Monkey/evaluator/evaluator.go

package evaluator

import (
	"fmt"
	"github/runnignwater/monkey/ast"
	"github/runnignwater/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.ExpressionStatement:
		return Eval(node.Expression, env)

	// Expression
	case *ast.IntegerLiteral:
		return &object.Integer{Value: node.Value}
	case *ast.Boolean:
		return nativeBooleanObject(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.BlockStatement:
		return evalBlockStatements(node, env)
	case *ast.IfExpression:
		return evalIfExpression(node, env)
	case *ast.ReturnStatement:
		val := Eval(node.ReturnValue, env)
		if isError(val) {
			return val
		}
		return &object.ReturnValue{Value: val}
	case *ast.Identifier:
		return evalIdentifier(node, env)
	case *ast.LetStatement:
		val := Eval(node.Value, env)
		if isError(val) {
			return val
		}
		env.Set(node.Name.Value, val)
	case *ast.FunctionLiteral:
		params := node.Parameters
		body := node.Body

		return &object.Function{Parameters: params, Body: body, Env: env}
	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)
	case *ast.StringLiteral:
		return &object.String{Value: node.Value}
	case *ast.ArrayLiteral:
		elements := evalExpressions(node.Element, env)

		if len(elements) == 1 && isError(elements[0]) {
			// return  object.Error
			return elements[0]
		}
		return &object.Array{Elements: elements}
	case *ast.IndexExpression:
		left := Eval(node.Left, env)
		if isError(left) {
			return left
		}
		index := Eval(node.Index, env)
		if isError(index) {
			return index
		}
		return evalIndexExpression(left, index)
	}

	return nil
}

func applyFunction(fn object.Object, args []object.Object) object.Object {
	switch fn := fn.(type) {
	case *object.Function:
		extendEnv := extendFunctionEnv(fn, args)
		evaluated := Eval(fn.Body, extendEnv)
		return unwrapReturnValue(evaluated)
	case *object.Builtin:
		return fn.Fn(args...)
	default:
		return newError("not a function: %s", fn.Type())
	}
}

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 newError(format string, a ...interface{}) *object.Error {
	return &object.Error{Msg: fmt.Sprintf(format, a...)}
}
func evalProgram(node *ast.Program, env *object.Environment) object.Object {
	var result object.Object

	for _, stmt := range node.Statements {
		result = Eval(stmt, env)
		// skip return 后面的语句
		switch result := result.(type) {
		case *object.ReturnValue:
			return result.Value
		case *object.Error:
			return result
		}
	}

	return result
}

func nativeBooleanObject(input bool) *object.Boolean {
	if input {
		return TRUE
	} else {
		return FALSE
	}
}

func evalPrefixExpression(operator string, right object.Object) object.Object {
	switch operator {
	case "!":
		return evalBangOperatorExpression(right)
	case "-":
		return evalMinusPreOperatorExpression(right)
	default:
		return newError("unknown operator: %s%s", operator, right.Type())
	}
}
func evalInfixExpression(operator string, left object.Object, right object.Object) object.Object {
	switch {
	case left.Type() == object.IntegerObj && right.Type() == object.IntegerObj:
		return evalIntegerInfixExpression(operator, left, right)
	case left.Type() == object.StringObj && right.Type() == object.StringObj:
		return evalStringInfixExpression(operator, left, right)
	case operator == "==":
		return nativeBooleanObject(left == right)
	case operator == "!=":
		return nativeBooleanObject(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 evalIfExpression(node *ast.IfExpression, env *object.Environment) object.Object {
	condition := Eval(node.Condition, env)
	if isError(condition) {
		return condition
	}
	if isTruthy(condition) {
		return Eval(node.Consequence, env)
	} else if node.Alternative != nil {
		return Eval(node.Alternative, env)
	} else {
		return NULL
	}
}
func evalBlockStatements(block *ast.BlockStatement, env *object.Environment) object.Object {
	var result object.Object

	for _, stmt := range block.Statements {
		result = Eval(stmt, env)

		if result != nil {
			rt := result.Type()
			if rt == object.ReturnValueObj || rt == object.ErrorObj {
				return result
			}
		}
	}
	return result
}
func evalIntegerInfixExpression(operator string,
	left object.Object, 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 nativeBooleanObject(leftVal < rightVal)
	case ">":
		return nativeBooleanObject(leftVal > rightVal)
	case "==":
		return nativeBooleanObject(leftVal == rightVal)
	case "!=":
		return nativeBooleanObject(leftVal != rightVal)
	default:
		return newError("unknown operator: %s %s %s", left.Type(), operator, right.Type())
	}
}
func evalStringInfixExpression(operator string,
	left object.Object, right object.Object) object.Object {
	// Only identifier + is valid
	if operator != "+" {
		return newError("unknown operator: %s %s %s", left.Type(), operator, right.Type())
	}
	leftVal := left.(*object.String).Value
	rightVal := right.(*object.String).Value
	return &object.String{Value: leftVal + rightVal}
}
func evalMinusPreOperatorExpression(right object.Object) object.Object {
	if right.Type() != object.IntegerObj {
		return newError("unknown operator: -%s", right.Type())
	}
	value := right.(*object.Integer).Value
	return &object.Integer{Value: -value}
}

func evalBangOperatorExpression(right object.Object) object.Object {
	switch right {
	case TRUE:
		return FALSE
	case FALSE:
		return TRUE
	default:
		return FALSE
	}
}

func evalIdentifier(node *ast.Identifier, env *object.Environment) object.Object {
	if val, ok := env.Get(node.Value); ok {
		return val
	}

	if builtin, ok := builtins[node.Value]; ok {
		return builtin
	}
	return newError("identifier not found: " + node.Value)
}

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 evalIndexExpression(left, index object.Object) object.Object {
	switch {
	case left.Type() == object.ArrayObj && index.Type() == object.IntegerObj:
		return evalArrayIndexExpression(left, index)
	default:
		return newError("index operator not supported: %s", left.Type())
	}
}

func evalArrayIndexExpression(array, index object.Object) object.Object {
	arrayObj := array.(*object.Array)
	idx := index.(*object.Integer).Value
	max := int64(len(arrayObj.Elements) - 1)

	if idx < 0 || idx > max {
		return NULL
	}
	return arrayObj.Elements[idx]
}

func isTruthy(obj object.Object) bool {
	switch obj {
	case NULL:
		return false
	case TRUE:
		return true
	case FALSE:
		return false
	default:
		return true
	}
}

func isError(obj object.Object) bool {
	if obj != nil {
		return obj.Type() == object.ErrorObj
	}
	return false
}