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Re: Functions returning functions in Emacs Lisp
From: |
Barry Margolin |
Subject: |
Re: Functions returning functions in Emacs Lisp |
Date: |
Wed, 16 Apr 2008 00:33:49 -0400 |
User-agent: |
MT-NewsWatcher/3.5.3b2 (Intel Mac OS X) |
In article
<c98a949b-5ada-490a-805f-298e84783e01@l28g2000prd.googlegroups.com>,
srinik001@hotmail.com wrote:
> Hi,
>
> I am trying to learn Emacs Lisp. I know a bit of (Common) Lisp and was
> trying the following on Emacs. This is to define a function called
> derivative, which returns as its result a function which is the
> derivative of the argument function fn, when the numerical calculation
> is done over delta. So I did the following.
>
> (defun derivative (fn delta)
> #'(lambda(x) (/ (- (funcall fn (+ x delta)) (funcall fn x)) delta)))
>
> --> this seemed to work (i.e. no error on C-x C-e)
>
> (setq c (derivative #'sin 0.001))
>
> --> this seemed to work (again, no error on evaluation)
>
> (funcall c (/ 3.1415 2))
>
> --> this threw up an error. The error is the following:
>
> Debugger entered--Lisp error: (void-variable fn)
> (funcall fn (+ x delta))
> (- (funcall fn (+ x delta)) (funcall fn x))
> (/ (- (funcall fn ...) (funcall fn x)) delta)
> (lambda (x) (/ (- ... ...) delta))(1.57075)
> funcall((lambda (x) (/ (- ... ...) delta)) 1.57075)
> eval((funcall c (/ 3.1415 2)))
> eval-last-sexp-1(nil)
> eval-last-sexp(nil)
> call-interactively(eval-last-sexp)
>
> I tried lambda expressions on mapcar, and it seemed to work on Emacs
> the way it does in Common Lisp. Could someone please tell me if I am
> doing something wrong vis-a-vis Emacs, or if Emacs does not support
> this? Thanks.
Emacs Lisp uses dynamic scoping, not lexical scoping, so it doesn't
create lexical closures.
You can get the effect using lexical-let:
(defun derivative (fn delta)
(lexical-let ((fn fn) (delta delta))
#'(lambda (x) (/ (- (funcall fn (+ x delta))
(funcall fn x))
delta))))
--
Barry Margolin, barmar@alum.mit.edu
Arlington, MA
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