Function expressions/higher order functions

lib/ahem/ast/function_definition.rb

@@ -14,6 +14,7 @@ class AST::FunctionDefinition
   end
 
   def execute(env)
-    env.set(@name.name, self)
+    env.set(@name.name, self) unless @name.nil?
+    self
   end
 end

lib/ahem/parser.rb

@@ -254,6 +254,8 @@ class Parser
         e = expression
         eat(TokenKinds::RPAREN)
         e
+      when TokenKinds::FUNCTION
+        function_expression
       else
         raise "Unexpected token #{token.type}"
       end
@@ -271,6 +273,13 @@ class Parser
     end
   end
 
+  def function_expression
+    eat(TokenKinds::FUNCTION)
+    params = parameters
+    body = block
+    AST::FunctionDefinition.new(nil, params, body)
+  end
+
   def identifier
     token = eat(TokenKinds::IDENTIFIER)
     AST::Identifier.new(token.value)

spec/ast/function_call_spec.rb

@@ -127,4 +127,60 @@ RSpec.describe AST::FunctionCall do
       ).execute(env)
     end.to raise_error('Undefined variable n')
   end
+
+  # let add_one = function(n) { n + 1; };
+  # let do = function(f, n) { f(n); };
+  # do(5);
+  it 'executes higher order functions' do
+    env = Environment.new
+    AST::VariableDeclaration.new(
+      AST::Identifier.new('add_one'),
+      AST::FunctionDefinition.new(
+        nil,
+        [
+          AST::Identifier.new('n'),
+        ],
+        AST::Block.new(
+          [
+            AST::Binary.new(
+              AST::Operators::ADD,
+              AST::Identifier.new('n'),
+              AST::Number.new(1.0)
+            )
+          ]
+        )
+      )
+    ).execute(env)
+
+    AST::VariableDeclaration.new(
+      AST::Identifier.new('do'),
+      AST::FunctionDefinition.new(
+        nil,
+        [
+          AST::Identifier.new('f'),
+          AST::Identifier.new('x'),
+        ],
+        AST::Block.new(
+          [
+            AST::FunctionCall.new(
+              AST::Identifier.new('f'),
+              [
+                AST::Identifier.new('x')
+              ]
+            )
+          ]
+        )
+      )
+    ).execute(env)
+
+    result = AST::FunctionCall.new(
+      AST::Identifier.new('do'),
+      [
+        AST::Identifier.new('add_one'),
+        AST::Number.new(5.0)
+      ]
+    ).execute(env)
+
+    expect(result).to eq(6)
+  end
 end

spec/parser_spec.rb

@@ -379,4 +379,27 @@ RSpec.describe Parser do
       ]
     )
   end
+
+  it 'parses a function expression' do
+    expect(parse('let sum = function(x, y) { x + y; };')).to eq(
+      [
+        AST::VariableDeclaration.new(
+          AST::Identifier.new('sum'),
+          AST::FunctionDefinition.new(
+            nil,
+            [AST::Identifier.new('x'), AST::Identifier.new('y')],
+            AST::Block.new(
+              [
+                AST::Binary.new(
+                  AST::Operators::ADD,
+                  AST::Identifier.new('x'),
+                  AST::Identifier.new('y'),
+                )
+              ]
+            )
+          )
+        )
+      ]
+    )
+  end
 end