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Functions returning functions in Emacs Lisp


From: srinik001
Subject: Functions returning functions in Emacs Lisp
Date: Tue, 15 Apr 2008 20:18:09 -0700 (PDT)
User-agent: G2/1.0

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.

Regards,

SK


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