[plt-scheme] Custom match expanders for eq?, eqv? and equal?

Could we add eq?, eqv?, and equal? as builtin match bindings, the way
we already interpret list and cons as patterns?  Is there a way for
users to add match interpretations for existing bindings?  Its a weird
use of syntax, but it has its uses... it would be nice to say, for
instance, (make-posn x y) instead of (struct posn (x y)).  Or, for
that matter, (list* a b) instead of (list-rest a b).  Right now, if a
constructor exists without the forethought of also adding it as a
match expander, we're out of luck.

--Carl

On Tue, Aug 19, 2008 at 1:38 PM, Sam TH <samth at ccs.neu.edu> wrote:
> I typically use `==' for this, to mean your `equal?'.  When I need a
> different comparison, I just use `?', since that's pretty rare.
>
> I would avoid changing the meaning of `eq?' etc, just because it
> potentially makes it harder to read your code.
>
> sam th
>
> On Tue, Aug 19, 2008 at 1:00 PM, Dave Gurnell <d.j.gurnell at gmail.com> wrote:
>> Hi PLTers,
>> I've often thought an equivalent of "eq?" or "equal?" to be a useful
>> addition for the match library. "?" does more or less what I want but it's a
>> little verbose:
>>   (define x 'a)
>>   (define y 'b)
>>   (match x
>>     [(? (cut eq? <> y)) "X is the same as Y."])
>> What I'd like to see is something like this:
>>   (match x
>>     [(eq? y) "X is the same as Y."])
>> I had a play this afternoon and wrote the a module called "my-match.ss"
>> (attached) that redefines eq?, eqv? and equal? to do what I've described
>> above:
>>   #lang scheme
>>
>>   (require (file "my-match.ss"))
>>
>>   ; and so on...
>> Now for my question. By supplying my own versions of these identifiers, I
>> have also changed how they are treated in an expression context (although
>> the end result of using them should be the same). Is it a "Bad Thing" (TM)
>> to do this to identifiers that are so fundamental to the core of Scheme?
>> Should I rename these guys, or make sure I only ever require them with a
>> prefix?
>> Many thanks,
>> -- Dave
>> ; my-match.ss ====================================
>> #lang scheme
>> (require srfi/26)
>> ; Match expanders --------------------------------
>> ; (_ expr pattern ...)
>> (define-match-expander match:eq?
>>   (syntax-rules ()
>>     [(_ value pattern ...)
>>      (? (cut eq? <> value) pattern ...)])
>>   eq?)
>> ; (_ expr pattern ...)
>> (define-match-expander match:eqv?
>>   (syntax-rules ()
>>     [(_ value pattern ...)
>>      (? (cut eqv? <> value) pattern ...)])
>>   eqv?)
>> ; (_ expr pattern ...)
>> (define-match-expander match:equal?
>>   (syntax-rules ()
>>     [(_ value pattern ...)
>>      (? (cut equal? <> value) pattern ...)])
>>   equal?)
>> ; Provide statements -----------------------------
>> (provide (rename-out [match:eq?    eq?]
>>                      [match:eqv?   eqv?]
>>                      [match:equal? equal?]))
>>