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Re: The Paul Graham's "Revenge of the nerds" cummulator function and the
From: |
Barry Margolin |
Subject: |
Re: The Paul Graham's "Revenge of the nerds" cummulator function and the solution in Emacs Lisp |
Date: |
Mon, 08 Jun 2009 20:27:41 -0400 |
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MT-NewsWatcher/3.5.3b3 (Intel Mac OS X) |
In article <mailman.243.1244499530.2239.help-gnu-emacs@gnu.org>,
Tiago Charters de Azevedo <tca@cii.fc.ul.pt> wrote:
> I've been reading Paul Graham's "Revenge of the nerds"
> (http://www.paulgraham.com/icad.html) particularly the appendix. It states
> the
> problem of writing "a function that generates accumulators-- a function that
> takes
> a number n, and returns a function that takes another number i and returns n
> incremented by i. (That's incremented by, not plus. An accumulator has to
> accumulate.)"
>
> Should not the function foo in Common Lisp work with Emacs Lisp?
Emacs Lisp has dynamic scoping, not lexical scoping. If you want to
emulate lexical scoping, (require 'cl-macs) and use lexical-let:
(defun foo (n)
(lexical-let ((lexn n))
(lambda (i)
(incf lexn n))))
>
> (defun foo (n)
> (lambda (i)
> (incf n i)))
>
> by setting
>
> (setq a (foo 3))
>
> followed by
>
> (funcall a 1)
>
> When I try to evaluate (funcall a 1) in Emacs (in emacs-lisp mode) I get:
>
> Debugger entered--Lisp error: (void-variable n)
> (+ n i)
> (setq n (+ n i))
> (incf n i)
> (lambda (i) (incf n i))(1)
> funcall((lambda (i) (incf n i)) 1)
> eval((funcall a 1))
> eval-last-sexp-1(nil)
> eval-last-sexp(nil)
> call-interactively(eval-last-sexp
>
>
> It seems that emacs considers that n is a local variable because it is inside
> a
> lambda expression?
>
> Is this a bug? Or I'm missing something?
>
> Thanks
>
> tca
--
Barry Margolin, barmar@alum.mit.edu
Arlington, MA
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