Aaron, I can't get your example to run. Pasted directly into DrRacket, and
with #lang r6rs at the top:
#lang r6rs
(library (scheme-extended-if)
(export <bindings of (scheme)>)
(import (rename (scheme) if %if))
(define-syntax if
(syntax-rules ()
[(_) (void)]
[(_ test cons rest ...) (%if test cons (if rest ...))])))
=>
import: bad `rename' form in: (rename (scheme) if %if)
I think the (old-name new-name) pairs are supposed to be paired with
parentheses, so replacing the rename form with:
(rename (scheme) (if %if))
and running again, yields:
module: identifier already imported from a different source in:
#%app
(rename (lib "scheme/main.rkt") #%app #%app)
r6rs/private/prelims
This same error message happens even when the program is pared down to:
#lang r6rs
(library (scheme-extended-if)
(export <bindings of (scheme)>)
(import (scheme)))
=>
module: identifier already imported from a different source in:
#%app
(lib "scheme/main.rkt")
r6rs/private/prelims
So I suspect this is either an error in Racket's R6RS or an error in your
suggested code.
--John Boyle
*Science is what we understand well enough to explain to a computer. Art is
everything else we do.* --Knuth
On Thu, Oct 11, 2012 at 6:46 PM, Aaron W. Hsu <[email protected]> wrote:
> On Thu, 11 Oct 2012 21:29:08 -0400, John Boyle
> <[email protected]> wrote:
>
> > While all Schemes I've tested
> > that support define-syntax do allow redefinition of "if"... on Racket, it
> > does not play well with low-level macros (using datum->syntax, or the
> > "mzlib/defmacro" library) due to their phase-separation. Ikarus does do
> > exactly what I want (I can redefine "if" and define low-level macros with
> > it); Chicken... does provide defmacro, in a separately downloadable
> > library
> > I found just now, which does work the way I want; Chibi and scsh/scheme48
> > don't seem to have low-level macros, while Gambit doesn't even have
> > define-syntax.
> >
> > I suppose this committee qua Scheme committee isn't responsible for
> > Common
> > Lisps, or technically even Racket. Restricting my view to the other
> > Schemes, they do seem to universally allow redefinition of built-in
> > operators without breaking everything, unlike the Common Lisps (and emacs
> > lisp). Clearly this is a good feature in this case, though I don't
> > believe
> > the standard guarantees it, and I wonder whether it'll be different for
> > new
> > Schemes or whether existing Schemes will change. Meanwhile, I'll
> > continue
> > using Racket as a compilation target and runtime.
>
> This is very surprising to me. I do not think I understand what your exact
> troubles are. I take it as a given that you would implement your new IF
> form as a hygienically safe macro, whether that is achieved by using
> something like SYNTAX-CASE or by using SYNTAX-RULES. Assuming that the
> macro is hygienic, any R6RS Scheme supports the use of the new IF macro in
> whatever other macro you want to use it in, and also in normal code. In
> other words, there should be literally no way to distinguish the new IF
> from the old one. For example, assuming that you have a library (scheme)
> that holds all of your normal Scheme environment, it is trivial to replace
> (scheme) with (scheme-extended-if); all other code should simply work, and
> your new IF form should work exactly as if it were already built that way.
>
> (library (scheme-extended-if)
> (export <bindings of (scheme)>)
> (import (rename (scheme) if %if))
> (define-syntax if
> (syntax-rules ()
> [(_) (void)]
> [(_ test cons rest ...) (%if test cons (if rest ...))])))
>
> After that, things just work, you cannot tell the difference between the
> built-in and the current in practice.
>
> --
> Aaron W. Hsu | [email protected] | http://www.sacrideo.us
> Programming is just another word for the Lost Art of Thinking.
>
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>
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