Greetings, all.
I'm terribly puzzled by a typed-racket error message I'm getting (I seem to be
rather abusing this list today -- apologies -- but I'm busy reminding myself
why it's useful not to do two interesting develoments with a project at one
time).
If I run the following in DrRacket:
#lang typed/racket
(: my-func (String String -> String))
(define (my-func s1 s2)
(string-append s1 ":" s2))
(my-func "Hello" "World")
(: call-func ((List String String) -> String))
(define (call-func string-list)
((inst apply String String) my-func string-list)
;(apply my-func string-list)
)
(call-func '("Hello" "There"))
...I get an error message
> Type Checker: Cannot instantiate non-polymorphic type ((String * -> String)
> (Listof String) -> String) in: apply
Dr Racket also tells me that:
> apply
- : (All (a b) ((a * -> b) (Listof a) -> b))
Which makes sense, in that it corresponds to the message I'm seeing.
Now, I don't _really_ follow the logic of Typed Racket in a fully formal way (I
know, I should have another go at The Little MLer), but the only way I can
really make sense of this message is if the type (String String -> String) is
not a subtype of (String * -> String), or if (List String String) is not a
subtype of (Listof String). Both of these would be a little unexpected.
Is that the right interpretation (in which case, where could I find the
hierarchy of types?); or is APPLY being more particular about its types than is
appropriate?
Thanks for any pointers.
Best wishes,
Norman
--
Norman Gray : http://nxg.me.uk
School of Physics and Astronomy, University of Glasgow, UK
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