diff --git a/builtin/core.wren b/builtin/core.wren
index 8031f8fe..91695011 100644
--- a/builtin/core.wren
+++ b/builtin/core.wren
@@ -34,9 +34,9 @@ class List is Sequence {
     return result
   }
 
-  + that {
+  +(other) {
     var result = this[0..-1]
-    for (element in that) {
+    for (element in other) {
       result.add(element)
     }
     return result
diff --git a/src/wren_compiler.c b/src/wren_compiler.c
index 80267b06..0844c9e1 100644
--- a/src/wren_compiler.c
+++ b/src/wren_compiler.c
@@ -1977,7 +1977,9 @@ void infixSignature(Compiler* compiler, char* name, int* length)
   name[(*length)++] = ' ';
 
   // Parse the parameter name.
+  consume(compiler, TOKEN_LEFT_PAREN, "Expect '(' after operator name.");
   declareNamedVariable(compiler);
+  consume(compiler, TOKEN_RIGHT_PAREN, "Expect ')' after parameter name.");
 }
 
 // Compiles a method signature for an unary operator (i.e. "!").
@@ -1990,14 +1992,15 @@ void unarySignature(Compiler* compiler, char* name, int* length)
 // infix (i.e. "-").
 void mixedSignature(Compiler* compiler, char* name, int* length)
 {
-  // If there is a parameter name, it's an infix operator, otherwise it's unary.
-  if (compiler->parser->current.type == TOKEN_NAME)
+  // If there is a parameter, it's an infix operator, otherwise it's unary.
+  if (match(compiler, TOKEN_LEFT_PAREN))
   {
     // Add a space for the RHS parameter.
     name[(*length)++] = ' ';
 
     // Parse the parameter name.
     declareNamedVariable(compiler);
+    consume(compiler, TOKEN_RIGHT_PAREN, "Expect ')' after parameter name.");
   }
 }
 
@@ -2011,7 +2014,9 @@ void namedSignature(Compiler* compiler, char* name, int* length)
     name[(*length)++] = ' ';
 
     // Parse the value parameter.
+    consume(compiler, TOKEN_LEFT_PAREN, "Expect '(' after '='.");
     declareNamedVariable(compiler);
+    consume(compiler, TOKEN_RIGHT_PAREN, "Expect ')' after parameter name.");
   }
   else
   {
diff --git a/src/wren_core.c b/src/wren_core.c
index d79f0539..3004de07 100644
--- a/src/wren_core.c
+++ b/src/wren_core.c
@@ -77,9 +77,9 @@ static const char* libSource =
 "    return result\n"
 "  }\n"
 "\n"
-"  + that {\n"
+"  +(other) {\n"
 "    var result = this[0..-1]\n"
-"    for (element in that) {\n"
+"    for (element in other) {\n"
 "      result.add(element)\n"
 "    }\n"
 "    return result\n"
diff --git a/test/constructor/infix_class_expression.wren b/test/constructor/infix_class_expression.wren
index 4274b58c..fc93e135 100644
--- a/test/constructor/infix_class_expression.wren
+++ b/test/constructor/infix_class_expression.wren
@@ -1,5 +1,5 @@
 class Foo {
-  + other { "Foo " + other }
+  +(other) { "Foo " + other }
 }
 
 IO.print(new Foo + "value") // expect: Foo value
diff --git a/test/implicit_receiver/inherited_methods.wren b/test/implicit_receiver/inherited_methods.wren
index c1edbfff..331cec20 100644
--- a/test/implicit_receiver/inherited_methods.wren
+++ b/test/implicit_receiver/inherited_methods.wren
@@ -3,7 +3,7 @@ class Foo {
     IO.print("getter")
   }
 
-  setter = value {
+  setter=(value) {
     IO.print("setter")
   }
 
diff --git a/test/implicit_receiver/instance_methods.wren b/test/implicit_receiver/instance_methods.wren
index 8092b415..3d32ba30 100644
--- a/test/implicit_receiver/instance_methods.wren
+++ b/test/implicit_receiver/instance_methods.wren
@@ -3,7 +3,7 @@ class Foo {
     IO.print("getter")
   }
 
-  setter = value {
+  setter=(value) {
     IO.print("setter")
   }
 
diff --git a/test/implicit_receiver/locals_shadow_setter.wren b/test/implicit_receiver/locals_shadow_setter.wren
index 3ca911eb..c5312240 100644
--- a/test/implicit_receiver/locals_shadow_setter.wren
+++ b/test/implicit_receiver/locals_shadow_setter.wren
@@ -1,5 +1,5 @@
 class Foo {
-  bar = value {
+  bar=(value) {
     IO.print("setter")
     return value
   }
diff --git a/test/implicit_receiver/nested_class.wren b/test/implicit_receiver/nested_class.wren
index 1d88e4d8..fc2cc209 100644
--- a/test/implicit_receiver/nested_class.wren
+++ b/test/implicit_receiver/nested_class.wren
@@ -3,7 +3,7 @@ class Outer {
     IO.print("outer getter")
   }
 
-  setter = value {
+  setter=(value) {
     IO.print("outer setter")
   }
 
@@ -21,7 +21,7 @@ class Outer {
         IO.print("inner getter")
       }
 
-      setter = value {
+      setter=(value) {
         IO.print("inner setter")
       }
 
diff --git a/test/implicit_receiver/static_methods.wren b/test/implicit_receiver/static_methods.wren
index 3c4594eb..95631ca8 100644
--- a/test/implicit_receiver/static_methods.wren
+++ b/test/implicit_receiver/static_methods.wren
@@ -3,7 +3,7 @@ class Foo {
     IO.print("getter")
   }
 
-  static setter = value {
+  static setter=(value) {
     IO.print("setter")
   }
 
diff --git a/test/method/operators.wren b/test/method/operators.wren
index c070b455..4e9c6059 100644
--- a/test/method/operators.wren
+++ b/test/method/operators.wren
@@ -1,17 +1,17 @@
 class Foo {
-  + other { "infix + " + other }
-  - other { "infix - " + other }
-  * other { "infix * " + other }
-  / other { "infix / " + other }
-  % other { "infix % " + other }
-  < other { "infix < " + other }
-  > other { "infix > " + other }
-  <= other { "infix <= " + other }
-  >= other { "infix >= " + other }
-  == other { "infix == " + other }
-  != other { "infix != " + other }
-  & other { "infix & " + other }
-  | other { "infix | " + other }
+  +(other) { "infix + " + other }
+  -(other) { "infix - " + other }
+  *(other) { "infix * " + other }
+  /(other) { "infix / " + other }
+  %(other) { "infix % " + other }
+  <(other) { "infix < " + other }
+  >(other) { "infix > " + other }
+  <=(other) { "infix <= " + other }
+  >=(other) { "infix >= " + other }
+  ==(other) { "infix == " + other }
+  !=(other) { "infix != " + other }
+  &(other) { "infix & " + other }
+  |(other) { "infix | " + other }
 
   ! { "prefix !" }
   - { "prefix -" }
diff --git a/test/setter/associativity.wren b/test/setter/associativity.wren
index 3df35eff..2c0e526f 100644
--- a/test/setter/associativity.wren
+++ b/test/setter/associativity.wren
@@ -1,7 +1,7 @@
 class Foo {
   new(value) { _value = value }
   toString { _value }
-  bar = value {
+  bar=(value) {
     _value = value
     return value
   }
diff --git a/test/setter/grouping.wren b/test/setter/grouping.wren
index f3c5625c..47fd97b8 100644
--- a/test/setter/grouping.wren
+++ b/test/setter/grouping.wren
@@ -1,5 +1,5 @@
 class Foo {
-  bar = value { value }
+  bar=(value) { value }
 }
 
 var foo = new Foo
diff --git a/test/setter/infix_operator.wren b/test/setter/infix_operator.wren
index aebbd0a6..e694c545 100644
--- a/test/setter/infix_operator.wren
+++ b/test/setter/infix_operator.wren
@@ -1,5 +1,5 @@
 class Foo {
-  bar = value { value }
+  bar=(value) { value }
 }
 
 var foo = new Foo
diff --git a/test/setter/instance.wren b/test/setter/instance.wren
index d61254d7..e2941fab 100644
--- a/test/setter/instance.wren
+++ b/test/setter/instance.wren
@@ -1,5 +1,5 @@
 class Foo {
-  bar = value {
+  bar=(value) {
     IO.print(value)
   }
 }
diff --git a/test/setter/is.wren b/test/setter/is.wren
index 812da466..1797bccb 100644
--- a/test/setter/is.wren
+++ b/test/setter/is.wren
@@ -1,5 +1,5 @@
 class Foo {
-  bar = value { value }
+  bar=(value) { value }
 }
 
 var foo = new Foo
diff --git a/test/setter/prefix_operator.wren b/test/setter/prefix_operator.wren
index 09e48fad..20b80542 100644
--- a/test/setter/prefix_operator.wren
+++ b/test/setter/prefix_operator.wren
@@ -1,5 +1,5 @@
 class Foo {
-  bar = value { value }
+  bar=(value) { value }
 }
 
 var foo = new Foo
diff --git a/test/setter/result.wren b/test/setter/result.wren
index e8db2fa6..ca153aa7 100644
--- a/test/setter/result.wren
+++ b/test/setter/result.wren
@@ -1,5 +1,5 @@
 class Foo {
-  bar = value { "result" }
+  bar=(value) { "result" }
 }
 
 var foo = new Foo
diff --git a/test/setter/same_name_as_method.wren b/test/setter/same_name_as_method.wren
index 82814c0b..c409328a 100644
--- a/test/setter/same_name_as_method.wren
+++ b/test/setter/same_name_as_method.wren
@@ -1,5 +1,5 @@
 class Foo {
-  bar = value { IO.print("set") }
+  bar=(value) { IO.print("set") }
   bar { IO.print("get") }
 }
 
diff --git a/test/setter/static.wren b/test/setter/static.wren
index 157d8847..e0c3850b 100644
--- a/test/setter/static.wren
+++ b/test/setter/static.wren
@@ -1,5 +1,5 @@
 class Foo {
-  static bar = value {
+  static bar=(value) {
     IO.print(value)
   }
 }