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Re: emacs lisp syntax rfc: (cond (EXPR => (lambda (X) ...)))

From: Harald Hanche-Olsen
Subject: Re: emacs lisp syntax rfc: (cond (EXPR => (lambda (X) ...)))
Date: Sun, 02 Jan 2011 23:52:34 +0100 (CET)

[Thien-Thi Nguyen <address@hidden> (2011-01-02 21:45:56 UTC)]

> In Scheme, the expression:
>   (cond (EXPR => (lambda (X) ...)))
> provides the lambda expression parameter X with the non-false
> value computed from evaluating EXPR.  [...]
> What do people think of adding this to Emacs Lisp?

If you think you need this, I suspect the need can just as easily be
covered using a macro:

(defmacro acond (&rest clauses)
  "Anaphoric `cond': Like `cond', except the value of the condition is bound
to the variable `it' during the execution of the corresponding body."
  ;; Bug: This macro does no error checking.
  `(let (it)
     (cond ,@(mapcar (lambda (clause) (cons (list 'setq 'it (car clause))
                                            (cdr clause)))

Example usage:

 ((foo) (do-something-with it))
 ((bar) (do-something-else-with it)))

I learned about anaphoric macros from Paul Graham's "On Lisp".
They can be very handy.

- Harald

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