[Haskell-cafe] extensible effects + classy lenses

[Mitchell Rosen]

Hi all,

I'm trying to combine an extensible effects style state with classy lenses. 
That is, instead of pinning the type of my state down, I'd like to only 
require the pieces I need.

For example,

{-# language FlexibleContexts #-}

import Control.Monad.Freer
import Control.Monad.Freer.State

class HasInt s where
  getInt :: s -> Int

foo :: (Member (State s) effs, HasInt s) => Eff effs Int
foo = get

However, this fails to typecheck:

    Overlapping instances for Member (State s0) effs
    Matching givens (or their superclasses):
      (Member (State s) effs)
        bound by the type signature for
                   foo :: (Member (State s) effs, HasInt s) => Eff effs Int
        at example.hs:9:8-56
    Matching instances:
      instance Data.Open.Union.Member'
                 t r (Data.Open.Union.FindElem t r) =>
               Member t r
        -- Defined in ‘Data.Open.Union’
    (The choice depends on the instantiation of ‘effs, s0’)
    In the ambiguity check for the type signature for ‘foo’:
      foo :: forall (effs :: [* -> *]) s.
             (Member (State s) effs, HasInt s) =>
             Eff effs Int
    To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
    In the type signature for ‘foo’:
      foo :: (Member (State s) effs, HasInt s) => Eff effs Int

Is this a weakness of extensible effects, or is there another way to 
express this function?

Thanks,
Mitchell
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