[racket] filters in typed racket

I have been working on fixing the feature that does the checking if a
certain type meets the expectations, and there are tons of bugs in it
for the edge cases. I'm guessing you are just triggering one of those
right now, so I would file a bug.

In terms of docs, they were just added and so are visible on the pre
release page:
http://www.cs.utah.edu/plt/snapshots/current/doc/ts-reference/type-ref.html#%28part._.Other_.Type_.Constructors%29

On Sat, May 10, 2014 at 8:30 AM, Alexander D. Knauth
<alexander at knauth.org> wrote:
> Is there a way of specifying filters in typed racket so that if it returns
> true, it knows it’s a certain type, but if it returns false, it doesn’t mean
> that it’s not that type (or the other way around).
>
> For example:
> (: string-with-length-1? (Any -> Boolean : #:if-true String #:if-false (U
> String Any)))
> (define (string-with-length-1? x)
>   (if (string? x)
>       (one? (string-length x))
>       #f))
>
> I know there’s something like this somewhere.
>
> When I tried it, it gave me an error message (I expected that), but that
> error message said something about #:+ and #:-
>
> After trying some stuff and guessing stuff from error messages, and just to
> try it and see what happened, I tried this:
> (: string-with-length-1? (Any -> Boolean : #:+ (String @ x) #:- ((U String
> Any) @ x)))
> (define (string-with-length-1? x)
>   (if (string? x)
>       (one? (string-length x))
>       #f))
> And it gave me this weird error message:
> . Type Checker: type mismatch;
>  mismatch in filter
>   expected: ((String @ x) | Top)
>   given: ((String @ x) | Top) in: (if (string? x) (one? (string-length x))
> #f)
>
> What?  If it’s expecting ((String @ x) | Top), and I’m giving it ((String @
> x) | Top),  then what’s the problem?
>
> And is there any documentation anywhere about this?
>
>
>
> ____________________
>   Racket Users list:
>   http://lists.racket-lang.org/users
>