Re: [Scheme-reports] Formal Response #382: Allow "if" to accept arbitrarily many if-then pairs

[John Boyle <johnthescavenger@x>]

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 <arcfide@x> wrote:

On Thu, 11 Oct 2012 21:29:08 -0400, John Boyle
<johnthescavenger@x> 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 | arcfide@x | http://www.sacrideo.us
Programming is just another word for the Lost Art of Thinking.

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