I was somewhat surprised to see today that I can’t use a predicate with both
positive and negative propositions in the way I would expect with partition:
> (:print-type partition)
(All (a b)
(case->
(-> (-> b Any : #:+ a) (Listof b) (values (Listof a) (Listof b)))
(-> (-> a Any) (Listof a) (values (Listof a) (Listof a)))))
Specifically, I would have expected the type to be something like this:
(All (a b c)
(case->
(-> (-> b Any : #:+ a) (Listof b) (values (Listof a) (Listof b)))
;; the second list must consist of 'c's:
(-> (-> b Any : #:+ a #:- c) (Listof b) (values (Listof a) (Listof c)))
(-> (-> a Any) (Listof a) (values (Listof a) (Listof a)))))
… so that if, say, I had a list of Elephants and Emus, that I could use
elephant? to split it into two lists: one of type (Listof Elephant) and one of
type (Listof Emu).
I tried to roll my own, and got pretty close:
(: my-partition
(All (a b c)
(case->
;; the second list must consist of 'c's:
(-> (-> b Any : #:+ a #:- c) (Listof b) (values (Listof a) (Listof c)))
)))
(define (my-partition my-pred elts)
(cond [(empty? elts)
(values '() '())]
[else
(define-values (stacks non-stacks)
(my-partition my-pred (rest elts)))
(define f (first elts))
(cond [(my-pred f)
(values (cons f stacks)
non-stacks)]
[else
(values stacks (cons f non-stacks))])]))
That is, I can do it for the case-> clause that I care about. Putting the other
two back in there causes it to fail type-checking. Is that the problem, that TR
can’t accommodate both flavors in the same type?
John
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