You can do it if you have `flatten` also accept a predicate for A; i.e.
an (-> Any Boolean : A). Be aware that such functions can't be exported
to untyped Racket, though.
Neil ⊥
On 02/03/2015 08:31 PM, J. Ian Johnson wrote:
What if A is (Listof Integer)?
Generally a type variable naked in a union is just not going to do what you
want. There's no way to keep it disjoint from the other types in the union.
-Ian
----- Original Message -----
From: "Matthew Butterick" <m...@mbtype.com>
To: "users" <users@racket-lang.org>
Sent: Tuesday, February 3, 2015 7:29:47 PM
Subject: [racket] Typed Racket: can this recursive typing become polymorphic?
I'm trying to create a polymorphic type for `flatten` that reflects its usual
behavior.
This monomorphic version works:
#lang typed/racket/base
(require/typed rackunit [check-equal? (Any Any . -> . Any)])
(require/typed racket/list [flatten (All (A) (Rec as (U Integer (Listof as)))
-> (Listof Integer))])
(define flatten-integers (inst flatten Integer))
(check-equal? (flatten-integers 1) '(1))
(check-equal? (flatten-integers '(1)) '(1))
(check-equal? (flatten-integers '(1 (2 3) 4)) '(1 2 3 4))
(check-equal? (flatten-integers '(1 (2 3 (4 5)) 6)) '(1 2 3 4 5 6))
But when I try to substitute the polymorphic type `A` for `Integer`, it stops
working:
#lang typed/racket/base
(require/typed rackunit [check-equal? (Any Any . -> . Any)])
(require/typed racket/list [flatten (All (A) (Rec as (U A (Listof as))) ->
(Listof A))])
(define flatten-integers (inst flatten Integer))
(check-equal? (flatten-integers 1) '(1))
(check-equal? (flatten-integers '(1)) '(1))
(check-equal? (flatten-integers '(1 (2 3) 4)) '(1 2 3 4))
(check-equal? (flatten-integers '(1 (2 3 (4 5)) 6)) '(1 2 3 4 5 6))
git/racket/racket/collects/racket/contract/private/blame.rkt:143:0: flatten:
contract violation; none of the branches of the or/c matched
Is it possible? If so, what am I missing?
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