Re: Functions returning functions in Emacs Lisp

[Tim X]

srinik001@hotmail.com writes:

> On Apr 15, 9:33 pm, Barry Margolin <bar...@alum.mit.edu> wrote:
>> In article
>> <c98a949b-5ada-490a-805f-298e84783...@l28g2000prd.googlegroups.com>,
>>
>>
>>
>>  srinik...@hotmail.com wrote:
>> > Hi,
>>
>> > I am trying to learnEmacsLisp. I know a bit of (Common) Lisp and was
>> > trying the following onEmacs. 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 onEmacs
>> > the way it does in Common Lisp. Could someone please tell me if I am
>> > doing something wrong vis-a-visEmacs, or ifEmacsdoes not support
>> > this? Thanks.
>>
>> EmacsLisp 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, bar...@alum.mit.edu
>> Arlington, MA
>> *** PLEASE post questions in newsgroups, not directly to me ***
>> *** PLEASE don't copy me on replies, I'll read them in the group ***
>
> Thanks for the reply. I noticed that lexical-let requires a common
> lisp extension. I will google for it and figure out how to install
> it.
>
> Regards,
>

Its already there, just do a (require 'cl)

Tim


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