[plt-scheme] Predicates from Types

On Thu, Apr 2, 2009 at 12:52 PM, Sam TH <samth at ccs.neu.edu> wrote:
> On Thu, Apr 2, 2009 at 8:35 AM, Paulo J. Matos <pocmatos at gmail.com> wrote:
>> On Wed, Apr 1, 2009 at 11:43 PM, Sam TH <samth at ccs.neu.edu> wrote:
>>> On Wed, Apr 1, 2009 at 7:01 PM, Paulo J. Matos <pocmatos at gmail.com> wrote:
>>>> Is there then a way to extract a predicate from a type? Given I assume
>>>> the answer is no, what's the best solution for this?
>>>> Right now I had to define :
>>>> (: Predicate? (Any -> Boolean))
>>>> (define (Predicate? u)
>>>>     (or (Quantifier? u) (Logic-Predicate? u) ...))
>>>
>>> This is the right solution, but you probably want the type:
>>>
>>> (: Predicate? (Any -> Boolean : Predicate))
>>>
>>
>> Now you lost me here... Why do I need the ": Predicate" part? and what
>> does it do?
>
> If a function has the type (Any -> Boolean : T), then that function is
> a "predicate" for T (no relation to the "Predicate" in your mail).
> This means that using that function checks if a value of of that type.
>  `number?' is an excellent example here.  For example:
>
> (if (number? x) (add1 x) 7)
>
> always has type `Number'.
>
> Similarly, if you give `Predicate?' the type I described, then you can
> use `Predicate?' to check membership in the type `Predicate' in the
> same way.

I got it, so it basically gives more information for the type
inference mechanism to infer types in certain situations but doesn't
really change the function itself, right?

> --
> sam th
> samth at ccs.neu.edu
>



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