Re: 'let' for functions ?

[Oliver Scholz]

Ed L Cashin <ecashin@uga.edu> writes:
[...]
> As an exercise I tried in vain to create a simple implementation of
> labels that wouldn't require loading cl.  I couldn't do it, though,
> and had to get on with what I was supposed to be doing.  ;)

You need not fear the cl package. If you put 
`(eval-when-compile (require 'cl)' into your code, the compiled
package won't require cl.

However, since this is, as you say, a nice exercise, I give it a
shot. (Although I am supposed to be doing something different, too
...)

> I'd be very interested, though, to see such a simple implementation of
> labels if there's an elisp guru who would enjoy trying to find it.
> The hard part for me was recursion support.

I am not a Lisp-Guru, but here is my version anyways. I'd like to hear
whether there is any problem with this approach:

(defmacro my-simple-labels (spec &rest body)
  ;; We `fset' the symbols temporarily and restore the original state
  ;; after the body form was executed.
  (let ((symbol (make-symbol "symbol"))
        (definition (make-symbol "definition"))
        (sfunc (make-symbol "sfunc")))
    `(progn
       (let (,symbol)
         (dolist (,definition ',spec)
           (setq ,symbol (car ,definition))
           ;; Store function definition, if necessary.
           (when (fboundp ,symbol)
             (put ,symbol 'my-simple-labels (symbol-function ,symbol)))
           ;; New function definition.
           (fset ,symbol (cons 'lambda (cdr ,definition)))))
       (unwind-protect (progn ,@body)
         ;; Clean-up
         (let (,sfunc)
           (dolist (,symbol ',(mapcar 'car spec))
             (setq ,sfunc (get ,symbol 'my-simple-labels))
             (if ,sfunc
                 (fset ,symbol ,sfunc)
               (fmakunbound ,symbol))))))))


    Oliver
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