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