Prologue.Control.Monad

Documentation

return :: Applicative m => a -> m a

Deprecated: Use pure instead

(<<) :: Monad m => m a -> m b -> m a infixr 1

(>>$) :: Monad m => m a -> m b -> m b infixr 0

(>>=>) :: Monad m => (t1 -> t2 -> m a) -> (a -> m b) -> t1 -> t2 -> m b infixr 1

(>>>=>) :: Monad m => (t1 -> t2 -> t3 -> m a) -> (a -> m b) -> t1 -> t2 -> t3 -> m b infixr 1

(>>>>=>) :: Monad m => (t1 -> t2 -> t3 -> t4 -> m a) -> (a -> m b) -> t1 -> t2 -> t3 -> t4 -> m b infixr 1

(<=<<) :: Monad m => (a -> m b) -> (t1 -> t2 -> m a) -> t1 -> t2 -> m b infixr 1

(<=<<<) :: Monad m => (a -> m b) -> (t1 -> t2 -> t3 -> m a) -> t1 -> t2 -> t3 -> m b infixr 1

(<=<<<<) :: Monad m => (a -> m b) -> (t1 -> t2 -> t3 -> t4 -> m a) -> t1 -> t2 -> t3 -> t4 -> m b infixr 1

(>>~) :: Monad m => m a -> (a -> m b) -> m a

(=<<&) :: MonadFix m => (a -> m b) -> m a -> m a infixr 1

void :: m a -> m ()

when :: (Applicative m, Mempty a) => Bool -> m a -> m a

unless :: (Applicative m, Mempty a) => Bool -> m a -> m a

when_ :: Applicative m => Bool -> m a -> m ()

unless_ :: Applicative m => Bool -> m a -> m ()

whenM :: (Monad m, Mempty a) => m Bool -> m a -> m a

unlessM :: (Monad m, Mempty a) => m Bool -> m a -> m a

whenM_ :: Monad m => m Bool -> m a -> m ()

unlessM_ :: Monad m => m Bool -> m a -> m ()

guard :: (MonadPlus m, ToBool' cond) => cond -> m ()

bind :: Monad m => (t1 -> m a) -> m t1 -> m a

bind2 :: Monad m => (t1 -> t2 -> m a) -> m t1 -> m t2 -> m a

bind3 :: Monad m => (t1 -> t2 -> t3 -> m a) -> m t1 -> m t2 -> m t3 -> m a

bind4 :: Monad m => (t1 -> t2 -> t3 -> t4 -> m a) -> m t1 -> m t2 -> m t3 -> m t4 -> m a

bind5 :: Monad m => (t1 -> t2 -> t3 -> t4 -> t5 -> m a) -> m t1 -> m t2 -> m t3 -> m t4 -> m t5 -> m a

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

The join function is the conventional monad join operator. It is used to remove one level of monadic structure, projecting its bound argument into the outer level.

Examples

A common use of join is to run an IO computation returned from an STM transaction, since STM transactions can't perform IO directly. Recall that

atomically :: STM a -> IO a

is used to run STM transactions atomically. So, by specializing the types of atomically and join to

atomically :: STM (IO b) -> IO (IO b)
join       :: IO (IO b)  -> IO b

we can compose them as

join . atomically :: STM (IO b) -> IO b

to run an STM transaction and the IO action it returns.

class Applicative m => Monad (m :: Type -> Type) where

The Monad class defines the basic operations over a monad, a concept from a branch of mathematics known as category theory. From the perspective of a Haskell programmer, however, it is best to think of a monad as an abstract datatype of actions. Haskell's do expressions provide a convenient syntax for writing monadic expressions.

Instances of Monad should satisfy the following laws:

• return a >>= k  =  k a

• m >>= return  =  m

• m >>= (\x -> k x >>= h)  =  (m >>= k) >>= h

Furthermore, the Monad and Applicative operations should relate as follows:

• pure = return

• (<*>) = ap

The above laws imply:

• fmap f xs  =  xs >>= return . f

• (>>) = (*>)

and that pure and (<*>) satisfy the applicative functor laws.

The instances of Monad for lists, Maybe and IO defined in the Prelude satisfy these laws.

Minimal complete definition

(>>=)

Methods

(>>=) :: m a -> (a -> m b) -> m b infixl 1

Sequentially compose two actions, passing any value produced by the first as an argument to the second.

(>>) :: m a -> m b -> m b infixl 1

Sequentially compose two actions, discarding any value produced by the first, like sequencing operators (such as the semicolon) in imperative languages.

Instances

Monad []

Since: base-2.1

Defined in GHC.Base

Methods

(>>=) :: [a] -> (a -> [b]) -> [b]

(>>) :: [a] -> [b] -> [b]

return :: a -> [a]

fail :: String -> [a]

Monad Maybe

Since: base-2.1

Defined in GHC.Base

Methods

(>>=) :: Maybe a -> (a -> Maybe b) -> Maybe b

(>>) :: Maybe a -> Maybe b -> Maybe b

return :: a -> Maybe a

fail :: String -> Maybe a

Monad IO

Since: base-2.1

Defined in GHC.Base

Methods

(>>=) :: IO a -> (a -> IO b) -> IO b

(>>) :: IO a -> IO b -> IO b

return :: a -> IO a

fail :: String -> IO a

Monad Par1

Since: base-4.9.0.0

Defined in GHC.Generics

Methods

(>>=) :: Par1 a -> (a -> Par1 b) -> Par1 b

(>>) :: Par1 a -> Par1 b -> Par1 b

return :: a -> Par1 a

fail :: String -> Par1 a

Monad Q

Defined in Language.Haskell.TH.Syntax

Methods

(>>=) :: Q a -> (a -> Q b) -> Q b

(>>) :: Q a -> Q b -> Q b

return :: a -> Q a

fail :: String -> Q a

Monad Complex

Since: base-4.9.0.0

Defined in Data.Complex

Methods

(>>=) :: Complex a -> (a -> Complex b) -> Complex b

(>>) :: Complex a -> Complex b -> Complex b

return :: a -> Complex a

fail :: String -> Complex a

Monad Min

Since: base-4.9.0.0

Defined in Data.Semigroup

Methods

(>>=) :: Min a -> (a -> Min b) -> Min b

(>>) :: Min a -> Min b -> Min b

return :: a -> Min a

fail :: String -> Min a

Monad Max

Since: base-4.9.0.0

Defined in Data.Semigroup

Methods

(>>=) :: Max a -> (a -> Max b) -> Max b

(>>) :: Max a -> Max b -> Max b

return :: a -> Max a

fail :: String -> Max a

Monad First

Since: base-4.9.0.0

Defined in Data.Semigroup

Methods

(>>=) :: First a -> (a -> First b) -> First b

(>>) :: First a -> First b -> First b

return :: a -> First a

fail :: String -> First a

Monad Last

Since: base-4.9.0.0

Defined in Data.Semigroup

Methods

(>>=) :: Last a -> (a -> Last b) -> Last b

(>>) :: Last a -> Last b -> Last b

return :: a -> Last a

fail :: String -> Last a

Monad Option

Since: base-4.9.0.0

Defined in Data.Semigroup

Methods

(>>=) :: Option a -> (a -> Option b) -> Option b

(>>) :: Option a -> Option b -> Option b

return :: a -> Option a

fail :: String -> Option a

Monad Identity

Since: base-4.8.0.0

Defined in Data.Functor.Identity

Methods

(>>=) :: Identity a -> (a -> Identity b) -> Identity b

(>>) :: Identity a -> Identity b -> Identity b

return :: a -> Identity a

fail :: String -> Identity a

Monad STM

Since: base-4.3.0.0

Defined in GHC.Conc.Sync

Methods

(>>=) :: STM a -> (a -> STM b) -> STM b

(>>) :: STM a -> STM b -> STM b

return :: a -> STM a

fail :: String -> STM a

Monad First

Since: base-4.8.0.0

Defined in Data.Monoid

Methods

(>>=) :: First a -> (a -> First b) -> First b

(>>) :: First a -> First b -> First b

return :: a -> First a

fail :: String -> First a

Monad Last

Since: base-4.8.0.0

Defined in Data.Monoid

Methods

(>>=) :: Last a -> (a -> Last b) -> Last b

(>>) :: Last a -> Last b -> Last b

return :: a -> Last a

fail :: String -> Last a

Monad Dual

Since: base-4.8.0.0

Defined in Data.Semigroup.Internal

Methods

(>>=) :: Dual a -> (a -> Dual b) -> Dual b

(>>) :: Dual a -> Dual b -> Dual b

return :: a -> Dual a

fail :: String -> Dual a

Monad Sum

Since: base-4.8.0.0

Defined in Data.Semigroup.Internal

Methods

(>>=) :: Sum a -> (a -> Sum b) -> Sum b

(>>) :: Sum a -> Sum b -> Sum b

return :: a -> Sum a

fail :: String -> Sum a

Monad Product

Since: base-4.8.0.0

Defined in Data.Semigroup.Internal

Methods

(>>=) :: Product a -> (a -> Product b) -> Product b

(>>) :: Product a -> Product b -> Product b

return :: a -> Product a

fail :: String -> Product a

Monad Down

Since: base-4.11.0.0

Defined in Data.Ord

Methods

(>>=) :: Down a -> (a -> Down b) -> Down b

(>>) :: Down a -> Down b -> Down b

return :: a -> Down a

fail :: String -> Down a

Monad ReadPrec

Since: base-2.1

Defined in Text.ParserCombinators.ReadPrec

Methods

(>>=) :: ReadPrec a -> (a -> ReadPrec b) -> ReadPrec b

(>>) :: ReadPrec a -> ReadPrec b -> ReadPrec b

return :: a -> ReadPrec a

fail :: String -> ReadPrec a

Monad ReadP

Since: base-2.1

Defined in Text.ParserCombinators.ReadP

Methods

(>>=) :: ReadP a -> (a -> ReadP b) -> ReadP b

(>>) :: ReadP a -> ReadP b -> ReadP b

return :: a -> ReadP a

fail :: String -> ReadP a

Monad NonEmpty

Since: base-4.9.0.0

Defined in GHC.Base

Methods

(>>=) :: NonEmpty a -> (a -> NonEmpty b) -> NonEmpty b

(>>) :: NonEmpty a -> NonEmpty b -> NonEmpty b

return :: a -> NonEmpty a

fail :: String -> NonEmpty a

Monad Put

Defined in Data.ByteString.Builder.Internal

Methods

(>>=) :: Put a -> (a -> Put b) -> Put b

(>>) :: Put a -> Put b -> Put b

return :: a -> Put a

fail :: String -> Put a

Monad Tree

Defined in Data.Tree

Methods

(>>=) :: Tree a -> (a -> Tree b) -> Tree b

(>>) :: Tree a -> Tree b -> Tree b

return :: a -> Tree a

fail :: String -> Tree a

Monad Seq

Defined in Data.Sequence.Internal

Methods

(>>=) :: Seq a -> (a -> Seq b) -> Seq b

(>>) :: Seq a -> Seq b -> Seq b

return :: a -> Seq a

fail :: String -> Seq a

Monad DList

Defined in Data.DList

Methods

(>>=) :: DList a -> (a -> DList b) -> DList b

(>>) :: DList a -> DList b -> DList b

return :: a -> DList a

fail :: String -> DList a

Monad Vector

Defined in Data.Vector

Methods

(>>=) :: Vector a -> (a -> Vector b) -> Vector b

(>>) :: Vector a -> Vector b -> Vector b

return :: a -> Vector a

fail :: String -> Vector a

Monad SmallArray

Defined in Data.Primitive.SmallArray

Methods

(>>=) :: SmallArray a -> (a -> SmallArray b) -> SmallArray b

(>>) :: SmallArray a -> SmallArray b -> SmallArray b

return :: a -> SmallArray a

fail :: String -> SmallArray a

Monad Array

Defined in Data.Primitive.Array

Methods

(>>=) :: Array a -> (a -> Array b) -> Array b

(>>) :: Array a -> Array b -> Array b

return :: a -> Array a

fail :: String -> Array a

Monad P

Since: base-2.1

Defined in Text.ParserCombinators.ReadP

Methods

(>>=) :: P a -> (a -> P b) -> P b

(>>) :: P a -> P b -> P b

return :: a -> P a

fail :: String -> P a

Monad OneTuple

Defined in Prologue.Data.OneTuple

Methods

(>>=) :: OneTuple a -> (a -> OneTuple b) -> OneTuple b

(>>) :: OneTuple a -> OneTuple b -> OneTuple b

return :: a -> OneTuple a

fail :: String -> OneTuple a

Monad (Either e)

Since: base-4.4.0.0

Defined in Data.Either

Methods

(>>=) :: Either e a -> (a -> Either e b) -> Either e b

(>>) :: Either e a -> Either e b -> Either e b

return :: a -> Either e a

fail :: String -> Either e a

Monad (U1 :: Type -> Type)

Since: base-4.9.0.0

Defined in GHC.Generics

Methods

(>>=) :: U1 a -> (a -> U1 b) -> U1 b

(>>) :: U1 a -> U1 b -> U1 b

return :: a -> U1 a

fail :: String -> U1 a

Monoid a => Monad ((,) a)

Since: base-4.9.0.0

Defined in GHC.Base

Methods

(>>=) :: (a, a0) -> (a0 -> (a, b)) -> (a, b)

(>>) :: (a, a0) -> (a, b) -> (a, b)

return :: a0 -> (a, a0)

fail :: String -> (a, a0)

Representable f => Monad (Co f)

Defined in Data.Functor.Rep

Methods

(>>=) :: Co f a -> (a -> Co f b) -> Co f b

(>>) :: Co f a -> Co f b -> Co f b

return :: a -> Co f a

fail :: String -> Co f a

Monad m => Monad (WrappedMonad m)

Since: base-4.7.0.0

Defined in Control.Applicative

Methods

(>>=) :: WrappedMonad m a -> (a -> WrappedMonad m b) -> WrappedMonad m b

(>>) :: WrappedMonad m a -> WrappedMonad m b -> WrappedMonad m b

return :: a -> WrappedMonad m a

fail :: String -> WrappedMonad m a

Monad (Proxy :: Type -> Type)

Since: base-4.7.0.0

Defined in Data.Proxy

Methods

(>>=) :: Proxy a -> (a -> Proxy b) -> Proxy b

(>>) :: Proxy a -> Proxy b -> Proxy b

return :: a -> Proxy a

fail :: String -> Proxy a

Monad m => Monad (MaybeT m)

Defined in Control.Monad.Trans.Maybe

Methods

(>>=) :: MaybeT m a -> (a -> MaybeT m b) -> MaybeT m b

(>>) :: MaybeT m a -> MaybeT m b -> MaybeT m b

return :: a -> MaybeT m a

fail :: String -> MaybeT m a

Alternative f => Monad (Cofree f)

Defined in Control.Comonad.Cofree

Methods

(>>=) :: Cofree f a -> (a -> Cofree f b) -> Cofree f b

(>>) :: Cofree f a -> Cofree f b -> Cofree f b

return :: a -> Cofree f a

fail :: String -> Cofree f a

Functor f => Monad (Free f)

Defined in Control.Monad.Free

Methods

(>>=) :: Free f a -> (a -> Free f b) -> Free f b

(>>) :: Free f a -> Free f b -> Free f b

return :: a -> Free f a

fail :: String -> Free f a

Monad (ImpossibleM1 :: Type -> Type)

Defined in Data.Impossible

Methods

(>>=) :: ImpossibleM1 a -> (a -> ImpossibleM1 b) -> ImpossibleM1 b

(>>) :: ImpossibleM1 a -> ImpossibleM1 b -> ImpossibleM1 b

return :: a -> ImpossibleM1 a

fail :: String -> ImpossibleM1 a

Monad m => Monad (Yoneda m)

Defined in Data.Functor.Yoneda

Methods

(>>=) :: Yoneda m a -> (a -> Yoneda m b) -> Yoneda m b

(>>) :: Yoneda m a -> Yoneda m b -> Yoneda m b

return :: a -> Yoneda m a

fail :: String -> Yoneda m a

Monad (ReifiedGetter s)

Defined in Control.Lens.Reified

Methods

(>>=) :: ReifiedGetter s a -> (a -> ReifiedGetter s b) -> ReifiedGetter s b

(>>) :: ReifiedGetter s a -> ReifiedGetter s b -> ReifiedGetter s b

return :: a -> ReifiedGetter s a

fail :: String -> ReifiedGetter s a

Monad (ReifiedFold s)

Defined in Control.Lens.Reified

Methods

(>>=) :: ReifiedFold s a -> (a -> ReifiedFold s b) -> ReifiedFold s b

(>>) :: ReifiedFold s a -> ReifiedFold s b -> ReifiedFold s b

return :: a -> ReifiedFold s a

fail :: String -> ReifiedFold s a

(Monad (Rep p), Representable p) => Monad (Prep p)

Defined in Data.Profunctor.Rep

Methods

(>>=) :: Prep p a -> (a -> Prep p b) -> Prep p b

(>>) :: Prep p a -> Prep p b -> Prep p b

return :: a -> Prep p a

fail :: String -> Prep p a

Monad f => Monad (Rec1 f)

Since: base-4.9.0.0

Defined in GHC.Generics

Methods

(>>=) :: Rec1 f a -> (a -> Rec1 f b) -> Rec1 f b

(>>) :: Rec1 f a -> Rec1 f b -> Rec1 f b

return :: a -> Rec1 f a

fail :: String -> Rec1 f a

Monad f => Monad (Ap f)

Since: base-4.12.0.0

Defined in Data.Monoid

Methods

(>>=) :: Ap f a -> (a -> Ap f b) -> Ap f b

(>>) :: Ap f a -> Ap f b -> Ap f b

return :: a -> Ap f a

fail :: String -> Ap f a

Monad f => Monad (Alt f)

Since: base-4.8.0.0

Defined in Data.Semigroup.Internal

Methods

(>>=) :: Alt f a -> (a -> Alt f b) -> Alt f b

(>>) :: Alt f a -> Alt f b -> Alt f b

return :: a -> Alt f a

fail :: String -> Alt f a

Monad m => Monad (IdentityT m)

Defined in Control.Monad.Trans.Identity

Methods

(>>=) :: IdentityT m a -> (a -> IdentityT m b) -> IdentityT m b

(>>) :: IdentityT m a -> IdentityT m b -> IdentityT m b

return :: a -> IdentityT m a

fail :: String -> IdentityT m a

(Applicative f, Monad f) => Monad (WhenMissing f x)

Equivalent to ReaderT k (ReaderT x (MaybeT f)).

Since: containers-0.5.9

Defined in Data.IntMap.Internal

Methods

(>>=) :: WhenMissing f x a -> (a -> WhenMissing f x b) -> WhenMissing f x b

(>>) :: WhenMissing f x a -> WhenMissing f x b -> WhenMissing f x b

return :: a -> WhenMissing f x a

fail :: String -> WhenMissing f x a

Monad m => Monad (ExceptT e m)

Defined in Control.Monad.Trans.Except

Methods

(>>=) :: ExceptT e m a -> (a -> ExceptT e m b) -> ExceptT e m b

(>>) :: ExceptT e m a -> ExceptT e m b -> ExceptT e m b

return :: a -> ExceptT e m a

fail :: String -> ExceptT e m a

(Functor f, Monad m) => Monad (FreeT f m)

Defined in Control.Monad.Trans.Free

Methods

(>>=) :: FreeT f m a -> (a -> FreeT f m b) -> FreeT f m b

(>>) :: FreeT f m a -> FreeT f m b -> FreeT f m b

return :: a -> FreeT f m a

fail :: String -> FreeT f m a

(Alternative f, Monad w) => Monad (CofreeT f w)

Defined in Control.Comonad.Trans.Cofree

Methods

(>>=) :: CofreeT f w a -> (a -> CofreeT f w b) -> CofreeT f w b

(>>) :: CofreeT f w a -> CofreeT f w b -> CofreeT f w b

return :: a -> CofreeT f w a

fail :: String -> CofreeT f w a

(Monad m, Error e) => Monad (ErrorT e m)

Defined in Control.Monad.Trans.Error

Methods

(>>=) :: ErrorT e m a -> (a -> ErrorT e m b) -> ErrorT e m b

(>>) :: ErrorT e m a -> ErrorT e m b -> ErrorT e m b

return :: a -> ErrorT e m a

fail :: String -> ErrorT e m a

Monad (Indexed i a)

Defined in Control.Lens.Internal.Indexed

Methods

(>>=) :: Indexed i a a0 -> (a0 -> Indexed i a b) -> Indexed i a b

(>>) :: Indexed i a a0 -> Indexed i a b -> Indexed i a b

return :: a0 -> Indexed i a a0

fail :: String -> Indexed i a a0

Monad (Tagged s)

Defined in Data.Tagged

Methods

(>>=) :: Tagged s a -> (a -> Tagged s b) -> Tagged s b

(>>) :: Tagged s a -> Tagged s b -> Tagged s b

return :: a -> Tagged s a

fail :: String -> Tagged s a

Monad ((->) r :: Type -> Type)

Since: base-2.1

Defined in GHC.Base

Methods

(>>=) :: (r -> a) -> (a -> r -> b) -> r -> b

(>>) :: (r -> a) -> (r -> b) -> r -> b

return :: a -> r -> a

fail :: String -> r -> a

(Monad f, Monad g) => Monad (f :*: g)

Since: base-4.9.0.0

Defined in GHC.Generics

Methods

(>>=) :: (f :*: g) a -> (a -> (f :*: g) b) -> (f :*: g) b

(>>) :: (f :*: g) a -> (f :*: g) b -> (f :*: g) b

return :: a -> (f :*: g) a

fail :: String -> (f :*: g) a

(Monad f, Monad g) => Monad (Product f g)

Since: base-4.9.0.0

Defined in Data.Functor.Product

Methods

(>>=) :: Product f g a -> (a -> Product f g b) -> Product f g b

(>>) :: Product f g a -> Product f g b -> Product f g b

return :: a -> Product f g a

fail :: String -> Product f g a

Monad (Cokleisli w a)

Defined in Control.Comonad

Methods

(>>=) :: Cokleisli w a a0 -> (a0 -> Cokleisli w a b) -> Cokleisli w a b

(>>) :: Cokleisli w a a0 -> Cokleisli w a b -> Cokleisli w a b

return :: a0 -> Cokleisli w a a0

fail :: String -> Cokleisli w a a0

(Monad f, Applicative f) => Monad (WhenMatched f x y)

Equivalent to ReaderT Key (ReaderT x (ReaderT y (MaybeT f)))

Since: containers-0.5.9

Defined in Data.IntMap.Internal

Methods

(>>=) :: WhenMatched f x y a -> (a -> WhenMatched f x y b) -> WhenMatched f x y b

(>>) :: WhenMatched f x y a -> WhenMatched f x y b -> WhenMatched f x y b

return :: a -> WhenMatched f x y a

fail :: String -> WhenMatched f x y a

(Applicative f, Monad f) => Monad (WhenMissing f k x)

Equivalent to ReaderT k (ReaderT x (MaybeT f)) .

Since: containers-0.5.9

Defined in Data.Map.Internal

Methods

(>>=) :: WhenMissing f k x a -> (a -> WhenMissing f k x b) -> WhenMissing f k x b

(>>) :: WhenMissing f k x a -> WhenMissing f k x b -> WhenMissing f k x b

return :: a -> WhenMissing f k x a

fail :: String -> WhenMissing f k x a

Monad f => Monad (M1 i c f)

Since: base-4.9.0.0

Defined in GHC.Generics

Methods

(>>=) :: M1 i c f a -> (a -> M1 i c f b) -> M1 i c f b

(>>) :: M1 i c f a -> M1 i c f b -> M1 i c f b

return :: a -> M1 i c f a

fail :: String -> M1 i c f a

(Monad f, Applicative f) => Monad (WhenMatched f k x y)

Equivalent to ReaderT k (ReaderT x (ReaderT y (MaybeT f)))

Since: containers-0.5.9

Defined in Data.Map.Internal

Methods

(>>=) :: WhenMatched f k x y a -> (a -> WhenMatched f k x y b) -> WhenMatched f k x y b

(>>) :: WhenMatched f k x y a -> WhenMatched f k x y b -> WhenMatched f k x y b

return :: a -> WhenMatched f k x y a

fail :: String -> WhenMatched f k x y a

class (Alternative m, Monad m) => MonadPlus (m :: Type -> Type) where

Monads that also support choice and failure.

Minimal complete definition

Nothing

Methods

mzero :: m a

The identity of mplus. It should also satisfy the equations

mzero >>= f  =  mzero
v >> mzero   =  mzero

The default definition is

mzero = empty

mplus :: m a -> m a -> m a

An associative operation. The default definition is

mplus = (<|>)

Instances

MonadPlus []

Since: base-2.1

Defined in GHC.Base

Methods

mzero :: [a]

mplus :: [a] -> [a] -> [a]

MonadPlus Maybe

Since: base-2.1

Defined in GHC.Base

Methods

mzero :: Maybe a

mplus :: Maybe a -> Maybe a -> Maybe a

MonadPlus IO

Since: base-4.9.0.0

Defined in GHC.Base

Methods

mzero :: IO a

mplus :: IO a -> IO a -> IO a

MonadPlus Option

Since: base-4.9.0.0

Defined in Data.Semigroup

Methods

mzero :: Option a

mplus :: Option a -> Option a -> Option a

MonadPlus STM

Since: base-4.3.0.0

Defined in GHC.Conc.Sync

Methods

mzero :: STM a

mplus :: STM a -> STM a -> STM a

MonadPlus ReadPrec

Since: base-2.1

Defined in Text.ParserCombinators.ReadPrec

Methods

mzero :: ReadPrec a

mplus :: ReadPrec a -> ReadPrec a -> ReadPrec a

MonadPlus ReadP

Since: base-2.1

Defined in Text.ParserCombinators.ReadP

Methods

mzero :: ReadP a

mplus :: ReadP a -> ReadP a -> ReadP a

MonadPlus Seq

Defined in Data.Sequence.Internal

Methods

mzero :: Seq a

mplus :: Seq a -> Seq a -> Seq a

MonadPlus DList

Defined in Data.DList

Methods

mzero :: DList a

mplus :: DList a -> DList a -> DList a

MonadPlus Vector

Defined in Data.Vector

Methods

mzero :: Vector a

mplus :: Vector a -> Vector a -> Vector a

MonadPlus SmallArray

Defined in Data.Primitive.SmallArray

Methods

mzero :: SmallArray a

mplus :: SmallArray a -> SmallArray a -> SmallArray a

MonadPlus Array

Defined in Data.Primitive.Array

Methods

mzero :: Array a

mplus :: Array a -> Array a -> Array a

MonadPlus P

Since: base-2.1

Defined in Text.ParserCombinators.ReadP

Methods

mzero :: P a

mplus :: P a -> P a -> P a

MonadPlus (U1 :: Type -> Type)

Since: base-4.9.0.0

Defined in GHC.Generics

Methods

mzero :: U1 a

mplus :: U1 a -> U1 a -> U1 a

MonadPlus (Proxy :: Type -> Type)

Since: base-4.9.0.0

Defined in Data.Proxy

Methods

mzero :: Proxy a

mplus :: Proxy a -> Proxy a -> Proxy a

Monad m => MonadPlus (MaybeT m)

Defined in Control.Monad.Trans.Maybe

Methods

mzero :: MaybeT m a

mplus :: MaybeT m a -> MaybeT m a -> MaybeT m a

(Functor v, MonadPlus v) => MonadPlus (Free v)

This violates the MonadPlus laws, handle with care.

Defined in Control.Monad.Free

Methods

mzero :: Free v a

mplus :: Free v a -> Free v a -> Free v a

MonadPlus m => MonadPlus (Yoneda m)

Defined in Data.Functor.Yoneda

Methods

mzero :: Yoneda m a

mplus :: Yoneda m a -> Yoneda m a -> Yoneda m a

MonadPlus (ReifiedFold s)

Defined in Control.Lens.Reified

Methods

mzero :: ReifiedFold s a

mplus :: ReifiedFold s a -> ReifiedFold s a -> ReifiedFold s a

MonadPlus f => MonadPlus (Rec1 f)

Since: base-4.9.0.0

Defined in GHC.Generics

Methods

mzero :: Rec1 f a

mplus :: Rec1 f a -> Rec1 f a -> Rec1 f a

MonadPlus f => MonadPlus (Ap f)

Since: base-4.12.0.0

Defined in Data.Monoid

Methods

mzero :: Ap f a

mplus :: Ap f a -> Ap f a -> Ap f a

MonadPlus f => MonadPlus (Alt f)

Since: base-4.8.0.0

Defined in Data.Semigroup.Internal

Methods

mzero :: Alt f a

mplus :: Alt f a -> Alt f a -> Alt f a

MonadPlus m => MonadPlus (IdentityT m)

Defined in Control.Monad.Trans.Identity

Methods

mzero :: IdentityT m a

mplus :: IdentityT m a -> IdentityT m a -> IdentityT m a

(Monad m, Monoid e) => MonadPlus (ExceptT e m)

Defined in Control.Monad.Trans.Except

Methods

mzero :: ExceptT e m a

mplus :: ExceptT e m a -> ExceptT e m a -> ExceptT e m a

(Functor f, MonadPlus m) => MonadPlus (FreeT f m)

Defined in Control.Monad.Trans.Free

Methods

mzero :: FreeT f m a

mplus :: FreeT f m a -> FreeT f m a -> FreeT f m a

(Monad m, Error e) => MonadPlus (ErrorT e m)

Defined in Control.Monad.Trans.Error

Methods

mzero :: ErrorT e m a

mplus :: ErrorT e m a -> ErrorT e m a -> ErrorT e m a

(MonadPlus f, MonadPlus g) => MonadPlus (f :*: g)

Since: base-4.9.0.0

Defined in GHC.Generics

Methods

mzero :: (f :*: g) a

mplus :: (f :*: g) a -> (f :*: g) a -> (f :*: g) a

(MonadPlus f, MonadPlus g) => MonadPlus (Product f g)

Since: base-4.9.0.0

Defined in Data.Functor.Product

Methods

mzero :: Product f g a

mplus :: Product f g a -> Product f g a -> Product f g a

MonadPlus f => MonadPlus (M1 i c f)

Since: base-4.9.0.0

Defined in GHC.Generics

Methods

mzero :: M1 i c f a

mplus :: M1 i c f a -> M1 i c f a -> M1 i c f a

replicateM :: Applicative m => Int -> m a -> m [a]

replicateM n act performs the action n times, gathering the results.

foldM_ :: (Foldable t, Monad m) => (b -> a -> m b) -> b -> t a -> m ()

Like foldM, but discards the result.

foldM :: (Foldable t, Monad m) => (b -> a -> m b) -> b -> t a -> m b

The foldM function is analogous to foldl, except that its result is encapsulated in a monad. Note that foldM works from left-to-right over the list arguments. This could be an issue where (>>) and the `folded function' are not commutative.

foldM f a1 [x1, x2, ..., xm]

==

do
  a2 <- f a1 x1
  a3 <- f a2 x2
  ...
  f am xm

If right-to-left evaluation is required, the input list should be reversed.

Note: foldM is the same as foldlM

zipWithM_ :: Applicative m => (a -> b -> m c) -> [a] -> [b] -> m ()

zipWithM_ is the extension of zipWithM which ignores the final result.

zipWithM :: Applicative m => (a -> b -> m c) -> [a] -> [b] -> m [c]

The zipWithM function generalizes zipWith to arbitrary applicative functors.

forever :: Applicative f => f a -> f b

Repeat an action indefinitely.

Examples

A common use of forever is to process input from network sockets, Handles, and channels (e.g. MVar and Chan).

For example, here is how we might implement an echo server, using forever both to listen for client connections on a network socket and to echo client input on client connection handles:

echoServer :: Socket -> IO ()
echoServer socket = forever $ do
  client <- accept socket
  forkFinally (echo client) (\_ -> hClose client)
  where
    echo :: Handle -> IO ()
    echo client = forever $
      hGetLine client >>= hPutStrLn client

(<=<) :: Monad m => (b -> m c) -> (a -> m b) -> a -> m c infixr 1

Right-to-left composition of Kleisli arrows. (>=>), with the arguments flipped.

Note how this operator resembles function composition (.):

(.)   ::            (b ->   c) -> (a ->   b) -> a ->   c
(<=<) :: Monad m => (b -> m c) -> (a -> m b) -> a -> m c

(>=>) :: Monad m => (a -> m b) -> (b -> m c) -> a -> m c infixr 1

Left-to-right composition of Kleisli arrows.

(=<<) :: Monad m => (a -> m b) -> m a -> m b infixr 1

Same as >>=, but with the arguments interchanged.