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Re: Placement of defines

From: Sebastian Tennant
Subject: Re: Placement of defines
Date: Thu, 18 Dec 2008 18:47:05 +0000
User-agent: Gnus/5.110011 (No Gnus v0.11) Emacs/22.2 (gnu/linux)

Quoth "Kjetil S. Matheussen" <address@hidden>:
> Yes, this limitation in Scheme is a horrible pest. There's no
> decent reason why you shouldn't be allowed to use define other
> places than right after a lambda, except the long discussion
> involved how it should behave in various situations. But even
> inconsistant behaviour between implementations would be better
> than the current situation where you either have to rewrite
> perfectly fine expressions or use various types of lets or
> lambdas in more or less ridiculous ways:
> - ((lambda (x) (set! x "baz\n") (define bar x) (display bar)) "foo\n")
> + ((lambda (x) (set! x "baz\n") (let () (define bar x) (display bar) "foo\n"))

Your 'let' workaround gave me an idea.

I use this macro to bind lots of variables dynamically within my
scripts by wrapping calls to it in a map procedure:

 (define-macro (definer var val)                  ;var must be a symbol
   `(module-define! (current-module) ,var ,val))

and it happens to work fine as a workaround for the restriction on
define placements in lambdas and lets :)

 guile> ((lambda (x) (set! x "baz\n") (definer 'bar x) (display bar)) "foo\n")

It (the macro) won't work properly if you need/want to compile your
scheme code though.

If that's the case you should use this macro instead:

 (define-macro (definer var val)                  ;var must be a string
   `(define ,(string->symbol var) ,val))

I prefer the former macro definition because I don't need to compile my
code and

 (definer "foo" "bar")

seems somehow uglier and less intuitive than:

 (definer 'foo "bar")

to me.

Incidentally, perhaps you can tell me why the first argument to this
compile-friendly macro has to be a string for it to work.

 guile> (define-macro (definer var val)
          `(define ,var ,val))
 guile> (definer 'foo "bar")

appears to have worked, but...

 guile> foo
 ERROR: Unbound variable: foo
 ABORT: (unbound-variable)

it hasn't.


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