scala 什么是“按名字呼唤”？
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What is "Call By Name"?
提问by forellana
I'm working on a homework assignment where we are asked to implement an evaluation strategy called "call by name" in a certain language that we developed (using Scheme).
我正在做一项家庭作业，要求我们用我们开发的某种语言（使用 Scheme）实施一种名为“按姓名呼唤”的评估策略。
We were given an example in Scala, but I don't understand how "call by name" works and how it is different to "call by need"?
回答by Doug Currie
Call-by-need is a memoized version of call-by-name (see wikipedia).
Call-by-need 是 call-by-name 的记忆版本（请参阅维基百科）。
In call-by-name, the argument is evaluated every time it is used, whereas in call-by-need, it is evaluated the first time it is used, and the value recorded so that subsequently it need not be re-evaluated.
在 call-by-name 中，每次使用时都会对参数进行评估，而在 call-by-need 中，在第一次使用时对其进行评估，并记录该值，以便随后无需重新评估。
回答by Richard Pennington
Call by name is a a parameter passing scheme where the parameter is evaluated when it is used, not when the function is called. Here's an example in pseudo-C:
按名称调用是一种参数传递方案，参数在使用时计算，而不是在调用函数时计算。这是伪 C 中的示例：
int i;
char array[3] = { 0, 1, 2 };
i = 0;
f(a[i]);
int f(int j)
{
int k = j; // k = 0
i = 2; // modify global i
k = j; // The argument expression (a[i]) is re-evaluated, giving 2.
}
The argument expression is lazily evaluated when accessed using the current values of the argument expression.
当使用参数表达式的当前值访问时，参数表达式被延迟计算。
回答by Vijay Mathew
Add this to the above answers:
将此添加到上述答案中：
Work through the SICP section on Streams. It gives a good explanation of both call-by-name and call-by-need. It also shows how to implement those in Scheme. BTW, if you are looking for a quick solution here is a basic call-by-need implemented in Scheme:
完成有关 Streams的SICP 部分。它很好地解释了按名称调用和按需求调用。它还展示了如何在 Scheme 中实现这些。顺便说一句，如果您正在寻找快速解决方案，这里是 Scheme 中实现的基本按需调用：
;; Returns a promise to execute a computation. (implements call-by-name)
;; Caches the result (memoization) of the computation on its first evaluation
;; and returns that value on subsequent calls. (implements call-by-need)
(define-syntax delay
(syntax-rules ()
((_ (expr ...))
(let ((proc (lambda () (expr ...)))
(already-evaluated #f)
(result null))
(lambda ()
(if (not already-evaluated)
(begin
(display "computing ...") (newline)
(set! result (proc))
(set! already-evaluated #t)))
result)))))
;; Forces the evaluation of a delayed computation created by 'delay'.
(define (my-force proc) (proc))
A sample run:
示例运行：
> (define lazy (delay (+ 3 4)))
> (force lazy)
computing ... ;; Computes 3 + 4 and memoizes the result.
7
> (my-force lazy)
7 ;; Returns the memoized value.
