Control.Monad.Compat

Documentation

class Applicative m => Monad (m :: * -> *) 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.

return :: a -> m a

Inject a value into the monadic type.

fail :: String -> m a

Fail with a message. This operation is not part of the mathematical definition of a monad, but is invoked on pattern-match failure in a do expression.

As part of the MonadFail proposal (MFP), this function is moved to its own class MonadFail (see Control.Monad.Fail for more details). The definition here will be removed in a future release.

Instances

Monad []

Since: 2.1

Methods

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

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

return :: a -> [a]

fail :: String -> [a]

Monad Maybe

Since: 2.1

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: 2.1

Methods

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

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

return :: a -> IO a

fail :: String -> IO a

Monad P

Since: 2.1

Methods

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

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

return :: a -> P a

fail :: String -> P a

Monad Complex

Since: 4.9.0.0

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: 4.9.0.0

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: 4.9.0.0

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: 4.9.0.0

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: 4.9.0.0

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: 4.9.0.0

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: 4.8.0.0

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: 4.3.0.0

Methods

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

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

return :: a -> STM a

fail :: String -> STM a

Monad Dual

Since: 4.8.0.0

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: 4.8.0.0

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: 4.8.0.0

Methods

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

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

return :: a -> Product a

fail :: String -> Product a

Monad First

Methods

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

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

return :: a -> First a

fail :: String -> First a

Monad Last

Methods

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

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

return :: a -> Last a

fail :: String -> Last a

Monad ReadPrec

Since: 2.1

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: 2.1

Methods

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

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

return :: a -> ReadP a

fail :: String -> ReadP a

Monad (Either e)

Since: 4.4.0.0

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

Monoid a => Monad ((,) a)

Since: 4.9.0.0

Methods

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

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

return :: a -> (a, a)

fail :: String -> (a, a)

Monad (ST s)

Since: 2.1

Methods

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

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

return :: a -> ST s a

fail :: String -> ST s a

Monad (ST s)

Since: 2.1

Methods

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

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

return :: a -> ST s a

fail :: String -> ST s a

Monad m => Monad (WrappedMonad m)

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 *)

Since: 4.7.0.0

Methods

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

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

return :: a -> Proxy * a

fail :: String -> Proxy * a

Monad f => Monad (Alt * f)

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 ((->) LiftedRep LiftedRep r)

Since: 2.1

Methods

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

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

return :: a -> (LiftedRep -> LiftedRep) r a

fail :: String -> (LiftedRep -> LiftedRep) r a

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

Since: 4.9.0.0

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

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

Monads that also support choice and failure.

Methods

mzero :: m a

the identity of mplus. It should also satisfy the equations

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

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

an associative operation

Instances

MonadPlus []

Since: 2.1

Methods

mzero :: [a]

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

MonadPlus Maybe

Since: 2.1

Methods

mzero :: Maybe a

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

MonadPlus IO

Since: 4.9.0.0

Methods

mzero :: IO a

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

MonadPlus P

Since: 2.1

Methods

mzero :: P a

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

MonadPlus Option

Since: 4.9.0.0

Methods

mzero :: Option a

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

MonadPlus STM

Since: 4.3.0.0

Methods

mzero :: STM a

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

MonadPlus ReadPrec

Since: 2.1

Methods

mzero :: ReadPrec a

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

MonadPlus ReadP

Since: 2.1

Methods

mzero :: ReadP a

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

MonadPlus (Proxy *)

Since: 4.9.0.0

Methods

mzero :: Proxy * a

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

MonadPlus f => MonadPlus (Alt * f)

Methods

mzero :: Alt * f a

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

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

Since: 4.9.0.0

Methods

mzero :: Product * f g a

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