Question

I am learning Haskell and I want to do TDD.
I am trying to test if a function raise an expected exception.
I am using HUnit and testpack.

testpack provides an assertRaises function but I don’t manage to compile my code 🙁

Here is my source code :

module Main where
import Test.HUnit
import Test.HUnit.Tools
import Control.Exception

foo n   | n > 2 = throw ( IndexOutOfBounds ( "Index out of bounds : " ++ ( show n ) ) )
foo n | otherwise = n

testException = TestCase( assertRaises "throw exception" ( IndexOutOfBounds "Index out of bounds : 4" ) ( foo 4 ) )

main = runTestTT ( TestList [ testException ] )

When I compile it with ghc I get the following error message :

test_exceptions.hs:10:107:
    No instance for (Ord (IO a0))
      arising from a use of `foo'
    Possible fix: add an instance declaration for (Ord (IO a0))
    In the third argument of `assertRaises', namely `(foo 4)'
    In the first argument of `TestCase', namely
      `(assertRaises
          "throw exception"
          (IndexOutOfBounds "Index out of bounds : 4")
          (foo 4))'
    In the expression:
      TestCase
        (assertRaises
           "throw exception"
           (IndexOutOfBounds "Index out of bounds : 4")
           (foo 4))

test_exceptions.hs:10:111:
    No instance for (Num (IO a0))
      arising from the literal `4'
    Possible fix: add an instance declaration for (Num (IO a0))
    In the first argument of `foo', namely `4'
    In the third argument of `assertRaises', namely `(foo 4)'
    In the first argument of `TestCase', namely
      `(assertRaises
          "throw exception"
          (IndexOutOfBounds "Index out of bounds : 4")
          (foo 4))'

What’s wrong ?

Answer 1

assertRaises expects its third argument to be an IO action (with type IO a), but the return type from foo is a number (with type (Num a, Ord a) => a), not an IO action.

Try replacing (foo 4) with (evaluate (foo 4)).