elisp函数作为参数和返回值 [英] elisp functions as parameters and as return value

问题描述

我有以下代码

(defun avg-damp(f) 
    #'(lambda(x) (/ (+ (funcall f x) x) 2.0)))

通话

(funcall (avg-damp #'(lambda(v) (* v v))) 10)

在SBCL中返回55.0(正确的值)，但在emacs lisp中的以下堆栈崩溃

returns 55.0 (the correct value) in SBCL but crashes with the following stack in emacs lisp

Debugger entered--Lisp error: (void-variable f)
  (funcall f x)
  (+ (funcall f x) x)
  (/ (+ (funcall f x) x) 2.0)
  (lambda (x) (/ (+ ... x) 2.0))(10)
  funcall((lambda (x) (/ (+ ... x) 2.0)) 10)
  eval((funcall (avg-damp (function ...)) 10))
  eval-last-sexp-1(nil)
  eval-last-sexp(nil)
  call-interactively(eval-last-sexp)

如何使其在Emacs Lisp中工作?

How can I make it work in Emacs lisp?

推荐答案

一个棘手的问题，但终于明白了.问题在于，avg-damp定义中的#'使编译器在编译avg-damp本身时在已知f的实际值之前编译lambda函数.您需要将该函数的编译延迟到以后的某个时间点，例如，在调用avg-damp 时，如下所示:

A tricky question, but finally got this figured out. The problem is that #' in the definition of avg-damp makes the compiler compile the lambda function at the time when avg-damp itself is compiled, before the actual value of f is known. You need to delay the compilation of this function to a later point in time, when avg-damp is called, like this:

(defun avg-damp (f)
   `(lambda(x) (/ (+ (funcall ,f x) x) 2.0)))

(funcall (avg-damp #'(lambda(v) (* v v))) 10)

使用引号可以解决问题.

Backquoting does the trick.

编辑:当然，如果您以非咖喱形式定义avg-damp，那么整个问题就会消失，例如:

Edit: Of course, the whole problem goes away if you define avg-damp in an uncurried form, such as this:

(defun avg-damp (f x)
   (/ (+ (funcall f x) x) 2.0))

(funcall 'avg-damp #'(lambda(v) (* v v)) 10)

但是我想您有理由不这样做.

But I guess you have your reasons not to do so.

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