John Boyle scripsit:

> I hope so.  All Common Lisp implementations I've tested flat-out disallow
> "redefinition of the special form "if"".

You need to shadow the cl:if symbol, and then you can redefine it with
defmacro.

> 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.

Chicken and scsh/scheme48 support explicit-renaming macros, which subsume
defmacro macros; Chibi supports syntactic-closure macros, which subsume
both of the other two.  In Gambit, the option "-:s" provides syntax-case
support.

> 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.

Standards >= R5RS do guarantee it.

--
Go, and never darken my towels again!           John Cowan
        --Rufus T. Firefly                      http://ccil.org/~cowan

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