import Data.Monoid
import Control.Applicative
import Control.Monad
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-- Функции с состоянием
--
--
a -> State s b
data State s a = State (s -> (a, s))
runState :: State s a -> s -> (a, s)
runState (State f) = f
instance Monad (State s) where
return a
= State $ \s -> (a, s)
ma >>= mf = State $ \s0 ->
let (b, s1) = runState ma s0
in
runState (mf b) s1
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-- Функции с окружением
--
--
a -> Reader env b
data Reader env a = Reader (env -> a)
runReader :: Reader env a -> env -> a
runReader (Reader f) = f
instance Monad (Reader env) where
return a
= Reader $ const a
ma >>= mf
= Reader $ \env ->
let b = runReader ma env
in
runReader (mf b) env
---------------------------------------------------
-- Функции-накопители
--
--
Monoid msg => a -> Writer msg b
data Writer msg a = Writer (a, msg)
deriving (Show)
runWriter :: Writer msg a -> (a, msg)
runWriter (Writer f) = f
instance Monoid msg => Monad (Writer msg) where
return a
= Writer (a, mempty)
ma >>= mf
= Writer (c, msgA ‘mappend‘ msgF)
where (b, msgA) = runWriter ma