[plt-scheme] Predicates from Types

What if X and p don't actually match?

On Thu, Apr 2, 2009 at 9:39 AM, Paulo J. Matos <pocmatos at gmail.com> wrote:
> On Thu, Apr 2, 2009 at 2:18 PM, Carl Eastlund <carl.eastlund at gmail.com> wrote:
>> On Thu, Apr 2, 2009 at 9:42 AM, Paulo J. Matos <pocmatos at gmail.com> wrote:
>>> On Thu, Apr 2, 2009 at 1:11 PM, Sam TH <samth at ccs.neu.edu> wrote:
>>>>
>>>> This isn't related to type inference, so that's not correct.  But it
>>>> doesn't change the behavior of the function (no type annotation ever
>>>> changes the behavior of a Typed Scheme function).
>>>
>>> So, I guess I didn't really get it. What is the annotation that a
>>> function is a predicate for a type useful for?
>>
>> Let's say you have some type X, and some predicate p of type (Any ->
>> Boolean : X).  Then you can write:
>>
>> (: f ((U X Y) -> X))
>> (define (f x-or-y)
>>  (if (p x-or-y) x-or-y (error 'f "I don't have an X")))
>>
>> Notice that x-or-y initially has the type (U X Y), but by the time f
>> returns it in the first branch of the if it has type X.  That's
>> because (p x-or-y) returns true, and the ": X" annotation tells Typed
>> Scheme that when p returns true, its input is of type X.  So that ":
>> X" is useful when you have unions involving X and conditionals that
>> distinguish them.
>>
>
> Ahhhhhh, beautiful!!!
>
> Thanks for the explanation. In fact, going also back to what Sam said
> before about the number?, now I got it!
>
>> --
>> Carl Eastlund
>>
>
>
>
> --
> Paulo Jorge Matos - pocmatos at gmail.com
> Webpage: http://www.personal.soton.ac.uk/pocm
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