mtl

mtl.hs

{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE FlexibleInstances #-}

module Experiment where

import           Control.Lens
import           Control.Lens.TH               (makeClassyPrisms)
import           Control.Monad.Error.Class



data BaseError = BaseFoo | BaseGoo deriving (Eq, Show)
makeClassyPrisms ''BaseError

data TopError = Base BaseError | TopGoo deriving (Eq, Show)
makeClassyPrisms ''TopError

instance AsBaseError TopError where
  _BaseError = _Base

type BaseContext e m = (MonadError e m, AsBaseError e)

type TopContext e m = (MonadError e m, AsTopError e)


doBaseThing :: (BaseContext e m) => Int -> m e
doBaseThing _ = undefined

doTopThing :: (TopContext e m) => Int -> m e
doTopThing i = undefined