Question

I’m trying acid-state. The documentation states that Update st is an instance of MonadState st. I tried different things, but my compiler doesn’t want to see that 🙁 I tried the HelloWorld.hs from examples, but got the same issue:

HelloWorld.hs:26:7:
    No instance for (MonadState
                       HelloWorldState (Update HelloWorldState))
      arising from a use of `put'
    Possible fix:
      add an instance declaration for
      (MonadState HelloWorldState (Update HelloWorldState))
    In the expression: put (HelloWorldState newValue)
    In an equation for `writeState':
        writeState newValue = put (HelloWorldState newValue)

HelloWorld.hs:29:43:
    No instance for (MonadReader
                       HelloWorldState (Query HelloWorldState))
      arising from a use of `ask'
    Possible fix:
      add an instance declaration for
      (MonadReader HelloWorldState (Query HelloWorldState))
    In a stmt of a 'do' block: HelloWorldState string <- ask
    In the expression:
      do { HelloWorldState string <- ask;
           return string }
    In an equation for `queryState':
        queryState
          = do { HelloWorldState string <- ask;
                 return string }

What I’m doing wrong? How to make it work?

I’m using
acid-state-0.6.4,
ghc-7.4.2,
debian-6.0.5 (amd64)

Thanks,
Yuras

ADDED:
Looks like it works in ghc-7.4.1 and doesn’t work in ghc-7.4.2. Could anybody both compilers?

I tried reinstalling acid-state, but it didn’t help. Either a bug in ghc-7.4.2 or my environment id broken.

Answer 1

Ok, I finally solved it.

The issue was with two different mtl versions installed. Hiding the wrong one fixed everything.

The interesting part is how I got it 🙂

Prelude Data.Acid Control.Monad.State> :i Update
newtype Update st a
  = acid-state-0.6.4:Data.Acid.Common.Update {acid-state-0.6.4:Data.Acid.Common.unUpdate :: transformers-0.2.2.0:Control.Monad.Trans.State.Lazy.State
                                                                                              st a}
    -- Defined in `acid-state-0.6.4:Data.Acid.Common'
instance Monad (Update st)
  -- Defined in `acid-state-0.6.4:Data.Acid.Common'
instance Functor (Update st)
  -- Defined in `acid-state-0.6.4:Data.Acid.Common'
Prelude Data.Acid Control.Monad.State>

Control.Monad.State reexports State, but you can see that ghci still shows transformers-0.2.2.0:Control.Monad.Trans.State.Lazy.State fully qualified.