Is it possible with HSpec (or HUnit) to attach further information to assertions that get printed in and only in case of failure?

Question

Similarly to how quickcheck supports counterexamples:

property \x ->
  counterexample ("Foo failed with: " ++ ...) $
    foo x

but in a way that it works with shouldBe, e.g.

failDetails (" details: " ++ baz a) $
  a `shouldBe` 2

And I would like it to print something along the lines of:

expected: 2
 but got: 3
 details: ...

Answer 1

Yes, it seems to be possible:

import Control.Exception
import Test.HUnit.Lang (HUnitFailure(..))

failDetails details assert = do
  assert `catch` \(HUnitFailure loc msg) -> do
    throw $ HUnitFailure loc $ msg ++ "\n" ++ details

We catch the exception thrown by shouldBe, amend the message, and rethrow it.

We can even use it like:

1 `shouldBe` 2
  $> failDetails "foobar"

if we define:

($>) = flip ($)
infixl 0 $>
{-# INLINE ($>) #-}