Monkey/evaluator/evaluator.go

package evaluator

import (
	"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) object.Object {
	switch node := node.(type) {
	// Statements
	case *ast.Program:
		return evalProgram(node)

	case *ast.ExpressionStatement:
		return Eval(node.Expression)

	// Expression
	case *ast.IntegerLiteral:
		return &object.Integer{Value: node.Value}
	case *ast.Boolean:
		return nativeBooleanObject(node.Value)
	case *ast.PrefixExpression:
		right := Eval(node.Right)
		return evalPrefixExpression(node.Operator, right)
	case *ast.InfixExpression:
		left := Eval(node.Left)
		right := Eval(node.Right)
		return evalInfixExpression(node.Operator, left, right)

	case *ast.BlockStatement:
		return evalBlockStatements(node)
	case *ast.IfExpression:
		return evalIfExpression(node)

	case *ast.ReturnStatement:
		val := Eval(node.ReturnValue)
		return &object.ReturnValue{Value: val}
	}

	return nil
}

func evalProgram(node *ast.Program) object.Object {
	var result object.Object

	for _, stmt := range node.Statements {
		result = Eval(stmt)
		// skip return 后面的语句
		if rVal, ok := result.(*object.ReturnValue); ok {
			return rVal.Value
		}
	}

	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 NULL
	}
}
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 operator == "==":
		return nativeBooleanObject(left == right)
	case operator == "!=":
		return nativeBooleanObject(left != right)
	default:
		return NULL
	}
}
func evalIfExpression(node *ast.IfExpression) object.Object {
	condition := Eval(node.Condition)
	if isTruthy(condition) {
		return Eval(node.Consequence)
	} else if node.Alternative != nil {
		return Eval(node.Alternative)
	} else {
		return NULL
	}
}
func evalBlockStatements(node *ast.BlockStatement) object.Object {
	var result object.Object

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

		if result != nil && result.Type() == object.ReturnValueObj {
			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 NULL
	}
}

func evalMinusPreOperatorExpression(right object.Object) object.Object {
	if right.Type() != object.IntegerObj {
		return NULL
	}
	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 isTruthy(obj object.Object) bool {
	switch obj {
	case NULL:
		return false
	case TRUE:
		return true
	case FALSE:
		return false
	default:
		return true
	}
}