Overlapping instances for HasAcidState error

[Pete Ryland]

I know this post is old, but I've finally worked out what I needed to do to
fix this issue. See inline comments below.

So I'm trying to update my app to use 0.5.1 version of
happstack-foundation but I've come across the following error.
As I think is necessary, I've changed my type aliases to refer to
type MyApp' = FoundationT' Route (AcidState MyAppState) () IO
type MyApp = XMLGenT MyApp'

So to avoid the overlapping instance, what I needed to do was to add in
another type to encapsulate the AcidState and provide the instance, like
CtrlV does:

data MyAppAcid = MyAppAcid { acidSt :: AcidState MyAppState }

type MyApp' = FoundationT' Route MyAppAcid () IO
type MyApp = XMLGenT MyApp'

instance (Functor m, Monad m) => HasAcidState (FoundationT' u MyAppAcid r
m) MyAppState where
getAcidState = acidSt <$> getAcidSt

I hope that helps someone else with the same problem.

Cheers,
Pete