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From: Per Bothner <>
To: Sascha Ziemann <>,
	kawa mailing list <>
Subject: Re: syntax-rules problem
Date: Fri, 10 Mar 2017 19:04:00 -0000	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

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 ...)
         ((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

  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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