StateRW.hs

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE NoImplicitPrelude #-}
{-# LANGUAGE TypeOperators #-}
-- |
-- Module:       Control.Monad.Freer.StateRW
-- Description:  State effects in terms of Reader and Writer.
-- Copyright:    (c) 2016 Allele Dev; 2017 Ixperta Solutions s.r.o.
-- License:      BSD3
-- Maintainer:   ixcom-core@ixperta.com
-- Stability:    experimental
-- Portability:  GHC specific language extensions.
--
-- Composable handler for 'State' effects in terms of 'Reader' and 'Writer'
-- effects. This module is more a tutorial on how to compose handlers. It is
-- slightly slower than a dedicated 'State' handler.
--
-- Using <http://okmij.org/ftp/Haskell/extensible/Eff1.hs> as a starting point.
module Control.Monad.Freer.StateRW
    ( runStateR
    , Reader
    , Writer
    , tell
    , ask
    )
  where

import Control.Monad (return)
import Data.Either (Either(Left, Right))

import Control.Monad.Freer.Reader (Reader(Reader), ask)
import Control.Monad.Freer.Writer (Writer(Writer), tell)
import Control.Monad.Freer.Internal (Eff(E, Val), decomp, qComp, tsingleton)


-- | State handler, using 'Reader' and 'Writer' effects.
runStateR :: Eff (Writer s ': Reader s ': effs) a -> s -> Eff effs (a, s)
runStateR m s = loop s m
  where
    loop :: s -> Eff (Writer s ': Reader s ': effs) a -> Eff effs (a, s)
    loop s' (Val x) = return (x, s')
    loop s' (E u q) = case decomp u of
        Right (Writer o) -> k o ()
        Left  u'  -> case decomp u' of
            Right Reader -> k s' s'
            Left u'' -> E u'' (tsingleton (k s'))
      where
        k s'' = qComp q (loop s'')