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?
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