Add an animals guessing game example.

example/animals.wren

@@ -0,0 +1,86 @@
+import "io" for Stdin
+
+// Implements the classic "Animals" guessing game. The user thinks of an animal.
+// The program asks a series of yes/no questions to try to guess the animal
+// they are thinking of. If the program fails, it asks the user for a new
+// question and adds the animal to its knowledge base.
+//
+// Internally, the program's brain is stored as a binary tree. Leaf nodes are
+// animals. Internal nodes are yes/no questions that choose which branch to
+// explore.
+
+class Node {
+  // Reads a "yes" or "no" (or something approximating) those and returns true
+  // if yes was entered.
+  promptYesNo(prompt) {
+    while (true) {
+      var line = promptString(prompt)
+
+      if (line.startsWith("y") || line.startsWith("Y")) return true
+      if (line.startsWith("n") || line.startsWith("N")) return false
+
+      // Quit.
+      if (line.startsWith("q") || line.startsWith("Q")) Fiber.yield()
+    }
+  }
+
+  // Writes a prompt and reads a string of input.
+  promptString(prompt) {
+    System.write("%(prompt) ")
+    return Stdin.readLine()
+  }
+}
+
+class Animal is Node {
+  construct new(name) {
+    _name = name
+  }
+
+  ask() {
+    // Hit a leaf, so see if we guessed it.
+    if (promptYesNo("Is it a %(_name)?")) {
+      System.print("I won! Let's play again!")
+      return null
+    }
+
+    // Nope, so add a new animal and turn this node into a branch.
+    var name = promptString("I lost! What was your animal?")
+    var question = promptString(
+        "What question would distinguish a %(_name) from a %(name)?")
+    var isYes = promptYesNo(
+        "Is the answer to the question 'yes' for a %(name)?")
+    System.print("I'll remember that. Let's play again!")
+
+    var animal = Animal.new(name)
+    return Question.new(question, isYes ? animal : this, isYes ? this : animal)
+  }
+}
+
+class Question is Node {
+  construct new(question, ifYes, ifNo) {
+    _question = question
+    _ifYes = ifYes
+    _ifNo = ifNo
+  }
+
+  ask() {
+    // Recurse into the branches.
+    if (promptYesNo(_question)) {
+      var result = _ifYes.ask()
+      if (result != null) _ifYes = result
+    } else {
+      var result = _ifNo.ask()
+      if (result != null) _ifNo = result
+    }
+
+    return null
+  }
+}
+
+var root = Question.new("Does it live in the water?",
+    Animal.new("frog"), Animal.new("goat"))
+
+// Play games until the user quits.
+Fiber.new {
+  while (true) root.ask()
+}.call()

util/test.py

@@ -356,8 +356,10 @@ def run_api_test(path):
 
 
 def run_example(path):
-  # The guess number requires user input.
+  # Don't run examples that require user input.
+  if "animals" in path: return
   if "guess_number" in path: return
+
   run_script(WREN_APP, path, "example")