如何在JavaScript中实现lambda/anonymous函数
所以我想用JavaScript实现LISP的一个子集。我被困在与lambda相关的两件事上。如何在JavaScript中实现lambda/anonymous函数
如何实现创建lambda的能力，并同时给它的参数，并立即评估？例如：
((lambda(x)(* x 2)) 3) 现在我硬编码这个功能在我的eval环这样的：
else if (isArray(expr)){ if (expr[0][0] === 'lambda' || expr[0][0] === 'string') {
console.log("This is a special lambda");
var lambdaFunc = evaluate(expr[0], env)
var lambdaArgs = [];
for(var i = 1; i < expr.length; i++){
lambdaArgs.push(expr[i]);
}
return lambdaFunc.apply(this, lambdaArgs);
}
现在这个工作，如果我写在参数上面拉姆达它会评估到6，但是，我想知道是否有更聪明的方式来实现这一点？
如果一个lambda改为绑定到一个符号，例如：
(define fib (lambda(n) (if (< n 2) 1
(+ (fib (- n 1))(fib (- n 2)))
)))
在这种情况下，（定义FIB）部分将由EVAL-环路第一被评估，正如如果FIB只是被分配一个号码：
else if (expr[0] === 'define') { // (define var value) console.log(expr + " is a define statement");
var newVar = expr[1];
var newVal = evaluate(expr[2], env);
env.add(newVar, newVal);
return env;
}
而λ-功能正在创建这样的：
else if (expr[0] === 'lambda') { // (lambda args body) console.log(expr + " is a lambda statement");
var args = expr[1];
var body = expr[2];
return createLambda(args, body, env);
}
独立的函数来创建拉姆达：
function createLambda(args, body, env){ // lambda args body function runLambda(){
var lambdaEnvironment = new environment(env, "lambda environment");
for (var i = 0; i < arguments.length; i++){
lambdaEnvironment.add(args[i], evaluate(arguments[i], env));
}
return evaluate(body, lambdaEnvironment);
}
return runLambda;
}
此工作正常的lambda如：
(define range (lambda (a b) (if (= a b) (quote())
(cons a (range (+ a 1) b)))))
(define fact (lambda (n)
(if (<= n 1) 1
(* n (fact (- n 1))))))
例如，（范围0 10）返回从0到10
列表但是，如果我在lambda内尝试lambda，它不起作用。例如：
(define twice (lambda (x) (* 2 x))) (define repeat (lambda (f) (lambda (x) (f (f x)))))
我希望下面的返回40：
((repeat twice) 10) 但是，相反，它返回一个列表看起来像这样：
function runLambda(){ var lambdaEnvironment = new environment(env, "lambda environment"); for (var i = 0; i < arguments.length; i++){
lambdaEnvironment.add(args[i], evaluate(arguments[i], env)); } return
evaluate(body, lambdaEnvironment); },10
任何想法可能会丢失这里？
完整的JavaScript;
//functions for parsing invoice String function parse(exp) {
return readFromTokes(tokenize(exp));//code
}
function isNumeric(arg){
return !isNaN(arg);
}
function isArray(obj){
return !!obj && obj.constructor === Array;
}
function readFromTokes(exp){
//Create abstract syntax tree
if (exp.length == 0) {
}
var token = exp.shift();
if (token == '('){
var L = [];
while (exp[0] != ')') {
L.push(readFromTokes(exp));
}
exp.shift(); //remove end paranthesis
return L;
} else {
if (token == ')') {
console.log("Unexpected)");
} else {
return atom(token);
}
}
}
function tokenize(exp){
//Convert a program in form of a string into an array (list)
var re = /\(/g;
var re2 = /\)/g;
exp = exp.replace(re, " (");
exp = exp.replace(re2, ") ");
exp = exp.replace(/\s+/g, ' ');
exp = exp.trim().split(" ");
return exp;
}
function atom(exp){
if (isNumeric(exp)) {
return parseInt(exp); //A number is a number
} else {
return exp; //Everything else is a symbol
}
}
function environment(parentEnvironment, name){
var bindings = [];
var parent = parentEnvironment;
var name = name;
function add(variable, value){
console.log("variable: " + variable + " value: " + value);
bindings.push([variable, value]);
}
function printName(){
console.log(name);
}
function print() {
console.log("printing environment: ")
for (var i = 0; i < bindings.length; i++){
console.log(bindings[i][0] + " " + bindings[i][1]);
}
}
function get(variable){
for (var i = 0; i < bindings.length; i++){
if (variable == bindings[i][0]){
return bindings[i][1];
}
}
if (parent != null){
return parent.get(variable);
} else {
console.log("No such variable");
return false;
}
}
function getParent(){
return parent;
}
this.add = add;
this.get = get;
this.getParent = getParent;
this.print = print;
this.printName = printName;
return this;
}
function addPrimitives(env){
env.add("+", function() {var s = 0; for (var i = 0; i<arguments.length;i++){ s += arguments[i];} return s});
env.add("-", function() {var s = arguments[0]; for (var i = 1; i<arguments.length;i++){ s -= arguments[i];} return s});
env.add("*", function() {var s = 1; for (var i = 0; i<arguments.length;i++){ s *= arguments[i];} return s});
env.add("/", function(x, y) { return x/y });
env.add("false", false);
env.add("true", true);
env.add(">", function(x, y){ return (x > y) });
env.add("<", function(x, y){ return (x < y) });
env.add("=", function(x, y){ return (x === y)});
env.add(">=", function(x, y){ if (x >= y){return true;} else {return false;}});
env.add("<=", function(x, y){ if (x <= y){return true;} else {return false;}});
env.add("eq?", function() {var s = arguments[0]; var t = true; for(var i = 1; i<arguments.length; i++){ if (arguments[i] != s) {t = false }} return t;});
env.add("cons", function(x, y) { var temp = [x]; return temp.concat(y); });
env.add("car", function(x) { return x[0]; });
env.add("cdr", function(x) { return x.slice(1); });
env.add("list", function() { return Array.prototype.slice.call(arguments); });
env.add("list?", function(x) {return isArray(x); });
env.add("null", null);
env.add("null?", function (x) { return (!x || x.length === 0); });
}
function createLambda(args, body, env){ // lambda args body
function runLambda(){
var lambdaEnvironment = new environment(env, "lambda environment");
for (var i = 0; i < arguments.length; i++){
lambdaEnvironment.add(args[i], evaluate(arguments[i], env));
}
return evaluate(body, lambdaEnvironment);
}
return runLambda;
}
function evaluate(expr, env) {
console.log(expr + " has entered evaluate loop");
if (typeof expr === 'string') {
console.log(expr + " is a symbol");
return env.get(expr);
} else if (typeof expr === 'number') {
console.log(expr + " is a number");
return expr;
} else if (expr[0] === 'define') { // (define var value)
console.log(expr + " is a define statement");
var newVar = expr[1];
var newVal = evaluate(expr[2], env);
env.add(newVar, newVal);
return env;
} else if (expr[0] === 'lambda') { // (lambda args body)
console.log(expr + " is a lambda statement");
var args = expr[1];
var body = expr[2];
return createLambda(args, body, env);
} else if (expr[0] === 'quote') {
return expr[1];
} else if (expr[0] === 'cond'){
console.log(expr + " is a conditional");
for (var i = 1; i < expr.length; i++){
var temp = expr[i];
if (evaluate(temp[0], env)) {
console.log(temp[0] + " is evaluated as true");
return evaluate(temp[1], env);
}
}
console.log("no case was evaluated as true");
return;
} else if (expr[0] === 'if') {
console.log(expr + "is an if case");
return function(test, caseyes, caseno, env){
if (test) {
return evaluate(caseyes, env);
} else {
return evaluate(caseno, env);
}
}(evaluate(expr[1], env), expr[2], expr[3], env);
} else if (typeof expr[0] === 'string'){
console.log(expr + " is a function call");
var lispFunc = env.get(expr[0]);
var lispFuncArgs = [];
for(var i = 1; i < expr.length; i++){
lispFuncArgs.push(evaluate(expr[i], env));
}
return lispFunc.apply(this, lispFuncArgs);
} else if (isArray(expr)){
if (expr[0][0] === 'lambda' || expr[0][0] === 'string') {
console.log("This is a special lambda");
var lambdaFunc = evaluate(expr[0], env)
var lambdaArgs= [];
for(var i = 1; i < expr.length; i++){
lambdaArgs.push(expr[i]);
}
return lambdaFunc.apply(this, lambdaArgs);
} else {
console.log(expr + " is a list");
var evaluatedList = [];
for(var i = 0; i < expr.length; i++){
evaluatedList.push(evaluate(expr[i], env));
}
return evaluatedList;
}
} else {
console.log(expr + " cannot be interpreted");
}
}
var globalEnvironment = new environment(null, "Global");
addPrimitives(globalEnvironment);
function start(string) {
return evaluate(parse(string), globalEnvironment);
}
var output = function (string) {
try {
document.getElementById('debugdiv').innerHTML = start(string);
} catch (e) {
document.getElementById('debugdiv').innerHTML = e.name + ': ' + e.message;
}
};
Full HTML;
<html> <head>
<meta content="text/html; charset=UTF-8" http-equiv="Content-Script-Type" content="text/javascript">
<title>LISP in JavaScript</title>
<script type="text/javascript" src="lisp.js"></script>
</head>
<body>
<form id="repl" name="repl" action="parse(prompt.value)">
lisp==>
<input id="prompt" size="200" value="" name="prompt" maxlength="512">
<br>
<input type=button style="width:60px;height:30px" name="btnEval" value="eval" onclick="output(prompt.value)">
<br>
</form>
<div id="debugdiv" style="background-color:orange;width=100px;height=20px">
</div>
</body>
</html>
进一步的想法，建议和意见当然也是受欢迎的。 谢谢！
回答：
您不检查操作数的结构以查看它是否为lambda您操作数eval。 eval的标准方法是检查它是否为原始类型，然后检查特殊形式和宏，然后在应用之前评估运算符和操作数。
只需删除其中expr[0][0] === 'lambda' || expr[0][0] === 'string'是真实的一部分，而不是只返回形式的评估需要apply操作数：
else if (isArray(expr)){ const fn = evaluate(expr[0], env);
const evaluatedList = [];
for(var i = 1; i < expr.length; i++){
evaluatedList.push(evaluate(expr[i], env));
}
return fn(...evaluatedList);
}
的createLambda是错误的，因为你正在评估中的参数错误的环境。因为参数已经评价这将是正确的：
function createLambda(args, body, env){ // lambda args body function runLambda(){
const lambdaEnvironment = new environment(env, "lambda environment");
for (let i = 0; i < arguments.length; i++){
lambdaEnvironment.add(args[i], arguments[i]);
}
return evaluate(body, lambdaEnvironment);
}
return runLambda;
}
以上是 如何在JavaScript中实现lambda/anonymous函数 的全部内容， 来源链接： utcz.com/qa/265220.html
