No, I don't think you should use Either/Maybe to anticipate bugs. If your code could _legitimately_ fail (it's just something that follows from your domain) then Maybe/Either are appropriate. Otherwise, some sort of exception or "error" is more appropriate. E.g., if your domain knowledge tells you that zeroes are impossible then you would not want every division to return a Maybe just because division by zero is not defined. _Because that should never happen._ On Tue, Jun 11, 2019 at 12:44 AM Michele Alzetta wrote:

As beginner to beginner, so take this with a kg of salt ... isn't what you're looking for a construction of the Either sort?

That way you get Right and Left and you can check for Left.

Il giorno mar 11 giu 2019 alle ore 05:45 Hilco Wijbenga ha scritto:

...

Hi all,

I have a function that may call "error" if the input represents a bug in my program. (Think something like PositiveInt -1; this should never happen so Maybe or Either are not applicable.)

f :: Whatever f = error "This should never happen."

spec :: Spec spec = describe "f" $ it "error" $ return f `shouldThrow` anyErrorCall

The above does not work, I get "did not get expected exception: ErrorCall". Using "anyException" doesn't work either.

How can I write a test for this? And (assuming I can get the test to work), how do I check that the error message is indeed what I expect?

Cheers, Hilco _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

_______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

Yes, "evaluate" does the trick. Thank you. On Tue, Jun 11, 2019 at 2:39 AM Francesco Ariis wrote:

Hello Hilco,

On Mon, Jun 10, 2019 at 08:44:30PM -0700, Hilco Wijbenga wrote:

...

f :: Whatever f = error "This should never happen."

spec :: Spec spec = describe "f" $ it "error" $ return f `shouldThrow` anyErrorCall

import qualified Control.Exception as E

and

E.evaluate tt' f `shouldThrow` anyErrorCall

should work. Does this work? -F _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners