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Re: Testing availabilty of a procedure

Roland Orre <> writes:

> I think it is time to consider a redefinition of "defined?" to
> make it a special form.
> *  defined? is still not standardized within the scheme standard
>    (thanks Bertrand!) which is a pity as it is an essential procedure
>    for everyone who tries to write code which is compatible between
>    different interpreters and versions of interpreters.
> *  In the current guile definition defined? is a procedure which
>    evaluates its arguments. This is rather counterintuitive when
>    both "define" and "undefine" are special forms. Ok "undefine" is
>    not yet standardized either, but should be considered being
>    standardized in the scheme standard as well, as non essential.
>    Long running applications and shells etc need to be able to clean up.
> *  For my own modules I have defined defined? to be a special form
>    to make it compatible with scm and when considering the
>    following answer on a request from 
>    Ingo Hohmann <>
>    Lalo Martins <> wrote:
>    > (if (and (defined? a) (procedure? a))
>    >   #t
>    >   (define (a) #t)
>    > )
>   It seems as there are more than I who prefer/expect defined? to be
>   a special form...

Or it could've just been a typo ;). Reasons I can see to keep it as a

1) All r5rs predicates are defined as procedures. 

2) It's closest standardized analogue is boundp in cl, which is
   defined in the same way.

3) Probably the most important, why add a special form when a
   procedure can serve just as well? It's pretty hard to forget after
   you screw it up once (and that probably wouldn't even be a problem
   with easily accessable documentation). In any case, having it
   as a special form makes other things a pita, like:

(define funs-to-check '(this is a test))
(map (lambda (x) (if (not (defined? x)) (error "Required function not