diff --git a/evaluator/builtins.go b/evaluator/builtins.go
index f028c05..065e210 100644
--- a/evaluator/builtins.go
+++ b/evaluator/builtins.go
@@ -3,6 +3,7 @@ package evaluator
 import (
 	"fmt"
 	"monkey/object"
+	"unicode/utf8"
 )
 
 var builtins = map[string]*object.Builtin{
@@ -16,7 +17,7 @@ var builtins = map[string]*object.Builtin{
 			case *object.Array:
 				return &object.Integer{Value: int64(len(arg.Elements))}
 			case *object.String:
-				return &object.Integer{Value: int64(len(arg.Value))}
+				return &object.Integer{Value: int64(utf8.RuneCountInString(arg.Value))}
 			default:
 				return newError("argument to `len` not supported, got %s", args[0].Type())
 
@@ -30,7 +31,7 @@ var builtins = map[string]*object.Builtin{
 				return newError("wrong number of arguments. got=%d, want=1", len(args))
 			}
 			if args[0].Type() != object.ARRAY_OBJ {
-				return newError("argument to `first` must be Array, got %s", args[0].Type())
+				return newError("argument to `first` must be ARRAY, got %s", args[0].Type())
 			}
 
 			arr := args[0].(*object.Array)
@@ -48,7 +49,7 @@ var builtins = map[string]*object.Builtin{
 				return newError("wrong number of arguments. got=%d, want=1", len(args))
 			}
 			if args[0].Type() != object.ARRAY_OBJ {
-				return newError("argument to `last` must be Array, got %s", args[0].Type())
+				return newError("argument to `last` must be ARRAY, got %s", args[0].Type())
 			}
 
 			arr := args[0].(*object.Array)
@@ -64,36 +65,18 @@ var builtins = map[string]*object.Builtin{
 	"rest": &object.Builtin{
 		Fn: func(args ...object.Object) object.Object {
 			if len(args) != 1 {
-				return newError("wrong number of arguments. got=%d, want=1", len(args))
+				return newError("wrong number of arguments. got=%d, want=1",
+					len(args))
 			}
 			if args[0].Type() != object.ARRAY_OBJ {
-				return newError("argument to `rest` must be Array, got %s", args[0].Type())
-			}
-
-			arr := args[0].(*object.Array)
-			length := len(arr.Elements)
-
-			newElements := make([]object.Object, length+1)
-			copy(newElements, arr.Elements)
-			newElements[length] = args[1]
-
-			return &object.Array{Elements: newElements}
-		},
-	},
-
-	"push": &object.Builtin{
-		Fn: func(args ...object.Object) object.Object {
-			if len(args) != 1 {
-				return newError("wrong number of arguments. got=%d, want=1", len(args))
-			}
-			if args[0].Type() != object.ARRAY_OBJ {
-				return newError("argument to `push` must be Array, got %s", args[0].Type())
+				return newError("argument to `rest` must be ARRAY, got %s",
+					args[0].Type())
 			}
 
 			arr := args[0].(*object.Array)
 			length := len(arr.Elements)
 			if length > 0 {
-				newElements := make([]object.Object, length-1)
+				newElements := make([]object.Object, length-1, length-1)
 				copy(newElements, arr.Elements[1:length])
 				return &object.Array{Elements: newElements}
 			}
@@ -102,6 +85,28 @@ var builtins = map[string]*object.Builtin{
 		},
 	},
 
+	"push": &object.Builtin{
+		Fn: func(args ...object.Object) object.Object {
+			if len(args) != 2 {
+				return newError("wrong number of arguments. got=%d, want=2",
+					len(args))
+			}
+			if args[0].Type() != object.ARRAY_OBJ {
+				return newError("argument to `push` must be ARRAY, got %s",
+					args[0].Type())
+			}
+
+			arr := args[0].(*object.Array)
+			length := len(arr.Elements)
+
+			newElements := make([]object.Object, length+1, length+1)
+			copy(newElements, arr.Elements)
+			newElements[length] = args[1]
+
+			return &object.Array{Elements: newElements}
+		},
+	},
+
 	"puts": &object.Builtin{
 		Fn: func(args ...object.Object) object.Object {
 			for _, arg := range args {
diff --git a/evaluator/evaluator_test.go b/evaluator/evaluator_test.go
index a69fb31..2e35f8b 100644
--- a/evaluator/evaluator_test.go
+++ b/evaluator/evaluator_test.go
@@ -330,7 +330,7 @@ func TestStringConcatenation(t *testing.T) {
 	}
 }
 
-func TestBuiltinFunction(t *testing.T) {
+func TestBuiltinFunctions(t *testing.T) {
 	tests := []struct {
 		input    string
 		expected interface{}
@@ -340,6 +340,20 @@ func TestBuiltinFunction(t *testing.T) {
 		{`len("hello world")`, 11},
 		{`len(1)`, "argument to `len` not supported, got INTEGER"},
 		{`len("one", "two")`, "wrong number of arguments. got=2, want=1"},
+		{`len("∑")`, 1},
+		{`len([1, 2, 3])`, 3},
+		{`len([])`, 0},
+		{`first([1, 2, 3])`, 1},
+		{`first([])`, nil},
+		{`first(1)`, "argument to `first` must be ARRAY, got INTEGER"},
+		{`last([1, 2, 3])`, 3},
+		{`last([])`, nil},
+		{`last(1)`, "argument to `last` must be ARRAY, got INTEGER"},
+		{`rest([1, 2, 3])`, []int{2, 3}},
+		{`rest([])`, nil},
+		{`push([], 1)`, []int{1}},
+		{`push(1, 1)`, "argument to `push` must be ARRAY, got INTEGER"},
+		{`puts("Hello World")`, nil},
 	}
 
 	for _, tt := range tests {
@@ -351,11 +365,13 @@ func TestBuiltinFunction(t *testing.T) {
 		case string:
 			errObj, ok := evaluated.(*object.Error)
 			if !ok {
-				t.Errorf("object is not Error. got=%T (%+v)", evaluated, evaluated)
+				t.Errorf("object is not Error. got=%T (%+v)",
+					evaluated, evaluated)
 				continue
 			}
 			if errObj.Message != expected {
-				t.Errorf("wrong error message. expected=%q, got=%q", expected, errObj.Message)
+				t.Errorf("wrong error message. expected=%q, got=%q",
+					expected, errObj.Message)
 			}
 		}
 	}