racket / racket/redex

Running some ellipsis patterns in define-judgment-form causes syntax error.

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Racket
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Description

There is a small issue with the parsing of ellipsis patterns in define-judgment-form. The following file gets the error in racket 6.2. There are some observations about the pattern in the use and test cases.

#lang racket

(require redex)

(define-language L
  (t ::= () (t ... -> t)))

(define-judgment-form L
  #:mode (≈ I I)
  #:contract (≈ t t)
  [-------------------- "≈Refl"
   (≈ () ())]
  [(≈ t_1 t_2)
   ;; I have the intuition that this should be the same as
   ;; (≈* (τ_1 ...) (γ_1 ...))
   (≈ t_n t_m) ... 
   ------------------------------------------ "≈->"
   (≈ (t_n ... -> t_1) (t_m ... -> t_2))
   ;; The following conclusion gets rid of this error.
   #;(≈ (t_n ..._n -> t_1) (t_m ..._n -> t_2))])

(define-judgment-form L
  #:mode (≈* I I)
  #:contract (≈* (t ...) (t ...))
  [--------- "≈*base"
   (≈* () ())]
  [(≈ t_1 t_2) (≈* (t_n ...) (t_m ...))
   ---------------------- "≈->"
   (≈* (t_1 t_n ...) (t_2 t_m ...))])

(module+ test
  (test-true  (judgment-holds (≈ () ())))
  (test-true  (judgment-holds (≈ (-> ()) (-> ()))))
  (test-true  (judgment-holds (≈ (() -> ()) (() -> ()))))
  (test-true  (judgment-holds (≈ (() () -> ()) (() () -> ()))))
  (test-false (judgment-holds (≈ () (-> ()))))
  ;; This test causes this pattern mentioned above to report
  ;; the following error.
  ;;syntax: incompatible ellipsis match counts for template in: ...
  (test-false (judgment-holds (≈ (-> ()) (() -> ()))))
  )

(module+ test (test-results))

;; Helpers to consider abstracting

(define-syntax-rule (test-true e)
  (test-equal e #t))

(define-syntax-rule (test-false e)
  (test-equal e #f))

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First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start by running the embedded #lang racket reproduction and its module+ test cases, focusing on define-judgment-form ellipsis handling. The work is done when the final test-false case no longer raises the incompatible ellipsis match-count syntax error and the existing judgment tests still pass.

Written by the indexing model from the issue text.

Assessment

Domain
compilers
Issue type
Bug
Difficulty
3/5
Estimated time
1-2 days
Activity status
Stale
Clarity
Clearly specified
Newbie friendliness
35/100

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