{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE NoMonomorphismRestriction #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE Safe #-}
-- | Lazy write-only state
module Control.Eff.Writer.Lazy ( Writer(..)
, tell
, censor
, runWriter
, runFirstWriter
, runLastWriter
, runListWriter
, runMonoidWriter
) where
import Control.Eff
import Data.OpenUnion
import Data.Monoid
import Control.Applicative ((<|>))
-- ------------------------------------------------------------------------
-- | The Writer monad
--
-- In MTL's Writer monad, the told value must have a |Monoid| type. Our
-- writer has no such constraints. If we write a |Writer|-like
-- interpreter to accumulate the told values in a monoid, it will have
-- the |Monoid w| constraint then
data Writer w v = Writer w v
-- | Write a new value.
tell :: Member (Writer w) r => w -> Eff r ()
tell w = send $ Writer w ()
-- | Transform the state being produced.
censor :: forall w a r. Member (Writer w) r => (w -> w) -> Eff r a -> Eff r a
censor f = interpose return h
where
h (Writer w v) k = tell (f w) >> k v
-- | Handle Writer requests, using a user-provided function to accumulate
-- values, hence no Monoid constraints.
runWriter :: (w -> b -> b) -> b -> Eff (Writer w ': r) a -> Eff r (a, b)
runWriter accum b = handle_relay
(\x -> return (x, b))
(\(Writer w v) k -> k v >>= \(x, l) -> return (x, w `accum` l))
-- the second arg to 'handle_relay' above is same as:
-- (\(Writer w v) k -> second (accum w) `fmap` k v)
-- where
-- second f (x, y) = (x, f y)
-- | Handle Writer requests, using a List to accumulate values.
runListWriter :: Eff (Writer w ': r) a -> Eff r (a,[w])
runListWriter = runWriter (:) []
-- | Handle Writer requests, using a Monoid instance to accumulate values.
runMonoidWriter :: (Monoid w) => Eff (Writer w ': r) a -> Eff r (a, w)
runMonoidWriter = runWriter (<>) mempty
-- | Handle Writer requests by taking the first value provided.
runFirstWriter :: Eff (Writer w ': r) a -> Eff r (a, Maybe w)
runFirstWriter = runWriter (\w b -> Just w <|> b) Nothing
-- | Handle Writer requests by overwriting previous values.
runLastWriter :: Eff (Writer w ': r) a -> Eff r (a, Maybe w)
runLastWriter = runWriter (\w b -> b <|> Just w) Nothing