[Haskell-cafe] extensible effects + classy lenses

8 Mar 2016

      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

[Haskell-cafe] extensible effects + classy lenses

Mitchell Rosen