[Haskell-cafe] Reflection functional dependency as an associated type?

[Richard Eisenberg]

Type families and functional dependencies play very poorly together. The internal GHC mechanisms to implement them are basically entirely separate, so I'm not surprised that this fails. I do think that what you want is type-safe, but just not supported right now.

Richard

On Oct 1, 2014, at 6:14 PM, Brandon Moore <brandon_m_moore at yahoo.com> wrote:

> I was trying to make a bridge like this
> 
> class MonadStateTF m where
>    type StateOf m :: *
> instance (MonadState s m) => MonadStateTF m where
>   type StateOF m = s
> 
> to allow writing constraints on state monads without a dangling type parameter
> type NumMonad m = (MonadStateTF m, Num (StateOf m))
> 
> Unfortunately, the declaration type StateOF m = s
> complains that "s" isn't mentioned on the left hand side rather
> than somehow making use of the fundep.
> 
> Brandon
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