Опубликован: 19.09.2008 | Доступ: свободный | Студентов: 656 / 70 | Оценка: 4.50 / 5.00 | Длительность: 21:25:00
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Лекция 21:

Утилиты работы с монадами

< Лекция 20 || Лекция 21: 12 || Лекция 22 >

```module Monad (
join, guard, when, unless, ap,
msum,
filterM, mapAndUnzipM, zipWithM, zipWithM_, foldM,
liftM, liftM2, liftM3, liftM4, liftM5,```

- ...и то, что экспортирует Prelude

```Monad((>>=), (>>), return, fail),
Functor(fmap),
mapM, mapM_, sequence, sequence_, (=<<),
) where```

```class  (Monad m) => MonadPlus m  where
mzero  :: m a
mplus  :: m a -> m a -> m a```

```instance  MonadPlus Maybe  where
mzero                 = Nothing

Nothing `mplus` ys    =  ys
xs      `mplus` ys    =  xs

mzero =  []
mplus = (++)```

- Функции

```msum  :: MonadPlus m => [m a] -> m a
msum xs  =  foldr mplus mzero xs

join             :: (Monad m) => m (m a) -> m a
join x           =  x >>= id

when             :: (Monad m) => Bool -> m () -> m ()
when p s         =  if p then s else return ()

unless           :: (Monad m) => Bool -> m () -> m ()
unless p s       =  when (not p) s

ap               :: (Monad m) => m (a -> b) -> m a -> m b
ap               =  liftM2 (\$)

guard            :: MonadPlus m => Bool -> m ()
guard p          =  if p then return () else mzero

mapAndUnzipM     :: (Monad m) => (a -> m (b,c)) -> [a] -> m ([b], [c])
mapAndUnzipM f xs = sequence (map f xs) >>= return . unzip

zipWithM         :: (Monad m) => (a -> b -> m c) -> [a] -> [b] -> m [c]
zipWithM f xs ys =  sequence (zipWith f xs ys)

zipWithM_         :: (Monad m) => (a -> b -> m c) -> [a] -> [b] -> m ()
zipWithM_ f xs ys =  sequence_ (zipWith f xs ys)

foldM            :: (Monad m) => (a -> b -> m a) -> a -> [b] -> m a
foldM f a []     =  return a
foldM f a (x:xs) =  f a x >>= \ y -> foldM f y xs

filterM :: Monad m => (a -> m Bool) -> [a] -> m [a]
filterM p []     = return []
filterM p (x:xs) = do { b  <- p x;
ys <- filterM p xs;
return (if b then (x:ys) else ys)
}

liftM            :: (Monad m) => (a -> b) -> (m a -> m b)
liftM f          =  \a -> do { a' <- a; return (f a') }

liftM2           :: (Monad m) => (a -> b -> c) -> (m a -> m b -> m c)
liftM2 f         =  \a b -> do { a' <- a; b' <- b; return (f a' b') }

liftM3           :: (Monad m) => (a -> b -> c -> d) ->
(m a -> m b -> m c -> m d)
liftM3 f         =  \a b c -> do { a' <- a; b' <- b; c' <- c;
return (f a' b' c') }

liftM4           :: (Monad m) => (a -> b -> c -> d -> e) ->
(m a -> m b -> m c -> m d -> m e)
liftM4 f         =  \a b c d -> do { a' <- a; b' <- b; c' <- c; d' <- d;
return (f a' b' c' d') }

liftM5           :: (Monad m) => (a -> b -> c -> d -> e -> f) ->
(m a -> m b -> m c -> m d -> m e -> m f)
liftM5 f         =  \a b c d e -> do { a' <- a; b' <- b; c' <- c; d' <- d;
e' <- e; return (f a' b' c' d' e') }```
< Лекция 20 || Лекция 21: 12 || Лекция 22 >