Evaluate function literals and function calls.

Signed-off-by: jmug <u.g.a.mariano@gmail.com>

pkg/evaluator/evaluator.go

@@ -59,6 +59,20 @@ func Eval(node ast.Node, env *object.Environment) object.Object {
 		env.Set(node.Name.Value, val)
 	case *ast.Identifier:
 		return evalIdentifier(node, env)
+	case *ast.FunctionLiteral:
+		params := node.Parameters
+		body := node.Body
+		return &object.Function{Parameters: params, Body: body, Env: env}
+	case *ast.CallExpression:
+		fn := Eval(node.Function, env)
+		if isError(fn) {
+			return fn
+		}
+		args := evalExpressions(node.Arguments, env)
+		if len(args) == 1 && isError(args[0]) {
+			return args[0]
+		}
+		return applyFunction(fn, args)
 	}
 	return nil
 }
@@ -184,6 +198,46 @@ func evalIdentifier(exp *ast.Identifier, env *object.Environment) object.Object
 	return val
 }
 
+func evalExpressions(
+	exps []ast.Expression,
+	env *object.Environment,
+) []object.Object {
+	var res []object.Object
+	for _, exp := range exps {
+		ev := Eval(exp, env)
+		if isError(ev) {
+			return []object.Object{ev}
+		}
+		res = append(res, ev)
+	}
+	return res
+}
+
+func applyFunction(fnObj object.Object, args []object.Object) object.Object {
+	fn, ok := fnObj.(*object.Function)
+	if !ok {
+		return newError("not a function: %s", fn.Type())
+	}
+	env := extendFunctionEnv(fn, args)
+	ret := Eval(fn.Body, env)
+	return unwrapReturnValue(ret)
+}
+
+func extendFunctionEnv(fn *object.Function, args []object.Object) *object.Environment {
+	env := object.NewEnclosedEnvironment(fn.Env)
+	for pi, param := range fn.Parameters {
+		env.Set(param.Value, args[pi])
+	}
+	return env
+}
+
+func unwrapReturnValue(obj object.Object) object.Object {
+	if ret, ok := obj.(*object.ReturnValue); ok {
+		return ret.Value
+	}
+	return obj
+}
+
 func isTruthy(obj object.Object) bool {
 	switch obj {
 	case _TRUE:

pkg/evaluator/evaluator_test.go

@@ -221,6 +221,49 @@ func TestLetStatements(t *testing.T) {
 	}
 }
 
+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 testNullObject(t *testing.T, obj object.Object) bool {
 	if obj != _NULL {
 		t.Errorf("object is not NULL. got=%T (%+v)", obj, obj)