Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!apple!bbn!jr@bbn.com
From: jr@bbn.com (John Robinson)
Newsgroups: comp.emacs
Subject: Re: apply, mapcar, lack of setenv
Summary: try eval, not funcall
Keywords: Emacs-lisp, hacking, environment
Message-ID: <40844@bbn.COM>
Date: 2 Jun 89 17:04:01 GMT
References: <CSUWN.89Jun2041413@orchid.warwick.ac.uk> <62353@yale-celray.yale.UUCP>
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Reply-To: jr@bbn.com (John Robinson)
Distribution: comp
Organization: BBN Systems and Technologies Corporation, Cambridge MA
Lines: 35
In-reply-to: Duchier-Denys@cs.yale.edu (Denys Duchier)

In article <62353@yale-celray.yale.UUCP>, Duchier-Denys@cs (Denys Duchier) writes:
>(defun reduce (fun val lst)
>  (while lst (setq val (funcall fun cal (car lst)) lst (cdr lst)))
>  val)
>
>(reduce (function (lambda (x y) (or x y))) (mapcar <your-function> <your-list>))
>
>(reduce '* 1 '(1 2 3 4 5)) => 120 ;(i.e. 5!)

Unfortunately, funcall also works only with functions, not subr's:

  (reduce 'or nil '(nil t nil)) => error "Invalid function #<subr or>"

Eval, on the other hand, is happy with subrs.  Hence,

(defun reduce (fun val lst)
  (while lst (setq val (eval (list fun val (car lst))) lst (cdr lst)))
  val)

then,

  (reduce 'or nil '(nil t nil)) => t

and, to make some examples from the original query,

  (apply (function (lambda (lst) (eval (cons 'or lst))))
         (list (mapcar (function (lambda (elem) (string-match "^TERM=" elem)))
                       process-environment)))

      => 0 (if TERM is in process-environment), else nil.

Now, some might complain that eval is too messy...
--
/jr, nee John Robinson   What a waste it is to lose one's mind--or not
jr@bbn.com or bbn!jr      to have a mind.  How true that is. -Dan Quayle