# [racket] Specifying the type of a function that contains a type test (but isn't one)

On Mon, Apr 18, 2011 at 3:02 PM, Jeremiah Willcock <jewillco at osl.iu.edu> wrote:
>* Given the following Typed Racket program:
*>*
*>* #lang typed/racket
*>*
*>* (: greater-than-5? (Any -> Boolean))
*>* (define (greater-than-5? x)
*>* (and (integer? x) (> x 5)))
*
First, you probably want to use `exact-integer?' here, see the TR docs
for `Integer'.
>* (: equal-to-7? (Any -> Boolean))
*>* (define (equal-to-7? x)
*>* (and (greater-than-5? x) (= x 7)))
*>*
*>* I receive the error (highlighting the x in (= x 7)):
*>*
*>* Type Checker: Expected Complex, but got Any in: x
*>*
*>* Is there a way to tag greater-than-5? to mark that it only returns #t on
*>* integers? Marking its type as (Any -> Boolean : Integer) causes an error:
*>*
*>* Type Checker: Expected result with filter ((Integer @ x) | (! Integer @ x)),
*>* got filter (((U Float Integer) @ x) | (OrFilter (! Integer @ x) ((U Float
*>* Integer) @ x))) in: (and (integer? x) (> x 5))
*
Second, there isn't currently a way to write this, because I haven't
found a notation I like. It's something I want to do fairly soon,
though.
--
sam th
samth at ccs.neu.edu