让混淆到lambda转换 [英] Confused by let to lambda conversion
问题描述
我目前正在查看关于Y组合器的精彩文章 ="http://users.cms.caltech.edu/~mvanier/" rel ="nofollow noreferrer">迈克·范尼尔.按照说明，以下行被删除:
I am currently, going through this great article on Y-combinator by Mike Vanier. Along the explanation the following line is dropped:
事实证明，可以使用以下等式将任何let表达式转换为等效的lambda表达式:
It turns out that any let expression can be converted into an equivalent lambda expression using this equation:
(let ((x <expr1>)) <expr2>)
==> ((lambda (x) <expr2>) <expr1>)
(let ((x <expr1>)) <expr2>)
==> ((lambda (x) <expr2>) <expr1>)
本文通过转换来说明此语句:
The article illustrates this statement by converting:
(define (part-factorial self)
(let ((f (self self)))
(lambda (n)
(if (= n 0)
1
(* n (f (- n 1)))))))
收件人:
(define (part-factorial self)
((lambda (f)
(lambda (n)
(if (= n 0)
1
(* n (f (- n 1))))))
(self self)))
现在，我理解了上面两个代码片段的方式和原因相同，尽管我无法理解将let转换为lambda的一般公式是这样的事实:
Now, I understand how and why two code snippets above are identical, though I can't get my head around the fact that general equation for converting let to lambda is:
(let ((x <expr1>)) <expr2>)
==> ((lambda (x) <expr2>) <expr1>)
我很感谢详尽的解释.
推荐答案
let使您可以打开一个可用变量的新环境.用编程语言来讲，我们说它打开了一个新的框架".
let lets you open a new environment in which variables are available. In programming language terms we say it "opens a new frame".
编写(let ((x 42)) <body>)时，将创建一个框架，其中<body>内的x可用，并为其分配值42.
When you write (let ((x 42)) <body>) you create a frame in which x is available inside <body>, and assign it the value 42.
嗯，还有另一个工具可以让您打开新框架.实际上，它通常是您可以用来构建更多抽象构造的基本模块:它称为lambda.
Well, there is another tool that lets you open new frames. In fact, it's usually the basic brick with which you can build more abstract constructs: it is called lambda.
lambda打开一个新框架，其中的参数可供其主体使用.
编写(lambda (x) <body>)时，会将x用于该功能的<body>.
lambda opens a new frame in which its arguments are available to its body.
When you write (lambda (x) <body>) you make x available to the <body> of the function.
lambda和let之间的唯一区别是let立即为x分配一个值，而lambda等待该值作为参数.
The only difference between lambda and let is that let immediately assigns a value to x, while lambda awaits the value as an argument.
因此，如果要使用lambda用直接分配的值包装<body>，则只需传递该值！
Therefore, if you want to wrap a <body> with a directly assigned value using lambda, you just have to pass that value!
((lambda (x) <body>) 42)
这完全等同于:
(let ((x 42)) <body>)
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