简介
书中对这部分的介绍是对ast进行求值，但我感觉应该叫语义分析更加合适一点，单纯的ast是没有意义的，需要对其进行相关解析，生成符合自己需求的结果，才更加合适。
符号表（应该算是）
对象定义
package object
import (
"bytes"
"fmt"
"monkey/ast"
"strings"
)
type ObjectType string
const (
NULL_OBJ = "NULL"
ERROR_OBJ = "ERROR"
INTEGER_OBJ = "INTEGER"
BOOLEAN_OBJ = "BOOLEAN"
RETURN_VALUE_OBJ = "RETURN_VALUE"
FUNCTION_OBJ = "FUNCTION"
)
type Object interface {
Type() ObjectType
Inspect() string
}
type Integer struct {
Value int64
}
func (i *Integer) Type() ObjectType { return INTEGER_OBJ }
func (i *Integer) Inspect() string { return fmt.Sprintf("%d", i.Value) }
type Boolean struct {
Value bool
}
func (b *Boolean) Type() ObjectType { return BOOLEAN_OBJ }
func (b *Boolean) Inspect() string { return fmt.Sprintf("%t", b.Value) }
type Null struct{}
func (n *Null) Type() ObjectType { return NULL_OBJ }
func (n *Null) Inspect() string { return "null" }
type ReturnValue struct {
Value Object
}
func (rv *ReturnValue) Type() ObjectType { return RETURN_VALUE_OBJ }
func (rv *ReturnValue) Inspect() string { return rv.Value.Inspect() }
type Error struct {
Message string
}
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
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("\n}")
return out.String()
}符号表定义
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, 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
}
func (e *Environment) Set(name string, val Object) Object {
e.store[name] = val
return val
}语义分析过程如下
package evaluator
import (
"fmt"
"monkey/ast"
"monkey/object"
)
var (
NULL = &object.Null{}
TRUE = &object.Boolean{Value: true}
FALSE = &object.Boolean{Value: false}
)
func Eval(node ast.Node, env *object.Environment) object.Object {
switch node := node.(type) {
// Statements
case *ast.Program:
return evalProgram(node, env)
case *ast.BlockStatement:
return evalBlockStatement(node, env)
case *ast.ExpressionStatement:
return Eval(node.Expression, env)
case *ast.ReturnStatement:
val := Eval(node.ReturnValue, env)
if isError(val) {
return val
}
return &object.ReturnValue{Value: val}
case *ast.LetStatement:
val := Eval(node.Value, env)
if isError(val) {
return val
}
env.Set(node.Name.Value, val)
// Expressions
case *ast.IntegerLiteral:
return &object.Integer{Value: node.Value}
case *ast.Boolean:
return nativeBoolToBooleanObject(node.Value)
case *ast.PrefixExpression:
right := Eval(node.Right, env)
if isError(right) {
return right
}
return evalPrefixExpression(node.Operator, right)
case *ast.InfixExpression:
left := Eval(node.Left, env)
if isError(left) {
return left
}
right := Eval(node.Right, env)
if isError(right) {
return right
}
return evalInfixExpression(node.Operator, left, right)
case *ast.IfExpression:
return evalIfExpression(node, env)
case *ast.Identifier:
return evalIdentifier(node, env)
case *ast.FunctionLiteral:
params := node.Parameters
body := node.Body
return &object.Function{Parameters: params, Env: env, Body: body}
case *ast.CallExpression:
function := Eval(node.Function, env)
if isError(function) {
return function
}
args := evalExpressions(node.Arguments, env)
if len(args) == 1 && isError(args[0]) {
return args[0]
}
return applyFunction(function, args)
}
return nil
}
func evalProgram(program *ast.Program, env *object.Environment) object.Object {
var result object.Object
for _, statement := range program.Statements {
result = Eval(statement, env)
switch result := result.(type) {
case *object.ReturnValue:
return result.Value
case *object.Error:
return result
}
}
return result
}
func evalBlockStatement(
block *ast.BlockStatement,
env *object.Environment,
) object.Object {
var result object.Object
for _, statement := range block.Statements {
result = Eval(statement, env)
if result != nil {
rt := result.Type()
if rt == object.RETURN_VALUE_OBJ || rt == object.ERROR_OBJ {
return result
}
}
}
return result
}
func nativeBoolToBooleanObject(input bool) *object.Boolean {
if input {
return TRUE
}
return FALSE
}
func evalPrefixExpression(operator string, right object.Object) object.Object {
switch operator {
case "!":
return evalBangOperatorExpression(right)
case "-":
return evalMinusPrefixOperatorExpression(right)
default:
return newError("unknown operator: %s%s", operator, right.Type())
}
}
func evalInfixExpression(
operator string,
left, right object.Object,
) object.Object {
switch {
case left.Type() == object.INTEGER_OBJ && right.Type() == object.INTEGER_OBJ:
return evalIntegerInfixExpression(operator, left, right)
case operator == "==":
return nativeBoolToBooleanObject(left == right)
case operator == "!=":
return nativeBoolToBooleanObject(left != right)
case left.Type() != right.Type():
return newError("type mismatch: %s %s %s",
left.Type(), operator, right.Type())
default:
return newError("unknown operator: %s %s %s",
left.Type(), operator, right.Type())
}
}
func evalBangOperatorExpression(right object.Object) object.Object {
switch right {
case TRUE:
return FALSE
case FALSE:
return TRUE
case NULL:
return TRUE
default:
return FALSE
}
}
func evalMinusPrefixOperatorExpression(right object.Object) object.Object {
if right.Type() != object.INTEGER_OBJ {
return newError("unknown operator: -%s", right.Type())
}
value := right.(*object.Integer).Value
return &object.Integer{Value: -value}
}
func evalIntegerInfixExpression(
operator string,
left, right object.Object,
) object.Object {
leftVal := left.(*object.Integer).Value
rightVal := right.(*object.Integer).Value
switch operator {
case "+":
return &object.Integer{Value: leftVal + rightVal}
case "-":
return &object.Integer{Value: leftVal - rightVal}
case "*":
return &object.Integer{Value: leftVal * rightVal}
case "/":
return &object.Integer{Value: leftVal / rightVal}
case "<":
return nativeBoolToBooleanObject(leftVal < rightVal)
case ">":
return nativeBoolToBooleanObject(leftVal > rightVal)
case "==":
return nativeBoolToBooleanObject(leftVal == rightVal)
case "!=":
return nativeBoolToBooleanObject(leftVal != rightVal)
default:
return newError("unknown operator: %s %s %s",
left.Type(), operator, right.Type())
}
}
func evalIfExpression(
ie *ast.IfExpression,
env *object.Environment,
) object.Object {
condition := Eval(ie.Condition, env)
if isError(condition) {
return condition
}
if isTruthy(condition) {
return Eval(ie.Consequence, env)
} else if ie.Alternative != nil {
return Eval(ie.Alternative, env)
} else {
return NULL
}
}
func evalIdentifier(
node *ast.Identifier,
env *object.Environment,
) object.Object {
val, ok := env.Get(node.Value)
if !ok {
return newError("identifier not found: " + node.Value)
}
return val
}
func isTruthy(obj object.Object) bool {
switch obj {
case NULL:
return false
case TRUE:
return true
case FALSE:
return false
default:
return true
}
}
func newError(format string, a ...interface{}) *object.Error {
return &object.Error{Message: fmt.Sprintf(format, a...)}
}
func isError(obj object.Object) bool {
if obj != nil {
return obj.Type() == object.ERROR_OBJ
}
return false
}
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 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
}测试用例如下
package evaluator
import (
"monkey/lexer"
"monkey/object"
"monkey/parser"
"testing"
)
func TestEvalIntegerExpression(t *testing.T) {
tests := []struct {
input string
expected int64
}{
{"5", 5},
{"10", 10},
{"-5", -5},
{"-10", -10},
{"5 + 5 + 5 + 5 - 10", 10},
{"2 * 2 * 2 * 2 * 2", 32},
{"-50 + 100 + -50", 0},
{"5 * 2 + 10", 20},
{"5 + 2 * 10", 25},
{"20 + 2 * -10", 0},
{"50 / 2 * 2 + 10", 60},
{"2 * (5 + 10)", 30},
{"3 * 3 * 3 + 10", 37},
{"3 * (3 * 3) + 10", 37},
{"(5 + 10 * 2 + 15 / 3) * 2 + -10", 50},
}
for _, tt := range tests {
evaluated := testEval(tt.input)
testIntegerObject(t, evaluated, tt.expected)
}
}
func TestEvalBooleanExpression(t *testing.T) {
tests := []struct {
input string
expected bool
}{
{"true", true},
{"false", false},
{"1 < 2", true},
{"1 > 2", false},
{"1 < 1", false},
{"1 > 1", false},
{"1 == 1", true},
{"1 != 1", false},
{"1 == 2", false},
{"1 != 2", true},
{"true == true", true},
{"false == false", true},
{"true == false", false},
{"true != false", true},
{"false != true", true},
{"(1 < 2) == true", true},
{"(1 < 2) == false", false},
{"(1 > 2) == true", false},
{"(1 > 2) == false", true},
}
for _, tt := range tests {
evaluated := testEval(tt.input)
testBooleanObject(t, evaluated, tt.expected)
}
}
func TestBangOperator(t *testing.T) {
tests := []struct {
input string
expected bool
}{
{"!true", false},
{"!false", true},
{"!5", false},
{"!!true", true},
{"!!false", false},
{"!!5", true},
}
for _, tt := range tests {
evaluated := testEval(tt.input)
testBooleanObject(t, evaluated, tt.expected)
}
}
func TestIfElseExpressions(t *testing.T) {
tests := []struct {
input string
expected interface{}
}{
{"if (true) { 10 }", 10},
{"if (false) { 10 }", nil},
{"if (1) { 10 }", 10},
{"if (1 < 2) { 10 }", 10},
{"if (1 > 2) { 10 }", nil},
{"if (1 > 2) { 10 } else { 20 }", 20},
{"if (1 < 2) { 10 } else { 20 }", 10},
}
for _, tt := range tests {
evaluated := testEval(tt.input)
integer, ok := tt.expected.(int)
if ok {
testIntegerObject(t, evaluated, int64(integer))
} else {
testNullObject(t, evaluated)
}
}
}
func TestReturnStatements(t *testing.T) {
tests := []struct {
input string
expected int64
}{
{"return 10;", 10},
{"return 10; 9;", 10},
{"return 2 * 5; 9;", 10},
{"9; return 2 * 5; 9;", 10},
{"if (10 > 1) { return 10; }", 10},
{
`
if (10 > 1) {
if (10 > 1) {
return 10;
}
return 1;
}
`,
10,
},
{
`
let f = fn(x) {
return x;
x + 10;
};
f(10);`,
10,
},
{
`
let f = fn(x) {
let result = x + 10;
return result;
return 10;
};
f(10);`,
20,
},
}
for _, tt := range tests {
evaluated := testEval(tt.input)
testIntegerObject(t, evaluated, tt.expected)
}
}
func TestErrorHandling(t *testing.T) {
tests := []struct {
input string
expectedMessage string
}{
{
"5 + true;",
"type mismatch: INTEGER + BOOLEAN",
},
{
"5 + true; 5;",
"type mismatch: INTEGER + BOOLEAN",
},
{
"-true",
"unknown operator: -BOOLEAN",
},
{
"true + false;",
"unknown operator: BOOLEAN + BOOLEAN",
},
{
"true + false + true + false;",
"unknown operator: BOOLEAN + BOOLEAN",
},
{
"5; true + false; 5",
"unknown operator: BOOLEAN + BOOLEAN",
},
{
"if (10 > 1) { true + false; }",
"unknown operator: BOOLEAN + BOOLEAN",
},
{
`
if (10 > 1) {
if (10 > 1) {
return true + false;
}
return 1;
}
`,
"unknown operator: BOOLEAN + BOOLEAN",
},
{
"foobar",
"identifier not found: foobar",
},
}
for _, tt := range tests {
evaluated := testEval(tt.input)
errObj, ok := evaluated.(*object.Error)
if !ok {
t.Errorf("no error object returned. got=%T(%+v)",
evaluated, evaluated)
continue
}
if errObj.Message != tt.expectedMessage {
t.Errorf("wrong error message. expected=%q, got=%q",
tt.expectedMessage, errObj.Message)
}
}
}
func TestLetStatements(t *testing.T) {
tests := []struct {
input string
expected int64
}{
{"let a = 5; a;", 5},
{"let a = 5 * 5; a;", 25},
{"let a = 5; let b = a; b;", 5},
{"let a = 5; let b = a; let c = a + b + 5; c;", 15},
}
for _, tt := range tests {
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 TestEnclosingEnvironments(t *testing.T) {
input := `
let first = 10;
let second = 10;
let third = 10;
let ourFunction = fn(first) {
let second = 20;
first + second + third;
};
ourFunction(20) + first + second;`
testIntegerObject(t, testEval(input), 70)
}
func testEval(input string) object.Object {
l := lexer.New(input)
p := parser.New(l)
program := p.ParseProgram()
env := object.NewEnvironment()
return Eval(program, env)
}
func testIntegerObject(t *testing.T, obj object.Object, expected int64) bool {
result, ok := obj.(*object.Integer)
if !ok {
t.Errorf("object is not Integer. got=%T (%+v)", obj, obj)
return false
}
if result.Value != expected {
t.Errorf("object has wrong value. got=%d, want=%d",
result.Value, expected)
return false
}
return true
}
func testBooleanObject(t *testing.T, obj object.Object, expected bool) bool {
result, ok := obj.(*object.Boolean)
if !ok {
t.Errorf("object is not Boolean. got=%T (%+v)", obj, obj)
return false
}
if result.Value != expected {
t.Errorf("object has wrong value. got=%t, want=%t",
result.Value, expected)
return false
}
return true
}
func testNullObject(t *testing.T, obj object.Object) bool {
if obj != NULL {
t.Errorf("object is not NULL. got=%T (%+v)", obj, obj)
return false
}
return true
}总结
eval求值的过程如上所示，结合上一节的代码，可以更好的帮助理解
代码引用自https://interpreterbook.com/waiig_code_1.7.zip，有兴趣的同学可以去下载全部代码。