> On Aug 8, 2016, at 6:01 PM, Stephen Chang <[email protected]> wrote:
>
> For this function:
>
> (define (f [x : (U Integer Boolean)])
> (if (integer? x) (add1 x) "1"))
>
> Typed Racket infers the type:
>
> (-> (U Boolean Integer) (U Integer String))
>
> and also recognizes this equivalent case-> type:
>
> (ann f : (case-> (-> Boolean (U Integer String))
> (-> Integer (U Integer String))))
>
> but does not recognize this correct type:
>
> (ann f : (case-> (-> Boolean String)
> (-> Integer Integer)))
> =>
> Type Checker: type mismatch
> expected: (case-> (-> Boolean String) (-> Integer Integer))
> given: (-> (U Boolean Integer) (U Integer String)) in: f
>
> Is there a reason TR cannot compute the last type? Is it a performance issue?
The only reason it couldn't assign that type is because of the type annotation
you put on x. If you take away the (U Integer Boolean) annotation an x this
works:
#lang typed/racket
(: f : (case-> (-> Boolean String)
(-> Integer Integer)))
(define (f x)
(if (integer? x) (add1 x) "1"))
Once you put the (U Integer Boolean) annotation, it can treat f as having any
super type of (-> (U Boolean Integer) (U Integer String)), but it won't infer
anything more specific than what the annotations say.
P.S.
> (case-> (-> Boolean (U Integer String))
> (-> Integer (U Integer String)))
This is a super-type of (-> (U Boolean Integer) (U Integer String)). It's a
super-type because both (-> Boolean (U Integer String)) and (-> Integer (U
Integer String)) are super-types. It's not equivalent under the current
sub-typing rules though, because you can't pass a value of type (U Integer
Boolean) to it.
Alex Knauth
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