Acid-state: MonadState instance for Update

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 { Hell…

Accepted Answer

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.

Acid-state: MonadState instance for Update

Solution

Problem