Re: srfe records in reworked match

[Ludovic Courtès]

Hello Stefan,

stefan <address@hidden> writes:

> I have started to code in some record recognition into the match construct

Excellent!  :-)

Are you hacking Wright’s match as currently in Guile or Alex Shinn’s
rewrite from <http://synthcode.com/scheme/>?

> I need first to make sure that I grok the intention of the syntax!

Do the comments in (ice-9 match) and the examples in the paper that in
match.tar.gz at
<http://www.cs.indiana.edu/scheme-repository/code.match.html> help?

You could ask on comp.lang.scheme too.  :-)

> Now, one can do ...
>
> * (define rtf      (make-record-type "n" '(x y z)))
> * (define make-n   (record-constructor rtf))
> * (define v        (make-n 1 2 3))
> * (define g        (record-accessor rtf 'x))
> * (define s        (record-modifier rtf 'x))

FWIW I’d really prefer if it could work with SRFI-9 (which is purely
syntactic, so there’s no run-time record type descriptor) rather than
with Guile’s records (as above).

> * (match v ((= g 1) 'yes))    ;; This is the old behavior
> yes
>
> ;; = allow for a getter and setter argument so that we can do ...
> * (match v ((= (g s) (and (set! x.set) 
> *                       (get! x.get) 
> *                       1)) 
> *         (begin (x.set 2) 
> *                (x.get))))
> 2
>
> ;;Now the $ syntax work, although a lot of unpacking of accessors and 
> modifiers
> ;;are done dynamically and not at compile time.
> * (match v (($ rtf x 2 3) x))
> 2

I think it should be:

  (match v (($ n x 2 3) x))

The original API assumes that when ‘n’ appears as the record-type above,
then there exists a type predicate called ‘n?’, a procedure called ‘n-x’
to access the ‘x’ field, etc.

> * (match v (  ($ rtf 
> *              x 
> *              (and (set! y.set) 
> *                   (get! y.get)) 
> *              3) 
> *         (begin (y.set 4) 
> *                (+ x (y.get)))))
> 6
>
> It's recursive.

OK.

So would the following work?

  (define x (make-n 1 2 (make-n (3 4 (make-n 5 6 7)))))
  (match x
    (($ n x y ($ n p q ($ n a b c)))
     (list a b c p q x y)))

As noted in Shinn’s match-cond-expand.scm, this record matching form is
not ideal:

  ;; Annoying unhygienic record matching.  Record patterns look like
  ;;   ($ record fields...)
  ;; where the record name simply assumes that the same name suffixed
  ;; with a "?" is the correct predicate.

Thanks!

Ludo’.