base-compat-0.9.3: A compatibility layer for base

Synopsis

# 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

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

 Methodsmzero :: [a] #mplus :: [a] -> [a] -> [a] # Methodsmzero :: Maybe a #mplus :: Maybe a -> Maybe a -> Maybe a # Methodsmzero :: IO a #mplus :: IO a -> IO a -> IO a # Methodsmzero :: U1 a #mplus :: U1 a -> U1 a -> U1 a # Methodsmzero :: P a #mplus :: P a -> P a -> P a # Methodsmzero :: STM a #mplus :: STM a -> STM a -> STM a # Methodsmplus :: ReadPrec a -> ReadPrec a -> ReadPrec a # Methodsmzero :: ReadP a #mplus :: ReadP a -> ReadP a -> ReadP a # MonadPlus f => MonadPlus (Rec1 f) Methodsmzero :: Rec1 f a #mplus :: Rec1 f a -> Rec1 f a -> Rec1 f a # Methodsmzero :: Proxy * a #mplus :: Proxy * a -> Proxy * a -> Proxy * a # (MonadPlus f, MonadPlus g) => MonadPlus ((:*:) f g) Methodsmzero :: (f :*: g) a #mplus :: (f :*: g) a -> (f :*: g) a -> (f :*: g) a # MonadPlus f => MonadPlus (Alt * f) Methodsmzero :: Alt * f a #mplus :: Alt * f a -> Alt * f a -> Alt * f a # MonadPlus f => MonadPlus (M1 i c f) Methodsmzero :: M1 i c f a #mplus :: M1 i c f a -> M1 i c f a -> M1 i c f a #