#lang scribble/doc Ha, you can run this email! In Redex, I have on several occasions found myself trying to use a pattern to constrain a sequence's member to match that pattern. i.e., I want to use a pattern bound at depth 0 to constrain a pattern at depth 1. Unfortunately, I can't do this, so it seems I must escape into Racket and enforce this constraint with andmap and equal?. I'm hoping someone can tell me there is a better way.
For example:
@(require
redex/reduction-semantics
racket/function)
@(define-language testL
[x ::= variable-not-otherwise-mentioned])
(define-judgment-form testL
#:mode (complex-match I)
#:contract (complex-match (x (x ...)))
[-----------------
(complex-match (x (x ...)))])
Gives this error:
; ....: define-judgment-form: found the same binder, x, at different depths, 0
and 1
; at: x
; in: (x (x ...))
So, I am forced to write this:
@(define-judgment-form testL
#:mode (complex-match_0 I)
#:contract (complex-match_0 (x (x ...)))
[;; NB: Ugh this should be possible with pattern matching alone ....
(side-condition ,(andmap (curry equal? (term x)) (term (x_0 ...))))
-----------------
(complex-match_0 (x (x_0 ...)))])
@(require rackunit)
@(check-true
(judgment-holds (complex-match_0 (x (x)))))
@(check-true
(judgment-holds (complex-match_0 (foo (foo foo)))))
@(check-false
(judgment-holds (complex-match_0 (x (y z))))))
This is particularly annoying, since I can easily do almost the
*opposite*, i.e., use a depth 0 pattern while constraining a sequence's
members to be distinct.
@(define-judgment-form testL
#:mode (complex-not-match I)
#:contract (complex-not-match (x (x ...)))
[-----------------
(complex-not-match (x_!_0 (x_!_0 ...)))])
@(check-false
(judgment-holds (complex-not-match (x (x)))))
@(check-false
(judgment-holds (complex-not-match (foo (foo foo)))))
@(check-true
(judgment-holds (complex-not-match (x (y z))))))
If there is no better way, would it be too much to ask for a new
pattern that does this? E.g., perhaps (x_=_0 (x_=_0 ...))?
--
William J. Bowman
Northeastern University
College of Computer and Information Science
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