On 24/02/11 12:31 AM, Edward Z. Yang wrote:
Excerpts from Xavier Shay's message of Tue Feb 22 20:25:28 -0500 2011:
On 23/02/11 10:35 AM, Edward Z. Yang wrote:
What you describe is one of the cases when a partial function is ok (though it's probably better to do something like:
giveMeThree x | x == 3 = True | otherwise = error "giveMeThree: not three"
It's frequently not possible, but if you can arrange your types so that the invalid values are not possible, even better.) ah that's good, I can give a helpful error message.
Still would like a way to test it though...
Check the section "Testing for expected errors" herE:
http://leiffrenzel.de/papers/getting-started-with-hunit.html Excellent. I had to update the code to use the new Control.Exception (rather than Control.OldException), and ended up with:
import Control.Exception TestCase $ do handle (\(_ :: ErrorCall) -> return ()) $ do evaluate ( myFunc 3 [False, False, True] ) assertFailure "must raise an error on invalid state" The only issue is that I now require the -XScopedTypeVariables flag, otherwise it fails to compile with: Illegal signature in pattern: ErrorCall Use -XScopedTypeVariables to permit it I am not sure whether this is an acceptable solution or not. I tried reading this: http://www.haskell.org/ghc/docs/latest/html/users_guide/other-type-extension... ... but I don't know enough about haskell for it to make any sense to me yet. Cheers, Xavier