3.9: Implement function call

evaluator/evaluator.go

@@ -67,12 +67,57 @@ func Eval(node ast.Node, env *object.Environment) object.Object {
 		return evalInfixExpression(node.Operator, left, right)
 	case *ast.IfExpression:
 		return evalIfExpression(node, env)
-
+	case *ast.FunctionLiteral:
+		params := node.Parameters
+		body := node.Body
+		return &object.Function{Parameters: params, Body: body, Env: env}
+	case *ast.CallExpression:
+		// eval function body
+		function := Eval(node.Function, env)
+		if isError(function) {
+			return function
+		}
+		// eval parameters
+		args := evalExpressions(node.Arguments, env)
+		if len(args) == 1 && isError(args[0]) {
+			return args[0]
+		}
+		// execute function
+		return applyFunction(function, args)
 	}
 
 	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)
+	evaluated := Eval(function.Body, extendedEnv)
+
+	return unwrapReturnValue(evaluated)
+}
+
+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 evalIdentifier(node *ast.Identifier, env *object.Environment) object.Object {
 	val, ok := env.Get(node.Value)
 	if !ok {
@@ -115,6 +160,20 @@ func evalBlockStatement(block *ast.BlockStatement, env *object.Environment) obje
 	return result
 }
 
+func evalExpressions(exps []ast.Expression, env *object.Environment) []object.Object {
+	var result []object.Object
+
+	for _, e := range exps {
+		evaluated := Eval(e, env)
+		if isError(evaluated) {
+			return []object.Object{evaluated}
+		}
+		result = append(result, evaluated)
+	}
+
+	return result
+}
+
 func evalPrefixExpression(operator string, right object.Object) object.Object {
 	switch operator {
 	case "!":

evaluator/evaluator_test.go

@@ -263,3 +263,58 @@ func TestLetStatements(t *testing.T) {
 		testIntegerObject(t, testEval(tt.input), tt.expected)
 	}
 }
+
+func TestFunctionObject(t *testing.T) {
+	input := "fn(x) { x + 2; };"
+
+	evaluated := testEval(input)
+	fn, ok := evaluated.(*object.Function)
+	if !ok {
+		t.Fatalf("object is not Function. got=%T (%+v)", evaluated, evaluated)
+	}
+
+	if len(fn.Parameters) != 1 {
+		t.Fatalf("function has wrong parameters. Parameters=%+v",
+			fn.Parameters)
+	}
+
+	if fn.Parameters[0].String() != "x" {
+		t.Fatalf("parameter is not 'x'. got=%q", fn.Parameters[0])
+	}
+
+	expectedBody := "(x + 2)"
+
+	if fn.Body.String() != expectedBody {
+		t.Fatalf("body is not %q. got=%q", expectedBody, fn.Body.String())
+	}
+}
+
+func TestFunctionApplication(t *testing.T) {
+	tests := []struct {
+		input    string
+		expected int64
+	}{
+		{"let identity = fn(x) { x; }; identity(5);", 5},
+		{"let identity = fn(x) { return x; }; identity(5);", 5},
+		{"let double = fn(x) { x * 2; }; double(5);", 10},
+		{"let add = fn(x, y) { x + y; }; add(5, 5);", 10},
+		{"let add = fn(x, y) { x + y; }; add(5 + 5, add(5, 5));", 20},
+		{"fn(x) { x; }(5)", 5},
+	}
+
+	for _, tt := range tests {
+		testIntegerObject(t, testEval(tt.input), tt.expected)
+	}
+}
+
+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)
+}

object/environment.go

@@ -2,15 +2,25 @@ package object
 
 func NewEnvironment() *Environment {
 	s := make(map[string]Object)
-	return &Environment{store: s}
+	return &Environment{store: s, outer: nil}
+}
+
+func NewEnclosedEnvironment(outer *Environment) *Environment {
+	env := NewEnvironment()
+	env.outer = outer
+	return env
 }
 
 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
 }
 

object/object.go

@@ -1,6 +1,11 @@
 package object
 
-import "fmt"
+import (
+	"bytes"
+	"fmt"
+	"monkey_interpreter/ast"
+	"strings"
+)
 
 type ObjectType string
 
@@ -10,6 +15,7 @@ const (
 	NULL_OBJ         = "NULL"
 	RETURN_VALUE_OBJ = "RETURN_VALUE"
 	ERROR_OBJ        = "ERROR"
+	FUNCTION_OBJ     = "FUNCTION"
 )
 
 type Object interface {
@@ -49,3 +55,28 @@ type Error struct {
 
 func (e *Error) Type() ObjectType { return ERROR_OBJ }
 func (e *Error) Inspect() string  { return "ERROR: " + e.Message }
+
+type Function struct {
+	Parameters []*ast.Identifier
+	Body       *ast.BlockStatement
+	Env        *Environment
+}
+
+func (f *Function) Type() ObjectType { return FUNCTION_OBJ }
+func (f *Function) Inspect() string {
+	var out bytes.Buffer
+
+	var params []string
+	for _, p := range f.Parameters {
+		params = append(params, p.String())
+	}
+
+	out.WriteString("fn")
+	out.WriteString("(")
+	out.WriteString(strings.Join(params, ", "))
+	out.WriteString(") {\n")
+	out.WriteString(f.Body.String())
+	out.WriteString("}")
+
+	return out.String()
+}