From: Per Bothner <per@bothner.com>
To: Sascha Ziemann <ceving@gmail.com>,
kawa mailing list <kawa@sourceware.org>
Subject: Re: syntax-rules problem
Date: Fri, 10 Mar 2017 19:04:00 -0000 [thread overview]
Message-ID: <d229c021-f1d4-ac99-6625-eaa4b793e723@bothner.com> (raw)
In-Reply-To: <CAGUt3y5kG3x462cDKCH4VueT8HLZmcKTvyTZ3thWL-9dFpCvSQ@mail.gmail.com>
On 03/10/2017 01:35 AM, Sascha Ziemann wrote:
> I have the following macro.
>
> (define-syntax define-facts
> (syntax-rules ()
> ((_ (name a0 a1 ...) ((v00 v01 ...) (v10 v11 ...) ...))
> '(define (name a0 a1 ...)
> (conde
> ((== a0 v00) (== a1 v01) ...)
> ((== a0 v10) (== a1 v11) ...)
> ...)))))
This got mangled a bit, but definition is clearly wrong.
I cleaned it up:
(define-syntax define-facts
(syntax-rules ()
((_ (name arg ...) ((value key ...) ...))
(define (name arg ...)
(cond
((and (equal? arg key) ...) value)
...)))))
However, this is still not valid R7RS Scheme:
Pattern variables that occur in subpatterns followed by one or more
instances of the identifier h<ellipsis> are allowed only in subtemplates
that are followed by as many instances of <ellipsis>.
The problem is that arg is singly-nested in the pattern, but
doubly-nested in the template.
Kawa does support an extension for different nesting levels, meant to support
things like (x (y ...) ...) => (((x y) ...) ...). However, in that case the x
varies by the outer "index":
((x0 (y00 y01)) (x1 (y10 y11 y12)))
==> (((x0 y00) (x0 y01)) ((x1 y10) (x1 y11) (x1 y12)))
But for define-facts the single-nested arg needs to vary with the
*inner* index. I.e. you would need Kawa to expand the example to:
==> (((x0 y00) (x1 y01)) ((x0 y10) (x1 y11) (x2 y12))) ; Oops no x2
But Kawa doesn't do that.
A solution that does work uses a list:
(define-syntax define-facts
(syntax-rules ()
((_ name ((value . keys) ...))
(define (name . args)
(cond ((equal? args 'keys) value) ...)))))
(define-facts father-of
(("Abraham" "Ismael")
("Abraham" "Isaac")
("Isaac" "Jacob")
("Jacob" "Benjamin")))
Obviously, Kawa should not throw ArrayIndexOutOfBoundsException,
but should instead report an error. So that is a Kawa bug.
--
--Per Bothner
per@bothner.com http://per.bothner.com/
next prev parent reply other threads:[~2017-03-10 19:04 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-03-10 9:35 Sascha Ziemann
2017-03-10 19:04 ` Per Bothner [this message]
2017-03-10 20:11 ` Per Bothner
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