# [plt-scheme] Predicates from Types

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