Comparing values that have IORef in them

[FranklinChen]

What is the recommended way to write a matcher to compare two values that have IORef in them and therefore have to be evaluated in IO both for equality comparison and showable diffs?

[sol]

Something like this:

(readIORef actual `shouldReturn`) =<< readIORef expected

[FranklinChen]

I meant, if I have

data List a = Nil | Cons (IORef a) (IORef (List a))

I want to write

shouldBeEqList :: (Show a, Eq a) => List a -> List a -> Expectation

[sol]

@FranklinChen sorry for the late replay, there is no standard way of achieving this.

Option 1

The easiest approach might be to convert your List to a regular Haskell list, say if you have

toList :: List a -> [a]

you can do something like this:

infix 1 `shouldBeEqList`

shouldBeEqList :: HasCallStack => (Eq a, Show a) => List a -> List a -> Expectation
shouldBeEqList actual expected = do
  (toList actual `shouldReturn`) =<< toList expected

Option 2

If you have functions:

eqList :: Eq a => List a -> List a -> IO Bool
showList :: Show a => List a -> List a -> IO String

then you can do something like:

Option 2.1

shouldBeEqList :: HasCallStack => Show a => List a -> List a -> Expectation
shouldBeEqList actual expected = do
  (showList actual `shouldReturn`) =<< showList expected

We are ignoring the Eq instance here and instead rely on the fact that lists that look equal are equal and that lists that are not equal look different.

If that is not the case for some reason, you can do this instead:

Option 2.2

shouldBeEqList :: HasCallStack => (Eq a, Show a) => List a -> List a -> Expectation
shouldBeEqList actual expected = do
  eq <- eqList actual expected
  unless eq $ throwIO (HUnitFailure location $ ExpectedButGot Nothing expectedMsg actualMsg)

location :: HasCallStack => Maybe SrcLoc
location = case reverse callStack of
  (_, loc) : _ -> Just loc
  [] -> Nothing

(look at https://hackage.haskell.org/package/HUnit-1.6.1.0/docs/src/Test.HUnit.Lang.html#assertEqual for the imports you will need)

We should probably export more things from HUnit to make this kind of things easier.

[sol]

I opened hspec/HUnit#29.