Re: [r6rs-discuss] [Scheme-reports] Scheme pattern matching: the case for (case)

[Peter Kourzanov <peter.kourzanov@x>]

On Tue, 2010-12-21 at 12:16 -0500, Jim Wise wrote:
> a.) existing match libraries do not have an analogue of case

I have seen no single library that didn't provide a construct
similar to (case) - its only called typically (match) and typically
allowing only one top-level alternative (all do support more verbose
or-patterns). There is one matcher that evan calls it explicitly
(match-case). They all expect a list of clauses where the (car) is the
pattern, the (car (reverse)) is the body and possibly a guard in 
between.

Extending this to cover (case), i.e., a list of possible patterns
rather than a single pattern is a very Schemey thing to do, in
my opinion.

> 
> b.) RNRS case's syntax does not lend itself to extending to the pattern
>     matching case in an upwardly compatible manner.  In particular, case
>     compares values with eqv? and performs no binding, while lambda and
>     the let forms are all binding forms (and compare with equal? in the
>     pattern-matching versions).

eqv? is immaterial here:

(let ([eqv? equal?]) (case "asd" (("asd") #t)))

About binding I think you're right, however the (else => exp) form is
sort-of breaking this already. How serious is this limitation that all
binding must be done by lambda, let, let*, letrec, do, case-lambda, 
let-values and define (and labels, and innumerable other extensions)?

> 
>     This means that making case destructure its arguments would change
>     the meaning of existing valid code, at least for non-list,
>     non-vector patterns:
> 
>                (case 'a
>                  ((a) (list a))
>                  (else #f))
> 
>                ==> (3)

Not necessarily so. One can consider a (case) pattern to be implicitly 
quasiquoted (just like traditional case is implicitly quoted). So, if
one needs "extended" (case) capabilities, one would write:

(case 'b
  ((,a) (list a)))

Otherwise the following would retain its meaning:

(case 'a
  ((a) (list a)))


> 
> Neither of these is necessarily compelling, but I think both argue that
> adding pattern matching support to case is more intrusive than adding it
> only to the environment-constructing forms.  On the other hand, the
> (match ...) form of (rnrs match) can be used in a manner similar to
> (case ...), but with no compatibility concerns for existing code.
> 
> Thoughts?

>From my perspective, (case) is a degenerate case of (match) and should 
be merged - we don't need two switch constructs in a simple and small 
language.


Regards,
Pjotr


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