[Haskell-beginners] HUnit - testing for failed pattern match

[Daniel Fischer]

On Saturday 26 February 2011 02:08:48, Xavier Shay wrote:
>
> 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.

ScopedTypeVariables is harmless, so it is accetable. But if you prefer, you 
can avoid it with a top-level handler

ignoreErrorCalls :: ErrorCall -> IO ()
ignoreErrorCalls _ = return ()

TestCase $
    ignoreErrorCalls $ do
        evaluate ( myFunc 3 [False, False, True] )
        assertFailure "must raise an error on invalid state"

or an explicit pattern in the handler,

TestCase $
    handle (\(ErrorCall _) -> return ()) $ do
        evaluate ...

I think (code untested).

> I tried reading this:
> http://www.haskell.org/ghc/docs/latest/html/users_guide/other-type-exten
>sions.html#pattern-type-sigs
>
> ... but I don't know enough about haskell for it to make any sense to me
> yet.
>
> Cheers,
> Xavier