Question

I’ve got a function:

unify :: [Constraint] -> [Substitution]

and in certain cases it throws exceptions with the error function:

error "Circular constraint"

I’m using Test.HUnit for unit testing and I’d like to make a test case that asserts these errors are thrown on certain inputs. I found this, which provides a way of testing for exceptions that are instances of Eq, but error seems to give an ErrorCall exception, which is not an instance of Eq, so I get the error:

No instance for (Eq ErrorCall)
  arising from a use of `assertException'

How can I write a TestCase that asserts that error was called and (preferably) checks the message?

Answer 1

Ideally I’d refactor your function into

unify' :: [Constraint] -> Maybe [Substitution]
unify' = -- your original function, but return Nothing instead of calling error,
         -- and return Just x when your original function would return x

unify = fromMaybe (error "Circular constraint") . unify'

I would then test unify' instead of testing unify.

If there was more than one possible error message, I would refactor it like this instead:

unify' :: [Constraint] -> Either String [Substitution]
    -- and return Left foo instead of calling error foo

unify = either error id . unify'

(Incidentally, if this is for a library other programmers will be using, some of them would prefer to call unify' instead of the partial function unify.)

---

If you can’t refactor your code, I’d modify the code you link to, replacing assertException with:

assertErrorCall :: String -> IO a -> IO ()
assertErrorCall desiredErrorMessage action
    = handleJust isWanted (const $ return ()) $ do
        action
        assertFailure $ "Expected exception: " ++ desiredErrorMessage
  where isWanted (ErrorCall actualErrorMessage)
            = guard $ actualErrorMessage == desiredErrorMessage