Reactive Banana 1.0.0 - Why is this old code breaking?

Here's code that used to work (truncated appropriately I hope)

makeNetworkDescription :: forall t . Frameworks t => Parameters -> Moment t ()
makeNetworkDescription params = do
  eInput <- fromAddHandler (input params)
  eTick <- fromAddHandler (tick params)
  ..
let
    bResourceMap :: Behavior t ResourceMap
    bResourceMap = accumB initRmap $
      adjustMarket <$>
      bMarketRolls <@
      eTick

But now the types have changed.
we have:
makeNetworkDescription :: Parameters -> MomentIO () and accumB :: MonadMoment m => a -> Event (a -> a) -> m (Behavior a)

say I change the definition of bResourceMap to

bResourceMap :: Behavior ResourceMap
bResourceMap = accumB initRmap $
               adjustMarket   <$>
               bMarketRolls   <@
               eTick

slightly off from the accumB definition, but let's see what happens.

ghc gives an error

Couldn't match type ‘Behavior ResourceMap’ with ‘ResourceMap’
Expected type: Behavior ResourceMap
  Actual type: Behavior (Behavior ResourceMap)

Right, because of the type of accumB the behavior needs to be within the context of a MonadMoment. Having a look at MonadMoment I find two instances

instance MonadMoment Moment where liftMoment = id
instance MonadMoment MomentIO where liftMoment = MIO . unM

So why did the actual type resolve to Behavior (Behavior ResourceMap), the outer type has to be a MonadMoment, which doesn't match.

I'd like advice on how to resolve this type of problem, it happens with all my Behavior definitions.

Solution

Adjusting your code to fit the new type of accumB should only take using a monadic bind rather than a let expression to define bResourceMap:

bResourceMap <- accumB initRmap (adjustMarket <$> bMarketRolls <@ eTick)

The type error you quote seems unrelated. My guess would be that initRmap was accidentally changed from being a ResourceMap to a Behavior ResourceMap, leading to the the type mismatch.