Fix closure environment binding

lib/ahem/ast/function_call.rb

@@ -21,6 +21,7 @@ class AST::FunctionCall
     end
 
     if function.is_a?(AST::FunctionDefinition)
+      env = function.env unless function.env.nil?
       if @arguments.size != function.parameters.size
         raise "Arity mismatch in call to function #{function.name}"
       end

lib/ahem/ast/function_definition.rb

@@ -1,10 +1,11 @@
 class AST::FunctionDefinition
-  attr_reader :name, :parameters, :body
+  attr_reader :name, :parameters, :body, :env
 
-  def initialize(name, parameters, body)
+  def initialize(name, parameters, body, env = nil)
     @name = name
     @parameters = parameters
     @body = body
+    @env = env
   end
 
   def ==(other)
@@ -14,7 +15,7 @@ class AST::FunctionDefinition
   end
 
   def execute(env)
-    env.set(@name.name, self) unless @name.nil?
-    self
+    return AST::FunctionDefinition.new(@name, @parameters, @body, env) if @name.nil?
+    env.set(@name.name, self)
   end
 end

spec/ast/function_call_spec.rb

@@ -200,4 +200,49 @@ RSpec.describe AST::FunctionCall do
     ).execute(Environment.new)
     expect(result).to eq(5.0)
   end
+
+  # function outer() {
+  #   let x = "hello world";
+  #   function () {
+  #     x;
+  #   };
+  # }
+  # let y = outer();
+  # y();
+  it 'keeps a reference to a closure\'s environment' do
+    env = Environment.new
+    AST::FunctionDefinition.new(
+      AST::Identifier.new('outer'),
+      [],
+      AST::Block.new(
+        [
+          AST::VariableDeclaration.new(
+            AST::Identifier.new('x'),
+            AST::String.new('hello world')
+          ),
+          AST::FunctionDefinition.new(
+            nil,
+            [],
+            AST::Block.new(
+              [
+                AST::Identifier.new('x'),
+              ]
+            )
+          )
+        ]
+      )
+    ).execute(env)
+    AST::VariableDeclaration.new(
+      AST::Identifier.new('y'),
+      AST::FunctionCall.new(
+        AST::Identifier.new('outer'),
+        []
+      )
+    ).execute(env)
+    result = AST::FunctionCall.new(
+      AST::Identifier.new('y'),
+      []
+    ).execute(env)
+    expect(result).to eq("hello world")
+  end
 end