  wrenCall(vm, noParams, NULL, "");
-  wrenCall(vm, zero, NULL, "");
-  wrenCall(vm, one, NULL, "i", 1);
-  wrenCall(vm, two, NULL, "ii", 1, 2);
-
-  WrenValue* getValue = wrenGetMethod(vm, "main", "Call", "getValue(_)");
+  wrenEnsureSlots(vm, 1);
+  wrenSetSlotValue(vm, 0, callClass);
+  wrenCall(vm, noParams);
+  
+  wrenEnsureSlots(vm, 1);
+  wrenSetSlotValue(vm, 0, callClass);
+  wrenCall(vm, zero);
+  
+  wrenEnsureSlots(vm, 2);
+  wrenSetSlotValue(vm, 0, callClass);
+  wrenSetSlotDouble(vm, 1, 1.0);
+  wrenCall(vm, one);
+  
+  wrenEnsureSlots(vm, 3);
+  wrenSetSlotValue(vm, 0, callClass);
+  wrenSetSlotDouble(vm, 1, 1.0);
+  wrenSetSlotDouble(vm, 2, 2.0);
+  wrenCall(vm, two);
   
   // Returning a value.
-  WrenValue* value = NULL;
-  wrenCall(vm, getValue, &value, "v", NULL);
+  WrenValue* getValue = wrenMakeCallHandle(vm, "getValue()");
+  wrenEnsureSlots(vm, 1);
+  wrenSetSlotValue(vm, 0, callClass);
+  wrenCall(vm, getValue);
+  WrenValue* value = wrenGetSlotValue(vm, 0);
   
   // Different argument types.
-  wrenCall(vm, two, NULL, "bb", true, false);
-  wrenCall(vm, two, NULL, "dd", 1.2, 3.4);
-  wrenCall(vm, two, NULL, "ii", 3, 4);
-  wrenCall(vm, two, NULL, "ss", "string", "another");
-  wrenCall(vm, two, NULL, "vv", NULL, value);
+  wrenEnsureSlots(vm, 3);
+  wrenSetSlotValue(vm, 0, callClass);
+  wrenSetSlotBool(vm, 1, true);
+  wrenSetSlotBool(vm, 2, false);
+  wrenCall(vm, two);
+
+  wrenEnsureSlots(vm, 3);
+  wrenSetSlotValue(vm, 0, callClass);
+  wrenSetSlotDouble(vm, 1, 1.2);
+  wrenSetSlotDouble(vm, 2, 3.4);
+  wrenCall(vm, two);
+  
+  wrenEnsureSlots(vm, 3);
+  wrenSetSlotValue(vm, 0, callClass);
+  wrenSetSlotString(vm, 1, "string");
+  wrenSetSlotString(vm, 2, "another");
+  wrenCall(vm, two);
+  
+  wrenEnsureSlots(vm, 3);
+  wrenSetSlotValue(vm, 0, callClass);
+  wrenSetSlotNull(vm, 1);
+  wrenSetSlotValue(vm, 2, value);
+  wrenCall(vm, two);
   
   // Truncate a string, or allow null bytes.
-  wrenCall(vm, two, NULL, "aa", "string", 3, "b\0y\0t\0e", 7);
+  wrenEnsureSlots(vm, 3);
+  wrenSetSlotValue(vm, 0, callClass);
+  wrenSetSlotBytes(vm, 1, "string", 3);
+  wrenSetSlotBytes(vm, 2, "b\0y\0t\0e", 7);
+  wrenCall(vm, two);
   
+  // Call ignores with extra temporary slots on stack.
+  wrenEnsureSlots(vm, 10);
+  wrenSetSlotValue(vm, 0, callClass);
+  for (int i = 1; i < 10; i++)
+  {
+    wrenSetSlotDouble(vm, i, i * 0.1);
+  }
+  wrenCall(vm, one);
+  
+  wrenReleaseValue(vm, callClass);
   wrenReleaseValue(vm, noParams);
   wrenReleaseValue(vm, zero);
   wrenReleaseValue(vm, one);
diff --git a/test/api/call.wren b/test/api/call.wren
index a7ee97ff..89f603a4 100644
--- a/test/api/call.wren
+++ b/test/api/call.wren
@@ -20,13 +20,7 @@ class Call {
     System.print("two %(one) %(two)")
   }
 
-  static def getValue(value) {
-    // Return a new value if we aren't given one.
-    if (value == null) return ["a", "b"]
-
-    // Otherwise print it.
-    System.print(value)
-  }
+  static def getValue() { ["a", "b"] }
 }
 
 // expect: noParams
@@ -36,7 +30,7 @@ class Call {
 
 // expect: two true false
 // expect: two 1.2 3.4
-// expect: two 3 4
 // expect: two string another
 // expect: two null [a, b]
 // expect: two str [98, 0, 121, 0, 116, 0, 101]
+// expect: one 0.1
diff --git a/test/api/foreign_class.c b/test/api/foreign_class.c
index cf9fbfeb..f77bf17d 100644
--- a/test/api/foreign_class.c
+++ b/test/api/foreign_class.c
@@ -7,7 +7,7 @@ static int finalized = 0;
 
 static void apiFinalized(WrenVM* vm)
 {
-  wrenReturnDouble(vm, finalized);
+  wrenSetSlotDouble(vm, 0, finalized);
 }
 
 static void counterAllocate(WrenVM* vm)
@@ -18,25 +18,25 @@ static void counterAllocate(WrenVM* vm)
 
 static void counterIncrement(WrenVM* vm)
 {
-  double* value = (double*)wrenGetArgumentForeign(vm, 0);
-  double increment = wrenGetArgumentDouble(vm, 1);
+  double* value = (double*)wrenGetSlotForeign(vm, 0);
+  double increment = wrenGetSlotDouble(vm, 1);
 
   *value += increment;
 }
 
 static void counterValue(WrenVM* vm)
 {
-  double value = *(double*)wrenGetArgumentForeign(vm, 0);
-  wrenReturnDouble(vm, value);
+  double value = *(double*)wrenGetSlotForeign(vm, 0);
+  wrenSetSlotDouble(vm, 0, value);
 }
 
 static void pointAllocate(WrenVM* vm)
 {
   double* coordinates = (double*)wrenAllocateForeign(vm, sizeof(double[3]));
 
-  // This gets called by both constructors, so sniff the argument count to see
+  // This gets called by both constructors, so sniff the slot count to see
   // which one was invoked.
-  if (wrenGetArgumentCount(vm) == 1)
+  if (wrenGetSlotCount(vm) == 1)
   {
     coordinates[0] = 0.0;
     coordinates[1] = 0.0;
@@ -44,36 +44,41 @@ static void pointAllocate(WrenVM* vm)
   }
   else
   {
-    coordinates[0] = wrenGetArgumentDouble(vm, 1);
-    coordinates[1] = wrenGetArgumentDouble(vm, 2);
-    coordinates[2] = wrenGetArgumentDouble(vm, 3);
+    coordinates[0] = wrenGetSlotDouble(vm, 1);
+    coordinates[1] = wrenGetSlotDouble(vm, 2);
+    coordinates[2] = wrenGetSlotDouble(vm, 3);
   }
 }
 
 static void pointTranslate(WrenVM* vm)
 {
-  double* coordinates = (double*)wrenGetArgumentForeign(vm, 0);
-  coordinates[0] += wrenGetArgumentDouble(vm, 1);
-  coordinates[1] += wrenGetArgumentDouble(vm, 2);
-  coordinates[2] += wrenGetArgumentDouble(vm, 3);
+  double* coordinates = (double*)wrenGetSlotForeign(vm, 0);
+  coordinates[0] += wrenGetSlotDouble(vm, 1);
+  coordinates[1] += wrenGetSlotDouble(vm, 2);
+  coordinates[2] += wrenGetSlotDouble(vm, 3);
 }
 
 static void pointToString(WrenVM* vm)
 {
-  double* coordinates = (double*)wrenGetArgumentForeign(vm, 0);
+  double* coordinates = (double*)wrenGetSlotForeign(vm, 0);
   char result[100];
   sprintf(result, "(%g, %g, %g)",
       coordinates[0], coordinates[1], coordinates[2]);
-  wrenReturnString(vm, result, (int)strlen(result));
+  wrenSetSlotString(vm, 0, result);
 }
 
 static void resourceAllocate(WrenVM* vm)
 {
-  wrenAllocateForeign(vm, 0);
+  int* value = (int*)wrenAllocateForeign(vm, sizeof(int));
+  *value = 123;
 }
 
-static void resourceFinalize(WrenVM* vm)
+static void resourceFinalize(void* data)
 {
+  // Make sure we get the right data back.
+  int* value = (int*)data;
+  if (*value != 123) exit(1);
+  
   finalized++;
 }
 
diff --git a/test/api/get_variable.c b/test/api/get_variable.c
new file mode 100644
index 00000000..c5237214
--- /dev/null
+++ b/test/api/get_variable.c
@@ -0,0 +1,44 @@
+#include <string.h>
+
+#include "get_variable.h"
+
+static void beforeDefined(WrenVM* vm)
+{
+  wrenGetVariable(vm, "main", "A", 0);
+}
+
+static void afterDefined(WrenVM* vm)
+{
+  wrenGetVariable(vm, "main", "A", 0);
+}
+
+static void afterAssigned(WrenVM* vm)
+{
+  wrenGetVariable(vm, "main", "A", 0);
+}
+
+static void otherSlot(WrenVM* vm)
+{
+  wrenEnsureSlots(vm, 3);
+  wrenGetVariable(vm, "main", "B", 2);
+  
+  // Move it into return position.
+  const char* string = wrenGetSlotString(vm, 2);
+  wrenSetSlotString(vm, 0, string);
+}
+
+static void otherModule(WrenVM* vm)
+{
+  wrenGetVariable(vm, "get_variable_module", "Variable", 0);
+}
+
+WrenForeignMethodFn getVariableBindMethod(const char* signature)
+{
+  if (strcmp(signature, "static GetVariable.beforeDefined()") == 0) return beforeDefined;
+  if (strcmp(signature, "static GetVariable.afterDefined()") == 0) return afterDefined;
+  if (strcmp(signature, "static GetVariable.afterAssigned()") == 0) return afterAssigned;
+  if (strcmp(signature, "static GetVariable.otherSlot()") == 0) return otherSlot;
+  if (strcmp(signature, "static GetVariable.otherModule()") == 0) return otherModule;
+
+  return NULL;
+}
diff --git a/test/api/get_variable.h b/test/api/get_variable.h
new file mode 100644
index 00000000..8150ee23
--- /dev/null
+++ b/test/api/get_variable.h
@@ -0,0 +1,3 @@
+#include "wren.h"
+
+WrenForeignMethodFn getVariableBindMethod(const char* signature);
diff --git a/test/api/get_variable.wren b/test/api/get_variable.wren
new file mode 100644
index 00000000..7d2c5c2f
--- /dev/null
+++ b/test/api/get_variable.wren
@@ -0,0 +1,24 @@
+import "get_variable_module"
+
+class GetVariable {
+  foreign static def beforeDefined()
+  foreign static def afterDefined()
+  foreign static def afterAssigned()
+  foreign static def otherSlot()
+  foreign static def otherModule()
+}
+
+System.print(GetVariable.beforeDefined()) // expect: null
+
+var A = "a"
+
+System.print(GetVariable.afterDefined()) // expect: a
+
+A = "changed"
+
+System.print(GetVariable.afterAssigned()) // expect: changed
+
+var B = "b"
+System.print(GetVariable.otherSlot()) // expect: b
+
+System.print(GetVariable.otherModule()) // expect: value
diff --git a/test/api/get_variable_module.wren b/test/api/get_variable_module.wren
new file mode 100644
index 00000000..17c96844
--- /dev/null
+++ b/test/api/get_variable_module.wren
@@ -0,0 +1,3 @@
+// nontest
+
+var Variable = "value"
diff --git a/test/api/lists.c b/test/api/lists.c
new file mode 100644
index 00000000..8a2f7a48
--- /dev/null
+++ b/test/api/lists.c
@@ -0,0 +1,46 @@
+#include <string.h>
+
+#include "lists.h"
+
+static void newList(WrenVM* vm)
+{
+  wrenSetSlotNewList(vm, 0);
+}
+
+// Helper function to store a double in a slot then insert it into the list at
+// slot zero.
+static void insertNumber(WrenVM* vm, int index, double value)
+{
+  wrenSetSlotDouble(vm, 1, value);
+  wrenInsertInList(vm, 0, index, 1);
+}
+
+static void insert(WrenVM* vm)
+{
+  wrenSetSlotNewList(vm, 0);
+  
+  wrenEnsureSlots(vm, 2);
+  
+  // Appending.
+  insertNumber(vm, 0, 1.0);
+  insertNumber(vm, 1, 2.0);
+  insertNumber(vm, 2, 3.0);
+  
+  // Inserting.
+  insertNumber(vm, 0, 4.0);
+  insertNumber(vm, 1, 5.0);
+  insertNumber(vm, 2, 6.0);
+  
+  // Negative indexes.
+  insertNumber(vm, -1, 7.0);
+  insertNumber(vm, -2, 8.0);
+  insertNumber(vm, -3, 9.0);
+}
+
+WrenForeignMethodFn listsBindMethod(const char* signature)
+{
+  if (strcmp(signature, "static Lists.newList()") == 0) return newList;
+  if (strcmp(signature, "static Lists.insert()") == 0) return insert;
+
+  return NULL;
+}
diff --git a/test/api/lists.h b/test/api/lists.h
new file mode 100644
index 00000000..9c74380b
--- /dev/null
+++ b/test/api/lists.h
@@ -0,0 +1,3 @@
+#include "wren.h"
+
+WrenForeignMethodFn listsBindMethod(const char* signature);
diff --git a/test/api/lists.wren b/test/api/lists.wren
new file mode 100644
index 00000000..d0e1d456
--- /dev/null
+++ b/test/api/lists.wren
@@ -0,0 +1,10 @@
+class Lists {
+  foreign static def newList()
+  foreign static def insert()
+}
+
+var list = Lists.newList()
+System.print(list is List) // expect: true
+System.print(list.count) // expect: 0
+
+System.print(Lists.insert()) // expect: [4, 5, 6, 1, 2, 3, 9, 8, 7]
diff --git a/test/api/main.c b/test/api/main.c
index 7674db05..7ec7f797 100644
--- a/test/api/main.c
+++ b/test/api/main.c
@@ -6,8 +6,10 @@
 
 #include "benchmark.h"
 #include "call.h"
+#include "get_variable.h"
 #include "foreign_class.h"
-#include "returns.h"
+#include "lists.h"
+#include "slots.h"
 #include "value.h"
 
 // The name of the currently executing API test.
@@ -32,11 +34,17 @@ static WrenForeignMethodFn bindForeignMethod(
   
   method = benchmarkBindMethod(fullName);
   if (method != NULL) return method;
-  
+
+  method = getVariableBindMethod(fullName);
+  if (method != NULL) return method;
+
   method = foreignClassBindMethod(fullName);
   if (method != NULL) return method;
   
-  method = returnsBindMethod(fullName);
+  method = listsBindMethod(fullName);
+  if (method != NULL) return method;
+  
+  method = slotsBindMethod(fullName);
   if (method != NULL) return method;
   
   method = valueBindMethod(fullName);
@@ -60,7 +68,7 @@ static WrenForeignClassMethods bindForeignClass(
 }
 
 static void afterLoad(WrenVM* vm) {
-  if (strstr(testName, "call.wren") != NULL) callRunTests(vm);
+  if (strstr(testName, "/call.wren") != NULL) callRunTests(vm);
 }
 
 int main(int argc, const char* argv[])
diff --git a/test/api/returns.c b/test/api/returns.c
deleted file mode 100644
index f622ded9..00000000
--- a/test/api/returns.c
+++ /dev/null
@@ -1,51 +0,0 @@
-#include <string.h>
-
-#include "returns.h"
-
-static void implicitNull(WrenVM* vm)
-{
-  // Do nothing.
-}
-
-static void returnInt(WrenVM* vm)
-{
-  wrenReturnDouble(vm, 123456);
-}
-
-static void returnFloat(WrenVM* vm)
-{
-  wrenReturnDouble(vm, 123.456);
-}
-
-static void returnTrue(WrenVM* vm)
-{
-  wrenReturnBool(vm, true);
-}
-
-static void returnFalse(WrenVM* vm)
-{
-  wrenReturnBool(vm, false);
-}
-
-static void returnString(WrenVM* vm)
-{
-  wrenReturnString(vm, "a string", -1);
-}
-
-static void returnBytes(WrenVM* vm)
-{
-  wrenReturnString(vm, "a\0b\0c", 5);
-}
-
-WrenForeignMethodFn returnsBindMethod(const char* signature)
-{
-  if (strcmp(signature, "static Returns.implicitNull") == 0) return implicitNull;
-  if (strcmp(signature, "static Returns.returnInt") == 0) return returnInt;
-  if (strcmp(signature, "static Returns.returnFloat") == 0) return returnFloat;
-  if (strcmp(signature, "static Returns.returnTrue") == 0) return returnTrue;
-  if (strcmp(signature, "static Returns.returnFalse") == 0) return returnFalse;
-  if (strcmp(signature, "static Returns.returnString") == 0) return returnString;
-  if (strcmp(signature, "static Returns.returnBytes") == 0) return returnBytes;
-
-  return NULL;
-}
diff --git a/test/api/returns.h b/test/api/returns.h
deleted file mode 100644
index 21fa57cd..00000000
--- a/test/api/returns.h
+++ /dev/null
@@ -1,3 +0,0 @@
-#include "wren.h"
-
-WrenForeignMethodFn returnsBindMethod(const char* signature);
diff --git a/test/api/returns.wren b/test/api/returns.wren
deleted file mode 100644
index 69f09027..00000000
--- a/test/api/returns.wren
+++ /dev/null
@@ -1,23 +0,0 @@
-class Returns {
-  foreign static def implicitNull
-
-  foreign static def returnInt
-  foreign static def returnFloat
-
-  foreign static def returnTrue
-  foreign static def returnFalse
-
-  foreign static def returnString
-  foreign static def returnBytes
-}
-
-System.print(Returns.implicitNull == null) // expect: true
-
-System.print(Returns.returnInt)  // expect: 123456
-System.print(Returns.returnFloat) // expect: 123.456
-
-System.print(Returns.returnTrue)  // expect: true
-System.print(Returns.returnFalse) // expect: false
-
-System.print(Returns.returnString) // expect: a string
-System.print(Returns.returnBytes.bytes.toList) // expect: [97, 0, 98, 0, 99]
diff --git a/test/api/slots.c b/test/api/slots.c
new file mode 100644
index 00000000..9e995b9b
--- /dev/null
+++ b/test/api/slots.c
@@ -0,0 +1,146 @@
+#include <stdio.h>
+#include <string.h>
+
+#include "slots.h"
+
+static void noSet(WrenVM* vm)
+{
+  // Do nothing.
+}
+
+static void getSlots(WrenVM* vm)
+{
+  bool result = true;
+  if (wrenGetSlotBool(vm, 1) != true) result = false;
+  // TODO: Test wrenGetSlotForeign().
+  
+  int length;
+  const char* bytes = wrenGetSlotBytes(vm, 2, &length);
+  if (length != 5) result = false;
+  if (memcmp(bytes, "by\0te", length) != 0) result = false;
+
+  if (wrenGetSlotDouble(vm, 3) != 12.34) result = false;
+  if (strcmp(wrenGetSlotString(vm, 4), "str") != 0) result = false;
+  
+  WrenValue* value = wrenGetSlotValue(vm, 5);
+
+  if (result)
+  {
+    // Otherwise, return the value so we can tell if we captured it correctly.
+    wrenSetSlotValue(vm, 0, value);
+    wrenReleaseValue(vm, value);
+  }
+  else
+  {
+    // If anything failed, return false.
+    wrenSetSlotBool(vm, 0, false);
+  }
+}
+
+static void setSlots(WrenVM* vm)
+{
+  WrenValue* value = wrenGetSlotValue(vm, 1);
+  
+  wrenSetSlotBool(vm, 1, true);
+  wrenSetSlotBytes(vm, 2, "by\0te", 5);
+  wrenSetSlotDouble(vm, 3, 12.34);
+  wrenSetSlotString(vm, 4, "str");
+  
+  // TODO: wrenSetSlotNull().
+  
+  // Read the slots back to make sure they were set correctly.
+  
+  bool result = true;
+  if (wrenGetSlotBool(vm, 1) != true) result = false;
+  
+  int length;
+  const char* bytes = wrenGetSlotBytes(vm, 2, &length);
+  if (length != 5) result = false;
+  if (memcmp(bytes, "by\0te", length) != 0) result = false;
+  
+  if (wrenGetSlotDouble(vm, 3) != 12.34) result = false;
+  if (strcmp(wrenGetSlotString(vm, 4), "str") != 0) result = false;
+
+  if (result)
+  {
+    // Move the value into the return position.
+    wrenSetSlotValue(vm, 0, value);
+    wrenReleaseValue(vm, value);
+  }
+  else
+  {
+    // If anything failed, return false.
+    wrenSetSlotBool(vm, 0, false);
+  }
+}
+
+static void ensure(WrenVM* vm)
+{
+  int before = wrenGetSlotCount(vm);
+  
+  wrenEnsureSlots(vm, 20);
+  
+  int after = wrenGetSlotCount(vm);
+  
+  // Use the slots to make sure they're available.
+  for (int i = 0; i < 20; i++)
+  {
+    wrenSetSlotDouble(vm, i, i);
+  }
+  
+  int sum = 0;
+
+  for (int i = 0; i < 20; i++)
+  {
+    sum += (int)wrenGetSlotDouble(vm, i);
+  }
+
+  char result[100];
+  sprintf(result, "%d -> %d (%d)", before, after, sum);
+  wrenSetSlotString(vm, 0, result);
+}
+
+static void ensureOutsideForeign(WrenVM* vm)
+{
+  // To test the behavior outside of a foreign method (which we're currently
+  // in), create a new separate VM.
+  WrenConfiguration config;
+  wrenInitConfiguration(&config);
+  WrenVM* otherVM = wrenNewVM(&config);
+
+  int before = wrenGetSlotCount(otherVM);
+
+  wrenEnsureSlots(otherVM, 20);
+
+  int after = wrenGetSlotCount(otherVM);
+
+  // Use the slots to make sure they're available.
+  for (int i = 0; i < 20; i++)
+  {
+    wrenSetSlotDouble(otherVM, i, i);
+  }
+
+  int sum = 0;
+
+  for (int i = 0; i < 20; i++)
+  {
+    sum += (int)wrenGetSlotDouble(otherVM, i);
+  }
+
+  wrenFreeVM(otherVM);
+
+  char result[100];
+  sprintf(result, "%d -> %d (%d)", before, after, sum);
+  wrenSetSlotString(vm, 0, result);
+}
+
+WrenForeignMethodFn slotsBindMethod(const char* signature)
+{
+  if (strcmp(signature, "static Slots.noSet") == 0) return noSet;
+  if (strcmp(signature, "static Slots.getSlots(_,_,_,_,_)") == 0) return getSlots;
+  if (strcmp(signature, "static Slots.setSlots(_,_,_,_)") == 0) return setSlots;
+  if (strcmp(signature, "static Slots.ensure()") == 0) return ensure;
+  if (strcmp(signature, "static Slots.ensureOutsideForeign()") == 0) return ensureOutsideForeign;
+
+  return NULL;
+}
diff --git a/test/api/slots.h b/test/api/slots.h
new file mode 100644
index 00000000..cf4392fa
--- /dev/null
+++ b/test/api/slots.h
@@ -0,0 +1,3 @@
+#include "wren.h"
+
+WrenForeignMethodFn slotsBindMethod(const char* signature);
diff --git a/test/api/slots.wren b/test/api/slots.wren
new file mode 100644
index 00000000..764307ae
--- /dev/null
+++ b/test/api/slots.wren
@@ -0,0 +1,24 @@
+class Slots {
+  foreign static def noSet
+  foreign static def getSlots(bool, num, string, bytes, value)
+  foreign static def setSlots(a, b, c, d)
+  foreign static def ensure()
+  foreign static def ensureOutsideForeign()
+}
+
+// If nothing is set in the return slot, it retains its previous value, the
+// receiver.
+System.print(Slots.noSet == Slots) // expect: true
+
+var value = ["value"]
+System.print(Slots.getSlots(true, "by\0te", 12.34, "str", value) == value)
+// expect: true
+
+System.print(Slots.setSlots(value, 0, 0, 0) == value)
+// expect: true
+
+System.print(Slots.ensure())
+// expect: 1 -> 20 (190)
+
+System.print(Slots.ensureOutsideForeign())
+// expect: 0 -> 20 (190)
diff --git a/test/api/value.c b/test/api/value.c
index 5b7b6225..837e8ce2 100644
--- a/test/api/value.c
+++ b/test/api/value.c
@@ -6,12 +6,12 @@ static WrenValue* value;
 
 static void setValue(WrenVM* vm)
 {
-  value = wrenGetArgumentValue(vm, 1);
+  value = wrenGetSlotValue(vm, 1);
 }
 
 static void getValue(WrenVM* vm)
 {
-  wrenReturnValue(vm, value);
+  wrenSetSlotValue(vm, 0, value);
   wrenReleaseValue(vm, value);
 }
 
diff --git a/test/benchmark/api_call.wren b/test/benchmark/api_call.wren
new file mode 100644
index 00000000..568f2549
--- /dev/null
+++ b/test/benchmark/api_call.wren
@@ -0,0 +1,9 @@
+class Benchmark {
+  foreign static call(iterations)
+}
+
+var result = Benchmark.call(1000000)
+// Returns false if it didn't calculate the right value. Otherwise returns the
+// elapsed time.
+System.print(result is Num)
+System.print("elapsed: %(result)")
diff --git a/test/io/directory/dir/a.txt b/test/io/directory/dir/a.txt
new file mode 100644
index 00000000..e068544d
--- /dev/null
+++ b/test/io/directory/dir/a.txt
@@ -0,0 +1 @@
+this is a text file
\ No newline at end of file
diff --git a/test/io/directory/dir/b.txt b/test/io/directory/dir/b.txt
new file mode 100644
index 00000000..e068544d
--- /dev/null
+++ b/test/io/directory/dir/b.txt
@@ -0,0 +1 @@
+this is a text file
\ No newline at end of file
diff --git a/test/io/directory/dir/c.txt b/test/io/directory/dir/c.txt
new file mode 100644
index 00000000..e068544d
--- /dev/null
+++ b/test/io/directory/dir/c.txt
@@ -0,0 +1 @@
+this is a text file
\ No newline at end of file
diff --git a/test/io/directory/list.wren b/test/io/directory/list.wren
new file mode 100644
index 00000000..bf61d759
--- /dev/null
+++ b/test/io/directory/list.wren
@@ -0,0 +1,8 @@
+import "io" for Directory
+
+var entries = Directory.list("test/io/directory/dir")
+
+// Ignore OS-specific dot files like ".DS_Store".
+entries = entries.where {|entry| !entry.startsWith(".") }.toList
+
+System.print(entries) // expect: [a.txt, b.txt, c.txt]
diff --git a/test/io/directory/list_file.wren b/test/io/directory/list_file.wren
new file mode 100644
index 00000000..a411eeb0
--- /dev/null
+++ b/test/io/directory/list_file.wren
@@ -0,0 +1,3 @@
+import "io" for Directory
+
+var entries = Directory.list("test/io/directory/dir/a.txt") // expect runtime error: not a directory
diff --git a/test/io/directory/list_nonexistent.wren b/test/io/directory/list_nonexistent.wren
new file mode 100644
index 00000000..024e7de3
--- /dev/null
+++ b/test/io/directory/list_nonexistent.wren
@@ -0,0 +1,3 @@
+import "io" for Directory
+
+Directory.list("nonexistent") // expect runtime error: no such file or directory
diff --git a/test/io/directory/list_wrong_arg_type.wren b/test/io/directory/list_wrong_arg_type.wren
new file mode 100644
index 00000000..03bedcd5
--- /dev/null
+++ b/test/io/directory/list_wrong_arg_type.wren
@@ -0,0 +1,3 @@
+import "io" for Directory
+
+Directory.list(123) // expect runtime error: Path must be a string.
diff --git a/test/io/file/finalize.wren b/test/io/file/finalize.wren
new file mode 100644
index 00000000..5502fbab
--- /dev/null
+++ b/test/io/file/finalize.wren
@@ -0,0 +1,10 @@
+import "io" for File
+
+// Don't store in a variable.
+File.open("test/io/file/finalize.wren")
+
+System.gc()
+// We can't really test what the finalizer *does* from Wren, since the object
+// is unreachable, but this at least ensures it doesn't crash.
+
+System.print("ok") // expect: ok
\ No newline at end of file
diff --git a/test/io/file/read_bytes.wren b/test/io/file/read_bytes.wren
index 44297cb4..a9d56a87 100644
--- a/test/io/file/read_bytes.wren
+++ b/test/io/file/read_bytes.wren
@@ -10,4 +10,10 @@ System.print(file.readBytes(7)) // expect: this is
 // Allows zero.
 System.print(file.readBytes(0).bytes.count) // expect: 0
 
+// A longer number reads the whole file.
+System.print(file.readBytes(100)) // expect: this is a text file
+
+// Reading past the end truncates the buffer.
+System.print(file.readBytes(100).bytes.count) // expect: 19
+
 file.close()
diff --git a/test/io/file/read_bytes_negative.wren b/test/io/file/read_bytes_count_negative.wren
similarity index 100%
rename from test/io/file/read_bytes_negative.wren
rename to test/io/file/read_bytes_count_negative.wren
diff --git a/test/io/file/read_bytes_not_integer.wren b/test/io/file/read_bytes_count_not_integer.wren
similarity index 100%
rename from test/io/file/read_bytes_not_integer.wren
rename to test/io/file/read_bytes_count_not_integer.wren
diff --git a/test/io/file/read_bytes_from.wren b/test/io/file/read_bytes_from.wren
new file mode 100644
index 00000000..a05cbc97
--- /dev/null
+++ b/test/io/file/read_bytes_from.wren
@@ -0,0 +1,20 @@
+import "io" for File
+
+var file = File.open("test/io/file/file.txt")
+
+// Zero starts at the beginning.
+System.print(file.readBytes(3, 0)) // expect: thi
+
+// Starts at the offset.
+System.print(file.readBytes(8, 3)) // expect: s is a t
+
+// Allows zero.
+System.print(file.readBytes(0, 4).bytes.count) // expect: 0
+
+// A longer number length reads until the end.
+System.print(file.readBytes(100, 2)) // expect: is is a text file
+
+// An offset past the end returns an empty string.
+System.print(file.readBytes(100, 30).bytes.count) // expect: 0
+
+file.close()
diff --git a/test/io/file/read_bytes_from_after_close.wren b/test/io/file/read_bytes_from_after_close.wren
new file mode 100644
index 00000000..4cc8a753
--- /dev/null
+++ b/test/io/file/read_bytes_from_after_close.wren
@@ -0,0 +1,6 @@
+import "io" for File
+
+var file = File.open("test/io/file/file.txt")
+file.close()
+
+file.readBytes(3, 0) // expect runtime error: File is not open.
diff --git a/test/io/file/read_bytes_from_count_negative.wren b/test/io/file/read_bytes_from_count_negative.wren
new file mode 100644
index 00000000..9d251303
--- /dev/null
+++ b/test/io/file/read_bytes_from_count_negative.wren
@@ -0,0 +1,4 @@
+import "io" for File
+
+var file = File.open("test/io/file/file.txt")
+file.readBytes(-1, 0) // expect runtime error: Count cannot be negative.
diff --git a/test/io/file/read_bytes_from_count_not_integer.wren b/test/io/file/read_bytes_from_count_not_integer.wren
new file mode 100644
index 00000000..8a2ba576
--- /dev/null
+++ b/test/io/file/read_bytes_from_count_not_integer.wren
@@ -0,0 +1,4 @@
+import "io" for File
+
+var file = File.open("test/io/file/file.txt")
+file.readBytes(1.2, 0) // expect runtime error: Count must be an integer.
diff --git a/test/io/file/read_bytes_from_count_not_num.wren b/test/io/file/read_bytes_from_count_not_num.wren
new file mode 100644
index 00000000..7783b522
--- /dev/null
+++ b/test/io/file/read_bytes_from_count_not_num.wren
@@ -0,0 +1,4 @@
+import "io" for File
+
+var file = File.open("test/io/file/file.txt")
+file.readBytes("not num", 0) // expect runtime error: Count must be an integer.
diff --git a/test/io/file/read_bytes_from_offset_negative.wren b/test/io/file/read_bytes_from_offset_negative.wren
new file mode 100644
index 00000000..5a303082
--- /dev/null
+++ b/test/io/file/read_bytes_from_offset_negative.wren
@@ -0,0 +1,4 @@
+import "io" for File
+
+var file = File.open("test/io/file/file.txt")
+file.readBytes(1, -1) // expect runtime error: Offset cannot be negative.
diff --git a/test/io/file/read_bytes_from_offset_not_integer.wren b/test/io/file/read_bytes_from_offset_not_integer.wren
new file mode 100644
index 00000000..dc573cea
--- /dev/null
+++ b/test/io/file/read_bytes_from_offset_not_integer.wren
@@ -0,0 +1,4 @@
+import "io" for File
+
+var file = File.open("test/io/file/file.txt")
+file.readBytes(1, 1.2) // expect runtime error: Offset must be an integer.
diff --git a/test/io/file/read_bytes_from_offset_not_num.wren b/test/io/file/read_bytes_from_offset_not_num.wren
new file mode 100644
index 00000000..528d0097
--- /dev/null
+++ b/test/io/file/read_bytes_from_offset_not_num.wren
@@ -0,0 +1,4 @@
+import "io" for File
+
+var file = File.open("test/io/file/file.txt")
+file.readBytes(1, "not num") // expect runtime error: Offset must be an integer.
diff --git a/test/io/file/size.wren b/test/io/file/size.wren
index 84e72eab..b1cc17fe 100644
--- a/test/io/file/size.wren
+++ b/test/io/file/size.wren
@@ -1,13 +1,6 @@
 import "io" for File
 import "scheduler" for Scheduler
 
-System.print(File.size("test/io/file/size.wren")) // expect: 270
-
-// Runs asynchronously.
-Scheduler.add {
-  System.print("async")
-}
-
-System.print(File.size("test/io/file/size.wren"))
-// expect: async
-// expect: 270
+var file = File.open("test/io/file/file.txt")
+System.print(file.size) // expect: 19
+file.close()
diff --git a/test/io/file/size_static.wren b/test/io/file/size_static.wren
new file mode 100644
index 00000000..c9b720f0
--- /dev/null
+++ b/test/io/file/size_static.wren
@@ -0,0 +1,13 @@
+import "io" for File
+import "scheduler" for Scheduler
+
+System.print(File.size("test/io/file/file.txt")) // expect: 19
+
+// Runs asynchronously.
+Scheduler.add {
+  System.print("async")
+}
+
+System.print(File.size("test/io/file/file.txt"))
+// expect: async
+// expect: 19
diff --git a/test/io/file/size_nonexistent.wren b/test/io/file/size_static_nonexistent.wren
similarity index 100%
rename from test/io/file/size_nonexistent.wren
rename to test/io/file/size_static_nonexistent.wren
diff --git a/test/io/file/stat_static.wren b/test/io/file/stat_static.wren
new file mode 100644
index 00000000..0ca59d3f
--- /dev/null
+++ b/test/io/file/stat_static.wren
@@ -0,0 +1,16 @@
+import "io" for File, Stat
+import "scheduler" for Scheduler
+
+var stat = File.stat("test/io/file/file.txt")
+
+System.print(stat is Stat)            // expect: true
+System.print(stat.device is Num)      // expect: true
+System.print(stat.inode is Num)       // expect: true
+System.print(stat.mode is Num)        // expect: true
+System.print(stat.linkCount)          // expect: 1
+System.print(stat.user is Num)        // expect: true
+System.print(stat.group is Num)       // expect: true
+System.print(stat.specialDevice)      // expect: 0
+System.print(stat.size)               // expect: 19
+System.print(stat.blockSize is Num)   // expect: true
+System.print(stat.blockCount is Num)  // expect: true
diff --git a/test/io/file/stat_static_directory.wren b/test/io/file/stat_static_directory.wren
new file mode 100644
index 00000000..b5aa8ad3
--- /dev/null
+++ b/test/io/file/stat_static_directory.wren
@@ -0,0 +1,16 @@
+import "io" for File, Stat
+import "scheduler" for Scheduler
+
+var stat = File.stat("test/io/directory/dir")
+
+System.print(stat is Stat)            // expect: true
+System.print(stat.device is Num)      // expect: true
+System.print(stat.inode is Num)       // expect: true
+System.print(stat.mode is Num)        // expect: true
+System.print(stat.linkCount)          // expect: 5
+System.print(stat.user is Num)        // expect: true
+System.print(stat.group is Num)       // expect: true
+System.print(stat.specialDevice)      // expect: 0
+System.print(stat.size)               // expect: 170
+System.print(stat.blockSize is Num)   // expect: true
+System.print(stat.blockCount is Num)  // expect: true
diff --git a/test/io/file/stat_static_nonexistent.wren b/test/io/file/stat_static_nonexistent.wren
new file mode 100644
index 00000000..8bc17802
--- /dev/null
+++ b/test/io/file/stat_static_nonexistent.wren
@@ -0,0 +1,3 @@
+import "io" for File
+
+File.stat("nonexistent") // expect runtime error: no such file or directory
diff --git a/util/benchmark.py b/util/benchmark.py
index 36e7952d..cb00a649 100755
--- a/util/benchmark.py
+++ b/util/benchmark.py
@@ -51,6 +51,8 @@ def BENCHMARK(name, pattern):
   regex = re.compile(pattern + "\n" + r"elapsed: (\d+\.\d+)", re.MULTILINE)
   BENCHMARKS.append([name, regex, None])
 
+BENCHMARK("api_call", "true")
+
 BENCHMARK("api_foreign_method", "100000000")
 
 BENCHMARK("binary_trees", """stretch tree of depth 13 check: -1
diff --git a/util/test.py b/util/test.py
index 080d2e60..b338cbd6 100755
--- a/util/test.py
+++ b/util/test.py
@@ -182,6 +182,7 @@ class Test:
 
     # Make sure the stack trace has the right line. Skip over any lines that
     # come from builtin libraries.
+    match = False
     stack_lines = error_lines[line + 1:]
     match = None
     for stack_line in stack_lines:
diff --git a/util/wren.mk b/util/wren.mk
index b48f5ca1..88e8b457 100644
--- a/util/wren.mk
+++ b/util/wren.mk
@@ -206,12 +206,15 @@ $(LIBUV): $(LIBUV_DIR)/build/gyp/gyp util/libuv.py
 
 # Wren modules that get compiled into the binary as C strings.
 src/optional/wren_opt_%.wren.inc: src/optional/wren_opt_%.wren util/wren_to_c_string.py
+	@ printf "%10s %-30s %s\n" str $<
 	@ ./util/wren_to_c_string.py $@ $<
 
 src/vm/wren_%.wren.inc: src/vm/wren_%.wren util/wren_to_c_string.py
+	@ printf "%10s %-30s %s\n" str $<
 	@ ./util/wren_to_c_string.py $@ $<
 
 src/module/%.wren.inc: src/module/%.wren util/wren_to_c_string.py
+	@ printf "%10s %-30s %s\n" str $<
 	@ ./util/wren_to_c_string.py $@ $<
 
 .PHONY: all cli test vm
diff --git a/util/wren_to_c_string.py b/util/wren_to_c_string.py
index 6715fb40..8129882f 100755
--- a/util/wren_to_c_string.py
+++ b/util/wren_to_c_string.py
@@ -50,7 +50,5 @@ def main():
   with open(args.output, "w") as f:
     f.write(c_source)
 
-  print("        str " + args.input)
-
 
 main()
diff --git a/util/xcode/wren.xcodeproj/project.pbxproj b/util/xcode/wren.xcodeproj/project.pbxproj
index fe7b67a4..c250b022 100644
--- a/util/xcode/wren.xcodeproj/project.pbxproj
+++ b/util/xcode/wren.xcodeproj/project.pbxproj
@@ -19,6 +19,7 @@
 		29205C9E1AB4E6430073018D /* wren_value.c in Sources */ = {isa = PBXBuildFile; fileRef = 29205C971AB4E6430073018D /* wren_value.c */; };
 		29205C9F1AB4E6430073018D /* wren_vm.c in Sources */ = {isa = PBXBuildFile; fileRef = 29205C981AB4E6430073018D /* wren_vm.c */; };
 		293D46961BB43F9900200083 /* call.c in Sources */ = {isa = PBXBuildFile; fileRef = 293D46941BB43F9900200083 /* call.c */; };
+		2949AA8D1C2F14F000B106BA /* get_variable.c in Sources */ = {isa = PBXBuildFile; fileRef = 2949AA8B1C2F14F000B106BA /* get_variable.c */; };
 		29512C811B91F8EB008C10E6 /* libuv.a in Frameworks */ = {isa = PBXBuildFile; fileRef = 29512C801B91F8EB008C10E6 /* libuv.a */; };
 		29512C821B91F901008C10E6 /* libuv.a in Frameworks */ = {isa = PBXBuildFile; fileRef = 29512C801B91F8EB008C10E6 /* libuv.a */; };
 		29729F311BA70A620099CA20 /* io.c in Sources */ = {isa = PBXBuildFile; fileRef = 29729F2E1BA70A620099CA20 /* io.c */; };
@@ -26,6 +27,7 @@
 		29729F331BA70A620099CA20 /* io.wren.inc in Sources */ = {isa = PBXBuildFile; fileRef = 29729F301BA70A620099CA20 /* io.wren.inc */; };
 		2986F6D71ACF93BA00BCE26C /* wren_primitive.c in Sources */ = {isa = PBXBuildFile; fileRef = 2986F6D51ACF93BA00BCE26C /* wren_primitive.c */; };
 		29932D511C20D8C900099DEE /* benchmark.c in Sources */ = {isa = PBXBuildFile; fileRef = 29932D4F1C20D8C900099DEE /* benchmark.c */; };
+		29932D541C210F8D00099DEE /* lists.c in Sources */ = {isa = PBXBuildFile; fileRef = 29932D521C210F8D00099DEE /* lists.c */; };
 		29A427341BDBE435001E6E22 /* wren_opt_meta.c in Sources */ = {isa = PBXBuildFile; fileRef = 29A4272E1BDBE435001E6E22 /* wren_opt_meta.c */; };
 		29A427351BDBE435001E6E22 /* wren_opt_meta.c in Sources */ = {isa = PBXBuildFile; fileRef = 29A4272E1BDBE435001E6E22 /* wren_opt_meta.c */; };
 		29A427361BDBE435001E6E22 /* wren_opt_meta.wren.inc in Sources */ = {isa = PBXBuildFile; fileRef = 29A427301BDBE435001E6E22 /* wren_opt_meta.wren.inc */; };
@@ -47,7 +49,7 @@
 		29DC14A91BBA302F008A8274 /* wren_vm.c in Sources */ = {isa = PBXBuildFile; fileRef = 29205C981AB4E6430073018D /* wren_vm.c */; };
 		29DC14AA1BBA3032008A8274 /* main.c in Sources */ = {isa = PBXBuildFile; fileRef = 29D009A61B7E3993000CE58C /* main.c */; };
 		29DC14AB1BBA3038008A8274 /* foreign_class.c in Sources */ = {isa = PBXBuildFile; fileRef = 29D009A81B7E39A8000CE58C /* foreign_class.c */; };
-		29DC14AC1BBA303D008A8274 /* returns.c in Sources */ = {isa = PBXBuildFile; fileRef = 29D009AA1B7E39A8000CE58C /* returns.c */; };
+		29DC14AC1BBA303D008A8274 /* slots.c in Sources */ = {isa = PBXBuildFile; fileRef = 29D009AA1B7E39A8000CE58C /* slots.c */; };
 		29DC14AD1BBA3040008A8274 /* value.c in Sources */ = {isa = PBXBuildFile; fileRef = 29D009AC1B7E39A8000CE58C /* value.c */; };
 /* End PBXBuildFile section */
 
@@ -97,6 +99,8 @@
 		29205CA81AB4E65E0073018D /* wren_vm.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; name = wren_vm.h; path = ../../src/vm/wren_vm.h; sourceTree = "<group>"; };
 		293D46941BB43F9900200083 /* call.c */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.c; name = call.c; path = ../../test/api/call.c; sourceTree = "<group>"; };
 		293D46951BB43F9900200083 /* call.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; name = call.h; path = ../../test/api/call.h; sourceTree = "<group>"; };
+		2949AA8B1C2F14F000B106BA /* get_variable.c */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.c; name = get_variable.c; path = ../../test/api/get_variable.c; sourceTree = "<group>"; };
+		2949AA8C1C2F14F000B106BA /* get_variable.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; name = get_variable.h; path = ../../test/api/get_variable.h; sourceTree = "<group>"; };
 		29512C7F1B91F86E008C10E6 /* api_test */ = {isa = PBXFileReference; explicitFileType = "compiled.mach-o.executable"; includeInIndex = 0; path = api_test; sourceTree = BUILT_PRODUCTS_DIR; };
 		29512C801B91F8EB008C10E6 /* libuv.a */ = {isa = PBXFileReference; lastKnownFileType = archive.ar; name = libuv.a; path = ../../build/libuv.a; sourceTree = "<group>"; };
 		296371B31AC713D000079FDA /* wren_opcodes.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; name = wren_opcodes.h; path = ../../src/vm/wren_opcodes.h; sourceTree = "<group>"; };
@@ -106,6 +110,8 @@
 		2986F6D61ACF93BA00BCE26C /* wren_primitive.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; name = wren_primitive.h; path = ../../src/vm/wren_primitive.h; sourceTree = "<group>"; };
 		29932D4F1C20D8C900099DEE /* benchmark.c */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.c; name = benchmark.c; path = ../../test/api/benchmark.c; sourceTree = "<group>"; };
 		29932D501C20D8C900099DEE /* benchmark.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; name = benchmark.h; path = ../../test/api/benchmark.h; sourceTree = "<group>"; };
+		29932D521C210F8D00099DEE /* lists.c */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.c; name = lists.c; path = ../../test/api/lists.c; sourceTree = "<group>"; };
+		29932D531C210F8D00099DEE /* lists.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; name = lists.h; path = ../../test/api/lists.h; sourceTree = "<group>"; };
 		29A4272E1BDBE435001E6E22 /* wren_opt_meta.c */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.c; name = wren_opt_meta.c; path = ../../src/optional/wren_opt_meta.c; sourceTree = "<group>"; };
 		29A4272F1BDBE435001E6E22 /* wren_opt_meta.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; name = wren_opt_meta.h; path = ../../src/optional/wren_opt_meta.h; sourceTree = "<group>"; };
 		29A427301BDBE435001E6E22 /* wren_opt_meta.wren.inc */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.pascal; name = wren_opt_meta.wren.inc; path = ../../src/optional/wren_opt_meta.wren.inc; sourceTree = "<group>"; };
@@ -118,8 +124,8 @@
 		29D009A61B7E3993000CE58C /* main.c */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.c; name = main.c; path = ../../test/api/main.c; sourceTree = "<group>"; };
 		29D009A81B7E39A8000CE58C /* foreign_class.c */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.c; name = foreign_class.c; path = ../../test/api/foreign_class.c; sourceTree = "<group>"; };
 		29D009A91B7E39A8000CE58C /* foreign_class.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; name = foreign_class.h; path = ../../test/api/foreign_class.h; sourceTree = "<group>"; };
-		29D009AA1B7E39A8000CE58C /* returns.c */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.c; name = returns.c; path = ../../test/api/returns.c; sourceTree = "<group>"; };
-		29D009AB1B7E39A8000CE58C /* returns.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; name = returns.h; path = ../../test/api/returns.h; sourceTree = "<group>"; };
+		29D009AA1B7E39A8000CE58C /* slots.c */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.c; name = slots.c; path = ../../test/api/slots.c; sourceTree = "<group>"; };
+		29D009AB1B7E39A8000CE58C /* slots.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; name = slots.h; path = ../../test/api/slots.h; sourceTree = "<group>"; };
 		29D009AC1B7E39A8000CE58C /* value.c */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.c; name = value.c; path = ../../test/api/value.c; sourceTree = "<group>"; };
 		29D009AD1B7E39A8000CE58C /* value.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; name = value.h; path = ../../test/api/value.h; sourceTree = "<group>"; };
 		29F384111BD19706002F84E0 /* io.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; name = io.h; path = ../../src/module/io.h; sourceTree = "<group>"; };
@@ -249,8 +255,12 @@
 				293D46951BB43F9900200083 /* call.h */,
 				29D009A81B7E39A8000CE58C /* foreign_class.c */,
 				29D009A91B7E39A8000CE58C /* foreign_class.h */,
-				29D009AA1B7E39A8000CE58C /* returns.c */,
-				29D009AB1B7E39A8000CE58C /* returns.h */,
+				2949AA8B1C2F14F000B106BA /* get_variable.c */,
+				2949AA8C1C2F14F000B106BA /* get_variable.h */,
+				29932D521C210F8D00099DEE /* lists.c */,
+				29932D531C210F8D00099DEE /* lists.h */,
+				29D009AA1B7E39A8000CE58C /* slots.c */,
+				29D009AB1B7E39A8000CE58C /* slots.h */,
 				29D009AC1B7E39A8000CE58C /* value.c */,
 				29D009AD1B7E39A8000CE58C /* value.h */,
 			);
@@ -358,7 +368,9 @@
 			files = (
 				29A427371BDBE435001E6E22 /* wren_opt_meta.wren.inc in Sources */,
 				29729F321BA70A620099CA20 /* io.c in Sources */,
+				29932D541C210F8D00099DEE /* lists.c in Sources */,
 				291647C81BA5EC5E006142EE /* modules.c in Sources */,
+				2949AA8D1C2F14F000B106BA /* get_variable.c in Sources */,
 				29DC14A11BBA2FEC008A8274 /* scheduler.c in Sources */,
 				29A427391BDBE435001E6E22 /* wren_opt_random.c in Sources */,
 				29932D511C20D8C900099DEE /* benchmark.c in Sources */,
@@ -376,7 +388,7 @@
 				293D46961BB43F9900200083 /* call.c in Sources */,
 				29A427351BDBE435001E6E22 /* wren_opt_meta.c in Sources */,
 				29DC14AB1BBA3038008A8274 /* foreign_class.c in Sources */,
-				29DC14AC1BBA303D008A8274 /* returns.c in Sources */,
+				29DC14AC1BBA303D008A8274 /* slots.c in Sources */,
 				29DC14AD1BBA3040008A8274 /* value.c in Sources */,
 			);
 			runOnlyForDeploymentPostprocessing = 0;