Predicate Inconsistency in with List, Listof in Polymorphic Functions

Questions & Answers
,
1

Hello,

If I define a function taking a heterogeneous list (List A ... A) and try to coerce the argument to a pair with (assert lst pair?) or an unless guard, the program typechecks, but the assertion fails even when I pass a non-empty list. However, print debugging shows that both (pair? lst) and (list? lst) return #t. Weirder still, if I define a different function that takes a homogeneous list (Listof A) with a body identical to the first function, there is no discrepancy; this second function returns just fine.

Is this an issue of my understanding of List in TR, or is this a TR bug (8.14 [cs])?

Example code:

#lang typed/racket/base

(: ttest-homo (All (A) (-> (Listof A) (Listof A))))
(define (ttest-homo lst)
  (display (format "pair? ~a; list? ~a~%" (pair? lst) (list? lst)))
  (unless (pair? lst)  ; or (assert inp pair?)
    (error 'wat))
  lst)

(: ttest-hetero (All (A  ...) (-> (List A ... A) (List A ... A))))
(define (ttest-hetero lst)
  (display (format "pair? ~a; list? ~a~%" (pair? lst) (list? lst)))
  (unless (pair? lst)
    (error 'wat))
  lst)
(define inp (ann '(1 2 3) (List Natural Natural Natural)))

(ttest-hetero inp)
;pair? #t; list? #t
;error: wat

(ttest-homo inp)
;pair? #t; list? #t
;'(1 2 3)

Background:
I am trying to grok polymorphic functions in TR and have been experimenting by trying to implement a simple function that copies a heterogeneous list. I ran into the above issue when trying to appease the TC. Incidentally, if anyone knows a way to implement the following function so that it typechecks with the given signature, please let me know.

(: list-copy (All (A ...) (-> (List A ... A) (List A ... A))))
(define (list-copy lst)
  (if (null? lst)
      lst
      (cons (car lst) (list-copy (cdr lst)))))

Thank you

edited to correct output

2

Stepping through the macros that TR uses here, we get to this strange expansion:

 (define-values:102 (ttest-hetero)
     (lambda:103 (lst)
       (#%app:105 display (#%app:106 format (quote "pair? ~a; list? ~a~%") (#%app:107 pair? lst) (#%app:108 list? lst)))
       (if (begin (#%app:109 pair? lst) (quote #f)) (let-values:110 () lst) (let-values:111 () lst))))

The (begin … (quote #f)) suggests something bad is happening here.

3

Well, that answers that part of the question. I guess I'll start working on an Issue on Github.

4

#1396. I looked through the issues and it looks similar to #412.