[Haskell-cafe] Do I Need To Write An Instance for Assertable Here?

Michael Litchard michael at schmong.org
Fri Nov 13 15:16:19 UTC 2015

Below is some test code I am writing for my game, the link to the
entire codebase, the error message and some discussion

main :: IO ()
main = defaultMain
  [ testGroup "EventNetwork Input"
    [testBuffer "bBuffer" Populated]

testBuffer :: String -> BufferState -> Test
testBuffer name Populated =
  testCase name $ assert $ bufferPopulated (UAC (PlayerCommand (Move
(ToPlanetName Mongo)) (AID (Data.Text.pack "100"))))
testBuffer name Empty =
  testCase name $ assert $ bufferEmptied (UAC (PlayerCommand (Move
(ToPlanetName Mongo)) (AID (Data.Text.pack "100"))))

bufferPopulated :: UAC -> MomentIO Bool
bufferPopulated ev = do
  let eInput = ev <$ never
      eValidated = toVAC <$> eInput
  bBufferMap <- (buffer eValidated eClear) :: MomentIO (Behavior BufferMap)
  let r2 = [(Just $ BufferMap $ M.insert (AID (Data.Text.pack "100"))
(toVAC ev) (M.empty :: M.Map AID VAC))]
  r1 <- liftIO $ ((interpret (eBuffer bBufferMap) []) :: IO [Maybe BufferMap])
  return $ r1 == r2

bufferEmptied :: UAC -> MomentIO Bool
bufferEmptied ev = undefined

eBuffer :: Behavior BufferMap -> Event a -> Event BufferMap
eBuffer bBufferMap nvr =
  bBufferMap <@ (() <$ nvr)

eClear = Clear <$ (() <$ never)

When I run stack build I get

    No instance for (Test.HUnit.Base.Assertable (MomentIO Bool))
      arising from a use of ‘assert’
In the expression: assert
In the second argument of ‘($)’, namely
   $ bufferPopulated
          (PlayerCommand (Move (ToPlanetName Mongo)) (AID (pack "100"))))’
In the expression:
  testCase name
  $ assert
    $ bufferPopulated
           (PlayerCommand (Move (ToPlanetName Mongo)) (AID (pack "100"))))

The problem lies with buffer. It relies on accumB which requires the
MomentIO monad. I considered writing an
instance for Assertable, but I think that's a red-herring and the
answer lies elsewhere. I need to reconcile the
fact that assert wants an IO Bool, but accumB wants a MomentIO. Maybe
I do need to write an instance for Assertable.

Here's the link to the
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