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

[Carl Eastlund]

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?]))
>>