[racket] Typed Racket: Defining a sequence predicate

From: Matthias Benkard (mailing-lists at mail.matthias.benkard.de)
Date: Thu Aug 11 09:32:39 EDT 2011


I recently discovered Typed Racket and am having a lot of fun with it.
 Occurrence typing is pretty amazing.

I've stumbled across a couple of problems when trying to use
dictionaries and sequences, though.  In particular, I'm currently
having trouble defining a predicate for the (Sequenceof Any) type.  I
tried importing sequence? with the type that I think it should have:

    (require/typed racket
      [sequence? (Any -> Boolean : (Sequenceof Any))])

This made Racket complain about not being able to create a contract
out of the type specification:

    Type Checker: Type (Any -> Boolean : (Sequenceof Any)) could not
be converted to a contract. in: (Any -> Boolean : (Sequenceof Any))

In addition, define-predicate seems to do something weird (type-incorrect even):

    > (define-predicate anyseq? (Sequenceof Any))
    > anyseq?
    - : (Any -> Boolean : (Sequenceof Any))
    > (anyseq? 100)
    procedure application: expected procedure, given:
#<make-contract>; arguments were: 100

Is this a bug, or am I misusing define-predicate?

I'm using Racket 5.1.2 on Mac OS X.



Posted on the users mailing list.