base-4.9.1.0: Basic libraries

Copyright(c) The University of Glasgow 2001
LicenseBSD-style (see the file libraries/base/LICENSE)
Maintainerlibraries@haskell.org
Stabilityprovisional
Portabilityportable
Safe HaskellTrustworthy
LanguageHaskell2010

Control.Monad

Contents

Description

The Functor, Monad and MonadPlus classes, with some useful operations on monads.

Synopsis

Functor and monad classes

class Functor f where #

The Functor class is used for types that can be mapped over. Instances of Functor should satisfy the following laws:

fmap id  ==  id
fmap (f . g)  ==  fmap f . fmap g

The instances of Functor for lists, Maybe and IO satisfy these laws.

Minimal complete definition

fmap

Methods

fmap :: (a -> b) -> f a -> f b #

Instances

Functor [] # 

Methods

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

(<$) :: a -> [b] -> [a] #

Functor Maybe # 

Methods

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

(<$) :: a -> Maybe b -> Maybe a #

Functor IO # 

Methods

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

(<$) :: a -> IO b -> IO a #

Functor V1 # 

Methods

fmap :: (a -> b) -> V1 a -> V1 b #

(<$) :: a -> V1 b -> V1 a #

Functor U1 # 

Methods

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

(<$) :: a -> U1 b -> U1 a #

Functor Par1 # 

Methods

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

(<$) :: a -> Par1 b -> Par1 a #

Functor ReadP # 

Methods

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

(<$) :: a -> ReadP b -> ReadP a #

Functor ReadPrec # 

Methods

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

(<$) :: a -> ReadPrec b -> ReadPrec a #

Functor Last # 

Methods

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

(<$) :: a -> Last b -> Last a #

Functor First # 

Methods

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

(<$) :: a -> First b -> First a #

Functor Product # 

Methods

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

(<$) :: a -> Product b -> Product a #

Functor Sum # 

Methods

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

(<$) :: a -> Sum b -> Sum a #

Functor Dual # 

Methods

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

(<$) :: a -> Dual b -> Dual a #

Functor STM # 

Methods

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

(<$) :: a -> STM b -> STM a #

Functor Handler # 

Methods

fmap :: (a -> b) -> Handler a -> Handler b #

(<$) :: a -> Handler b -> Handler a #

Functor ZipList # 

Methods

fmap :: (a -> b) -> ZipList a -> ZipList b #

(<$) :: a -> ZipList b -> ZipList a #

Functor ArgDescr # 

Methods

fmap :: (a -> b) -> ArgDescr a -> ArgDescr b #

(<$) :: a -> ArgDescr b -> ArgDescr a #

Functor OptDescr # 

Methods

fmap :: (a -> b) -> OptDescr a -> OptDescr b #

(<$) :: a -> OptDescr b -> OptDescr a #

Functor ArgOrder # 

Methods

fmap :: (a -> b) -> ArgOrder a -> ArgOrder b #

(<$) :: a -> ArgOrder b -> ArgOrder a #

Functor Complex # 

Methods

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

(<$) :: a -> Complex b -> Complex a #

Functor NonEmpty # 

Methods

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

(<$) :: a -> NonEmpty b -> NonEmpty a #

Functor Option # 

Methods

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

(<$) :: a -> Option b -> Option a #

Functor Last # 

Methods

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

(<$) :: a -> Last b -> Last a #

Functor First # 

Methods

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

(<$) :: a -> First b -> First a #

Functor Max # 

Methods

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

(<$) :: a -> Max b -> Max a #

Functor Min # 

Methods

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

(<$) :: a -> Min b -> Min a #

Functor Identity # 

Methods

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

(<$) :: a -> Identity b -> Identity a #

Functor ((->) r) # 

Methods

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

(<$) :: a -> (r -> b) -> r -> a #

Functor (Either a) # 

Methods

fmap :: (a -> b) -> Either a a -> Either a b #

(<$) :: a -> Either a b -> Either a a #

Functor f => Functor (Rec1 f) # 

Methods

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

(<$) :: a -> Rec1 f b -> Rec1 f a #

Functor (URec Char) # 

Methods

fmap :: (a -> b) -> URec Char a -> URec Char b #

(<$) :: a -> URec Char b -> URec Char a #

Functor (URec Double) # 

Methods

fmap :: (a -> b) -> URec Double a -> URec Double b #

(<$) :: a -> URec Double b -> URec Double a #

Functor (URec Float) # 

Methods

fmap :: (a -> b) -> URec Float a -> URec Float b #

(<$) :: a -> URec Float b -> URec Float a #

Functor (URec Int) # 

Methods

fmap :: (a -> b) -> URec Int a -> URec Int b #

(<$) :: a -> URec Int b -> URec Int a #

Functor (URec Word) # 

Methods

fmap :: (a -> b) -> URec Word a -> URec Word b #

(<$) :: a -> URec Word b -> URec Word a #

Functor (URec (Ptr ())) # 

Methods

fmap :: (a -> b) -> URec (Ptr ()) a -> URec (Ptr ()) b #

(<$) :: a -> URec (Ptr ()) b -> URec (Ptr ()) a #

Functor ((,) a) # 

Methods

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

(<$) :: a -> (a, b) -> (a, a) #

Functor (ST s) # 

Methods

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

(<$) :: a -> ST s b -> ST s a #

Functor (Proxy *) # 

Methods

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

(<$) :: a -> Proxy * b -> Proxy * a #

Arrow a => Functor (ArrowMonad a) # 

Methods

fmap :: (a -> b) -> ArrowMonad a a -> ArrowMonad a b #

(<$) :: a -> ArrowMonad a b -> ArrowMonad a a #

Monad m => Functor (WrappedMonad m) # 

Methods

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

(<$) :: a -> WrappedMonad m b -> WrappedMonad m a #

Functor (ST s) # 

Methods

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

(<$) :: a -> ST s b -> ST s a #

Functor (Arg a) # 

Methods

fmap :: (a -> b) -> Arg a a -> Arg a b #

(<$) :: a -> Arg a b -> Arg a a #

Functor (K1 i c) # 

Methods

fmap :: (a -> b) -> K1 i c a -> K1 i c b #

(<$) :: a -> K1 i c b -> K1 i c a #

(Functor g, Functor f) => Functor ((:+:) f g) # 

Methods

fmap :: (a -> b) -> (f :+: g) a -> (f :+: g) b #

(<$) :: a -> (f :+: g) b -> (f :+: g) a #

(Functor g, Functor f) => Functor ((:*:) f g) # 

Methods

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

(<$) :: a -> (f :*: g) b -> (f :*: g) a #

(Functor g, Functor f) => Functor ((:.:) f g) # 

Methods

fmap :: (a -> b) -> (f :.: g) a -> (f :.: g) b #

(<$) :: a -> (f :.: g) b -> (f :.: g) a #

Functor f => Functor (Alt * f) # 

Methods

fmap :: (a -> b) -> Alt * f a -> Alt * f b #

(<$) :: a -> Alt * f b -> Alt * f a #

Functor (Const * m) # 

Methods

fmap :: (a -> b) -> Const * m a -> Const * m b #

(<$) :: a -> Const * m b -> Const * m a #

Arrow a => Functor (WrappedArrow a b) # 

Methods

fmap :: (a -> b) -> WrappedArrow a b a -> WrappedArrow a b b #

(<$) :: a -> WrappedArrow a b b -> WrappedArrow a b a #

Functor f => Functor (M1 i c f) # 

Methods

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

(<$) :: a -> M1 i c f b -> M1 i c f a #

(Functor f, Functor g) => Functor (Product * f g) # 

Methods

fmap :: (a -> b) -> Product * f g a -> Product * f g b #

(<$) :: a -> Product * f g b -> Product * f g a #

(Functor f, Functor g) => Functor (Sum * f g) # 

Methods

fmap :: (a -> b) -> Sum * f g a -> Sum * f g b #

(<$) :: a -> Sum * f g b -> Sum * f g a #

(Functor f, Functor g) => Functor (Compose * * f g) # 

Methods

fmap :: (a -> b) -> Compose * * f g a -> Compose * * f g b #

(<$) :: a -> Compose * * f g b -> Compose * * f g a #

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:

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

The above laws imply:

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

(>>=) :: forall a b. 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.

(>>) :: forall a b. 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 [] # 

Methods

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

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

return :: a -> [a] #

fail :: String -> [a] #

Monad Maybe # 

Methods

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

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

return :: a -> Maybe a #

fail :: String -> Maybe a #

Monad IO # 

Methods

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

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

return :: a -> IO a #

fail :: String -> IO a #

Monad U1 # 

Methods

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

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

return :: a -> U1 a #

fail :: String -> U1 a #

Monad Par1 # 

Methods

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

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

return :: a -> Par1 a #

fail :: String -> Par1 a #

Monad ReadP # 

Methods

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

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

return :: a -> ReadP a #

fail :: String -> ReadP a #

Monad ReadPrec # 

Methods

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

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

return :: a -> ReadPrec a #

fail :: String -> ReadPrec 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 First # 

Methods

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

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

return :: a -> First a #

fail :: String -> First a #

Monad Product # 

Methods

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

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

return :: a -> Product a #

fail :: String -> Product a #

Monad Sum # 

Methods

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

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

return :: a -> Sum a #

fail :: String -> Sum a #

Monad Dual # 

Methods

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

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

return :: a -> Dual a #

fail :: String -> Dual a #

Monad STM # 

Methods

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

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

return :: a -> STM a #

fail :: String -> STM a #

Monad Complex # 

Methods

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

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

return :: a -> Complex a #

fail :: String -> Complex a #

Monad NonEmpty # 

Methods

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

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

return :: a -> NonEmpty a #

fail :: String -> NonEmpty a #

Monad Option # 

Methods

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

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

return :: a -> Option a #

fail :: String -> Option 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 First # 

Methods

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

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

return :: a -> First a #

fail :: String -> First a #

Monad Max # 

Methods

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

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

return :: a -> Max a #

fail :: String -> Max a #

Monad Min # 

Methods

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

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

return :: a -> Min a #

fail :: String -> Min a #

Monad Identity # 

Methods

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

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

return :: a -> Identity a #

fail :: String -> Identity a #

Monad ((->) r) # 

Methods

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

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

return :: a -> r -> a #

fail :: String -> r -> a #

Monad (Either e) # 

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 f => Monad (Rec1 f) # 

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 #

Monoid a => Monad ((,) a) # 

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) # 

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 (Proxy *) # 

Methods

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

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

return :: a -> Proxy * a #

fail :: String -> Proxy * a #

ArrowApply a => Monad (ArrowMonad a) # 

Methods

(>>=) :: ArrowMonad a a -> (a -> ArrowMonad a b) -> ArrowMonad a b #

(>>) :: ArrowMonad a a -> ArrowMonad a b -> ArrowMonad a b #

return :: a -> ArrowMonad a a #

fail :: String -> ArrowMonad a 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 (ST s) # 

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 f, Monad g) => Monad ((:*:) f g) # 

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 (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 f => Monad (M1 i c f) # 

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

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 [] # 

Methods

mzero :: [a] #

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

MonadPlus Maybe # 

Methods

mzero :: Maybe a #

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

MonadPlus IO # 

Methods

mzero :: IO a #

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

MonadPlus U1 # 

Methods

mzero :: U1 a #

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

MonadPlus ReadP # 

Methods

mzero :: ReadP a #

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

MonadPlus ReadPrec # 

Methods

mzero :: ReadPrec a #

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

MonadPlus STM # 

Methods

mzero :: STM a #

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

MonadPlus Option # 

Methods

mzero :: Option a #

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

MonadPlus f => MonadPlus (Rec1 f) # 

Methods

mzero :: Rec1 f a #

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

MonadPlus (Proxy *) # 

Methods

mzero :: Proxy * a #

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

(ArrowApply a, ArrowPlus a) => MonadPlus (ArrowMonad a) # 

Methods

mzero :: ArrowMonad a a #

mplus :: ArrowMonad a a -> ArrowMonad a a -> ArrowMonad a a #

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

Methods

mzero :: (f :*: g) a #

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

MonadPlus f => MonadPlus (Alt * f) # 

Methods

mzero :: Alt * f a #

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

MonadPlus f => MonadPlus (M1 i c f) # 

Methods

mzero :: M1 i c f a #

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

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

Methods

mzero :: Product * f g a #

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

Functions

Naming conventions

The functions in this library use the following naming conventions:

  • A postfix 'M' always stands for a function in the Kleisli category: The monad type constructor m is added to function results (modulo currying) and nowhere else. So, for example,
 filter  ::              (a ->   Bool) -> [a] ->   [a]
 filterM :: (Monad m) => (a -> m Bool) -> [a] -> m [a]
  • A postfix '_' changes the result type from (m a) to (m ()). Thus, for example:
 sequence  :: Monad m => [m a] -> m [a]
 sequence_ :: Monad m => [m a] -> m ()
  • A prefix 'm' generalizes an existing function to a monadic form. Thus, for example:
 sum  :: Num a       => [a]   -> a
 msum :: MonadPlus m => [m a] -> m a

Basic Monad functions

mapM :: (Traversable t, Monad m) => (a -> m b) -> t a -> m (t b) #

Map each element of a structure to a monadic action, evaluate these actions from left to right, and collect the results. For a version that ignores the results see mapM_.

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

Map each element of a structure to a monadic action, evaluate these actions from left to right, and ignore the results. For a version that doesn't ignore the results see mapM.

As of base 4.8.0.0, mapM_ is just traverse_, specialized to Monad.

forM :: (Traversable t, Monad m) => t a -> (a -> m b) -> m (t b) #

forM is mapM with its arguments flipped. For a version that ignores the results see forM_.

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

forM_ is mapM_ with its arguments flipped. For a version that doesn't ignore the results see forM.

As of base 4.8.0.0, forM_ is just for_, specialized to Monad.

sequence :: (Traversable t, Monad m) => t (m a) -> m (t a) #

Evaluate each monadic action in the structure from left to right, and collect the results. For a version that ignores the results see sequence_.

sequence_ :: (Foldable t, Monad m) => t (m a) -> m () #

Evaluate each monadic action in the structure from left to right, and ignore the results. For a version that doesn't ignore the results see sequence.

As of base 4.8.0.0, sequence_ is just sequenceA_, specialized to Monad.

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

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

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

Left-to-right Kleisli composition of monads.

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

Right-to-left Kleisli composition of monads. (>=>), 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

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

forever act repeats the action infinitely.

void :: Functor f => f a -> f () #

void value discards or ignores the result of evaluation, such as the return value of an IO action.

Examples

Replace the contents of a Maybe Int with unit:

>>> void Nothing
Nothing
>>> void (Just 3)
Just ()

Replace the contents of an Either Int Int with unit, resulting in an Either Int '()':

>>> void (Left 8675309)
Left 8675309
>>> void (Right 8675309)
Right ()

Replace every element of a list with unit:

>>> void [1,2,3]
[(),(),()]

Replace the second element of a pair with unit:

>>> void (1,2)
(1,())

Discard the result of an IO action:

>>> mapM print [1,2]
1
2
[(),()]
>>> void $ mapM print [1,2]
1
2

Generalisations of list functions

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.

msum :: (Foldable t, MonadPlus m) => t (m a) -> m a #

The sum of a collection of actions, generalizing concat. As of base 4.8.0.0, msum is just asum, specialized to MonadPlus.

mfilter :: MonadPlus m => (a -> Bool) -> m a -> m a #

Direct MonadPlus equivalent of filter filter = (mfilter:: (a -> Bool) -> [a] -> [a] applicable to any MonadPlus, for example mfilter odd (Just 1) == Just 1 mfilter odd (Just 2) == Nothing

filterM :: Applicative m => (a -> m Bool) -> [a] -> m [a] #

This generalizes the list-based filter function.

mapAndUnzipM :: Applicative m => (a -> m (b, c)) -> [a] -> m ([b], [c]) #

The mapAndUnzipM function maps its first argument over a list, returning the result as a pair of lists. This function is mainly used with complicated data structures or a state-transforming monad.

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

The zipWithM function generalizes zipWith to arbitrary applicative functors.

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

zipWithM_ is the extension of zipWithM which ignores the final 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

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

Like foldM, but discards the result.

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

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

replicateM_ :: Applicative m => Int -> m a -> m () #

Like replicateM, but discards the result.

Conditional execution of monadic expressions

guard :: Alternative f => Bool -> f () #

guard b is pure () if b is True, and empty if b is False.

when :: Applicative f => Bool -> f () -> f () #

Conditional execution of Applicative expressions. For example,

when debug (putStrLn "Debugging")

will output the string Debugging if the Boolean value debug is True, and otherwise do nothing.

unless :: Applicative f => Bool -> f () -> f () #

The reverse of when.

Monadic lifting operators

liftM :: Monad m => (a1 -> r) -> m a1 -> m r #

Promote a function to a monad.

liftM2 :: Monad m => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r #

Promote a function to a monad, scanning the monadic arguments from left to right. For example,

   liftM2 (+) [0,1] [0,2] = [0,2,1,3]
   liftM2 (+) (Just 1) Nothing = Nothing

liftM3 :: Monad m => (a1 -> a2 -> a3 -> r) -> m a1 -> m a2 -> m a3 -> m r #

Promote a function to a monad, scanning the monadic arguments from left to right (cf. liftM2).

liftM4 :: Monad m => (a1 -> a2 -> a3 -> a4 -> r) -> m a1 -> m a2 -> m a3 -> m a4 -> m r #

Promote a function to a monad, scanning the monadic arguments from left to right (cf. liftM2).

liftM5 :: Monad m => (a1 -> a2 -> a3 -> a4 -> a5 -> r) -> m a1 -> m a2 -> m a3 -> m a4 -> m a5 -> m r #

Promote a function to a monad, scanning the monadic arguments from left to right (cf. liftM2).

ap :: Monad m => m (a -> b) -> m a -> m b #

In many situations, the liftM operations can be replaced by uses of ap, which promotes function application.

      return f `ap` x1 `ap` ... `ap` xn

is equivalent to

      liftMn f x1 x2 ... xn

Strict monadic functions

(<$!>) :: Monad m => (a -> b) -> m a -> m b infixl 4 #

Strict version of <$>.

Since: 4.8.0.0