[plt-scheme] Predicates from Types

On Thu, Apr 2, 2009 at 3:59 PM, Robby Findler
<robby at eecs.northwestern.edu> wrote:
> What if X and p don't actually match?
>

Hell breaks loose?

> 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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>>
>



-- 
Paulo Jorge Matos - pocmatos at gmail.com
Webpage: http://www.personal.soton.ac.uk/pocm