functions and closures

3 files changed, 57 insertions, 1 deletions

diff --git a/evaluator/evaluator.go b/evaluator/evaluator.go
index f765d02..87bfc48 100644
--- a/evaluator/evaluator.go
+++ b/evaluator/evaluator.go
@@ -75,6 +75,8 @@ func Eval(node ast.Node, env *object.Environment) object.Object {
 			return args[0]
 		}
 
+		return applyFunction(function, args)
+
 	case *ast.FunctionLiteral:
 		params := node.Parameters
 		body := node.Body
@@ -92,6 +94,36 @@ func Eval(node ast.Node, env *object.Environment) object.Object {
 	return nil
 }
 
+func applyFunction(fn object.Object, args []object.Object) object.Object {
+	function, ok := fn.(*object.Function)
+
+	if !ok {
+		return newError("not a function: %s", fn.Type())
+	}
+
+	extendedEnv := extendFunctionEnv(function, args)
+	evaulated := Eval(function.Body, extendedEnv)
+	return unwrapReturnValue(evaulated)
+}
+
+func extendFunctionEnv(fn *object.Function, args []object.Object) *object.Environment {
+	env := object.NewEnclosedEnvironment(fn.Env)
+
+	for paramIdx, param := range fn.Parameters {
+		env.Set(param.Value, args[paramIdx])
+	}
+
+	return env
+}
+
+func unwrapReturnValue(obj object.Object) object.Object {
+	if returnValue, ok := obj.(*object.ReturnValue); ok {
+		return returnValue.Value
+	}
+
+	return obj
+}
+
 func evalProgram(program *ast.Program, env *object.Environment) object.Object {
 	var result object.Object
 
diff --git a/evaluator/evaluator_test.go b/evaluator/evaluator_test.go
index f83cf7f..75480dc 100644
--- a/evaluator/evaluator_test.go
+++ b/evaluator/evaluator_test.go
@@ -258,6 +258,19 @@ func TestFunctionApplication(t *testing.T) {
 	}
 }
 
+func TestClosures(t *testing.T) {
+	input := `
+	let newAdder = fn(x) {
+		fn(y) { x + y };
+	};
+
+	let addTwo = newAdder(2);
+	addTwo(2);
+	`
+
+	testIntegerObject(t, testEval(input), 4)
+}
+
 func testEval(input string) object.Object {
 	l := lexer.New(input)
 	p := parser.New(l)
diff --git a/object/environment.go b/object/environment.go
index bd89056..74b6ed2 100644
--- a/object/environment.go
+++ b/object/environment.go
@@ -1,16 +1,27 @@
 package object
 
+func NewEnclosedEnvironment(outer *Environment) *Environment {
+	env := NewEnvironment()
+	env.outer = outer
+	return env
+}
+
 func NewEnvironment() *Environment {
 	s := make(map[string]Object)
-	return &Environment{store: s}
+	return &Environment{store: s, outer: nil}
 }
 
 type Environment struct {
 	store map[string]Object
+	outer *Environment
 }
 
 func (e *Environment) Get(name string) (Object, bool) {
 	obj, ok := e.store[name]
+	if !ok && e.outer != nil {
+		obj, ok = e.outer.Get(name)
+	}
+
 	return obj, ok
 }