Haskell HUnit Function Testing

Question

I'm trying to set up a series of tests for a simple tic tac toe program I've written in Haskell and I'm unable to get past my first test due to a strange error being thrown reading:

Tests.hs:11:61: error:
    * Couldn't match expected type `Int' with actual type `IO Int'
    * In the third argument of `assertEqual', namely
        `(test_user_value x)'
      In the first argument of `TestCase', namely
        `(assertEqual "for (test_user_value 3)," f (test_user_value x))'
      In the expression:
        TestCase
          (assertEqual "for (test_user_value 3)," f (test_user_value x))
   |
11 | test1 = TestCase (assertEqual "for (test_user_value 3)," f (test_user_value x))
   |                                                             ^^^^^^^^^^^^^^^^^
Failed, one module loaded.

The value 'x' is an int but Haskell is reading it as 'IO Int' which is wrong, as I've specified "x:: Int". The function being tested has already been specified as "test_user_value:: Int -> IO Int" so I'm unsure why it is interpreting the variable incorrectly. Any suggestions would be appreciated.

Answer 1

It's not actually complaining about x , it's complaining about the expression test_user_value x - note that it is this expression that is underlined in the error message.

Since, as you have indicated, test_user_value:: Int -> IO Int and x:: Int , it follows that test_user_value x:: IO Int - and that's exactly what the compiler tells you shouldn't be there. It's expecting an Int , but you gave it IO Int .

You can get an Int out of it though. To do that, I think the do notation would be the clearest:

test1 = TestCase $ do
    v <- test_user_value x
    assertEqual "for (test_user_value 3)," f v

Or you could write it a bit shorter using the =<< operator:

test1 = TestCase (assertEqual "for (test_user_value 3)," f =<< test_user_value x)