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

[Michael Litchard]

 I think I'm on the right track. I have commented out all test harness code
and have changed the signature for bufferPopulated

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

I believe this should work, but here's the error

tests/Spec.hs:35:17:
    No instance for (MonadMoment IO) arising from a use of ‘liftMoment’
    In a stmt of a 'do' block:
      bBufferMap <- liftMoment
                      ((buffer eValidated eClear) :: Moment (Behavior
 BufferMap))

Let's take a look at MonadMoment from Reactive.Banana.Combinators
class Monad m => MonadMoment m where
 An instance of the MonadMoment class denotes a computation that happens at
one particular moment in time.
 Unlike the Moment monad, it need not be pure anymore.
 Methods
   liftMoment :: Moment a -> m a
 Instances
 MonadMoment MomentIO
 MonadMoment Moment

m can be any Monad, IO is a Monad. so liftMoment should lift the Moment
Behavior (BufferMap) to IO Behavior (BufferMap) , why doesn't it. What's
wrong with my reasoning?


On Fri, Nov 13, 2015 at 7:16 AM, Michael Litchard <michael at schmong.org>
wrote:

> 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
>
> tests/Spec.hs:26:19:
>     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
>   ‘assert
>    $ bufferPopulated
>        (UAC
>           (PlayerCommand (Move (ToPlanetName Mongo)) (AID (pack "100"))))’
> In the expression:
>   testCase name
>   $ assert
>     $ bufferPopulated
>         (UAC
>            (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 project:https://github.com/mlitchard/emporos/tree/banana-1.0.0/src
>
>
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