{-# LANGUAGE Safe #-}
{-
This module is part of Chatty.
Copyleft (c) 2014 Marvin Cohrs
All wrongs reversed. Sharing is an act of love, not crime.
Please share Chatty with everyone you like.
Chatty is free software: you can redistribute it and/or modify
it under the terms of the GNU Affero General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
Chatty is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU Affero General Public License for more details.
You should have received a copy of the GNU Affero General Public License
along with Chatty. If not, see .
-}
-- | Provides a counter monad.
module Data.Chatty.Counter where
import Control.Applicative
import Control.Arrow
import Control.Monad
import Control.Monad.Trans.Class
-- | A counter monad.
newtype CounterT m a = Counter { runCounterT :: Int -> m (a,Int) }
instance Functor m => Functor (CounterT m) where
fmap f a = Counter $ \s -> fmap (first f) $ runCounterT a s
instance (Functor m,Monad m) => Applicative (CounterT m) where
pure = return
(<*>) = ap
instance Monad m => Monad (CounterT m) where
return a = Counter $ \s -> return (a,s)
m >>= f = Counter $ \s -> do (a,s') <- runCounterT m s; runCounterT (f a) s'
instance MonadTrans CounterT where
lift m = Counter $ \s -> do a <- m; return (a,s)
-- | Typeclass for all counter monads.
class Monad m => ChCounter m where
-- | Tell the current number and increment it
countOn :: m Int
instance Monad m => ChCounter (CounterT m) where
countOn = Counter $ \s -> return (s,s+1)
-- | Run the given function inside a counter
withCounter :: (Monad m,Functor m) => CounterT m a -> m a
withCounter m = fmap fst $ runCounterT m 0