{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE DeriveFunctor #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE Safe #-}
-- | Create unique Enumerable values.
module Control.Eff.Fresh( Fresh (..)
, fresh
, runFresh'
) where
import Control.Eff
-- There are three possible implementations
-- The first one uses State Fresh where
-- newtype Fresh = Fresh Int
-- We get the `private' effect layer (State Fresh) that does not interfere
-- with with other layers.
-- This is the easiest implementation.
-- The second implementation defines a new effect Fresh
-- | Create unique Enumerable values.
data Fresh v where
Fresh :: Fresh Int
-- | Produce a value that has not been previously produced.
fresh :: Member Fresh r => Eff r Int
fresh = send Fresh
-- | Run an effect requiring unique values.
runFresh' :: Eff (Fresh ': r) w -> Int -> Eff r w
runFresh' m s =
handle_relay_s s (\_s x -> return x)
(\s' Fresh k -> (k $! s' + 1) s')
m
{-
-- Finally, the worst implementation but the one that answers
-- reviewer's question: implementing Fresh in terms of State
-- but not revealing that fact.
runFresh :: Eff (Fresh :> r) w -> Int -> Eff r w
runFresh m s = runState m' s >>= return . fst
where
m' = loop m
loop (Val x) = return x
loop (E u q) = case decomp u of
Right Fresh -> do
n <- get
put (n+1::Int)
k n
Left u -> send (\k -> weaken $ fmap k u) >>= loop
tfresh = runTrace $ flip runFresh 0 $ do
n <- fresh
-- (x::Int) <- get
trace $ "Fresh " ++ show n
n <- fresh
trace $ "Fresh " ++ show n
{-
If we try to meddle with the encapsulated state, by uncommenting the
get statement above, we get:
No instance for (Member (State Int) Void)
arising from a use of `get'
-}
-}
-- Encapsulation of effects
-- The example suggested by a reviewer
{- The reviewer outlined an MTL implementation below, writing
``This hides the state effect and I can layer another state effect on
top without getting into conflict with the class system.''
class Monad m => MonadFresh m where
fresh :: m Int
newtype FreshT m a = FreshT { unFreshT :: State Int m a }
deriving (Functor, Monad, MonadTrans)
instance Monad m => MonadFresh (FreshT m) where
fresh = FreshT $ do n <- get; put (n+1); return n
See EncapsMTL.hs for the complete code.
-}