Relude.Monad.Reexport

Reexports functions to work with monads.

Reexport transformers

newtype ExceptT e (m :: Type -> Type) a

A monad transformer that adds exceptions to other monads.

ExceptT constructs a monad parameterized over two things:

• e - The exception type.
• m - The inner monad.

The return function yields a computation that produces the given value, while >>= sequences two subcomputations, exiting on the first exception.

Constructors

ExceptT (m (Either e a))

Instances

MonadState s m => MonadState s (ExceptT e m)

Since: mtl-2.2

Defined in Control.Monad.State.Class

Methods

get :: ExceptT e m s

put :: s -> ExceptT e m ()

state :: (s -> (a, s)) -> ExceptT e m a

MonadReader r m => MonadReader r (ExceptT e m)

Since: mtl-2.2

Defined in Control.Monad.Reader.Class

Methods

ask :: ExceptT e m r

local :: (r -> r) -> ExceptT e m a -> ExceptT e m a

reader :: (r -> a) -> ExceptT e m a

MonadTrans (ExceptT e)

Defined in Control.Monad.Trans.Except

Methods

lift :: Monad m => m a -> ExceptT e m 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

Functor m => Functor (ExceptT e m)

Defined in Control.Monad.Trans.Except

Methods

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

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

MonadFix m => MonadFix (ExceptT e m)

Defined in Control.Monad.Trans.Except

Methods

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

MonadFail m => MonadFail (ExceptT e m)

Defined in Control.Monad.Trans.Except

Methods

fail :: String -> ExceptT e m a

(Functor m, Monad m) => Applicative (ExceptT e m)

Defined in Control.Monad.Trans.Except

Methods

pure :: a -> ExceptT e m a

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

liftA2 :: (a -> b -> c) -> ExceptT e m a -> ExceptT e m b -> ExceptT e m c

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

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

Foldable f => Foldable (ExceptT e f)

Defined in Control.Monad.Trans.Except

Methods

fold :: Monoid m => ExceptT e f m -> m

foldMap :: Monoid m => (a -> m) -> ExceptT e f a -> m

foldMap' :: Monoid m => (a -> m) -> ExceptT e f a -> m

foldr :: (a -> b -> b) -> b -> ExceptT e f a -> b

foldr' :: (a -> b -> b) -> b -> ExceptT e f a -> b

foldl :: (b -> a -> b) -> b -> ExceptT e f a -> b

foldl' :: (b -> a -> b) -> b -> ExceptT e f a -> b

foldr1 :: (a -> a -> a) -> ExceptT e f a -> a

foldl1 :: (a -> a -> a) -> ExceptT e f a -> a

toList :: ExceptT e f a -> [a]

null :: ExceptT e f a -> Bool

length :: ExceptT e f a -> Int

elem :: Eq a => a -> ExceptT e f a -> Bool

maximum :: Ord a => ExceptT e f a -> a

minimum :: Ord a => ExceptT e f a -> a

sum :: Num a => ExceptT e f a -> a

product :: Num a => ExceptT e f a -> a

Traversable f => Traversable (ExceptT e f)

Defined in Control.Monad.Trans.Except

Methods

traverse :: Applicative f0 => (a -> f0 b) -> ExceptT e f a -> f0 (ExceptT e f b)

sequenceA :: Applicative f0 => ExceptT e f (f0 a) -> f0 (ExceptT e f a)

mapM :: Monad m => (a -> m b) -> ExceptT e f a -> m (ExceptT e f b)

sequence :: Monad m => ExceptT e f (m a) -> m (ExceptT e f a)

Contravariant m => Contravariant (ExceptT e m)

Defined in Control.Monad.Trans.Except

Methods

contramap :: (a' -> a) -> ExceptT e m a -> ExceptT e m a'

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

(Eq e, Eq1 m) => Eq1 (ExceptT e m)

Defined in Control.Monad.Trans.Except

Methods

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

(Ord e, Ord1 m) => Ord1 (ExceptT e m)

Defined in Control.Monad.Trans.Except

Methods

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

(Read e, Read1 m) => Read1 (ExceptT e m)

Defined in Control.Monad.Trans.Except

Methods

liftReadsPrec :: (Int -> ReadS a) -> ReadS [a] -> Int -> ReadS (ExceptT e m a)

liftReadList :: (Int -> ReadS a) -> ReadS [a] -> ReadS [ExceptT e m a]

liftReadPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec (ExceptT e m a)

liftReadListPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec [ExceptT e m a]

(Show e, Show1 m) => Show1 (ExceptT e m)

Defined in Control.Monad.Trans.Except

Methods

liftShowsPrec :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> Int -> ExceptT e m a -> ShowS

liftShowList :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> [ExceptT e m a] -> ShowS

MonadZip m => MonadZip (ExceptT e m)

Defined in Control.Monad.Trans.Except

Methods

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

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

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

MonadIO m => MonadIO (ExceptT e m)

Defined in Control.Monad.Trans.Except

Methods

liftIO :: IO a -> ExceptT e m a

(Functor m, Monad m, Monoid e) => Alternative (ExceptT e m)

Defined in Control.Monad.Trans.Except

Methods

empty :: ExceptT e m a

(<|>) :: ExceptT e m a -> ExceptT e m a -> ExceptT e m a

some :: ExceptT e m a -> ExceptT e m [a]

many :: ExceptT e m a -> ExceptT e 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

(Eq e, Eq1 m, Eq a) => Eq (ExceptT e m a)

Defined in Control.Monad.Trans.Except

Methods

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

(/=) :: ExceptT e m a -> ExceptT e m a -> Bool

(Ord e, Ord1 m, Ord a) => Ord (ExceptT e m a)

Defined in Control.Monad.Trans.Except

Methods

compare :: ExceptT e m a -> ExceptT e m a -> Ordering

(<) :: ExceptT e m a -> ExceptT e m a -> Bool

(<=) :: ExceptT e m a -> ExceptT e m a -> Bool

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

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

max :: ExceptT e m a -> ExceptT e m a -> ExceptT e m a

min :: ExceptT e m a -> ExceptT e m a -> ExceptT e m a

(Read e, Read1 m, Read a) => Read (ExceptT e m a)

Defined in Control.Monad.Trans.Except

Methods

readsPrec :: Int -> ReadS (ExceptT e m a)

readList :: ReadS [ExceptT e m a]

readPrec :: ReadPrec (ExceptT e m a)

readListPrec :: ReadPrec [ExceptT e m a]

(Show e, Show1 m, Show a) => Show (ExceptT e m a)

Defined in Control.Monad.Trans.Except

Methods

showsPrec :: Int -> ExceptT e m a -> ShowS

show :: ExceptT e m a -> String

showList :: [ExceptT e m a] -> ShowS

runExceptT :: ExceptT e m a -> m (Either e a)

The inverse of ExceptT.

asks

Arguments

:: MonadReader r m

=> (r -> a)

The selector function to apply to the environment.

-> m a

Retrieves a function of the current environment.

class Monad m => MonadReader r (m :: Type -> Type) | m -> r where

See examples in Control.Monad.Reader. Note, the partially applied function type (->) r is a simple reader monad. See the instance declaration below.

Minimal complete definition

(ask | reader), local

Methods

ask :: m r

Retrieves the monad environment.

local

Arguments

:: (r -> r)

The function to modify the environment.

-> m a

Reader to run in the modified environment.

-> m a

Executes a computation in a modified environment.

reader

Arguments

:: (r -> a)

The selector function to apply to the environment.

-> m a

Retrieves a function of the current environment.

Instances

MonadReader r m => MonadReader r (MaybeT m)

Defined in Control.Monad.Reader.Class

Methods

ask :: MaybeT m r

local :: (r -> r) -> MaybeT m a -> MaybeT m a

reader :: (r -> a) -> MaybeT m a

MonadReader r m => MonadReader r (ListT m)

Defined in Control.Monad.Reader.Class

Methods

ask :: ListT m r

local :: (r -> r) -> ListT m a -> ListT m a

reader :: (r -> a) -> ListT m a

(Monoid w, MonadReader r m) => MonadReader r (WriterT w m)

Defined in Control.Monad.Reader.Class

Methods

ask :: WriterT w m r

local :: (r -> r) -> WriterT w m a -> WriterT w m a

reader :: (r -> a) -> WriterT w m a

(Monoid w, MonadReader r m) => MonadReader r (WriterT w m)

Defined in Control.Monad.Reader.Class

Methods

ask :: WriterT w m r

local :: (r -> r) -> WriterT w m a -> WriterT w m a

reader :: (r -> a) -> WriterT w m a

MonadReader r m => MonadReader r (StateT s m)

Defined in Control.Monad.Reader.Class

Methods

ask :: StateT s m r

local :: (r -> r) -> StateT s m a -> StateT s m a

reader :: (r -> a) -> StateT s m a

MonadReader r m => MonadReader r (StateT s m)

Defined in Control.Monad.Reader.Class

Methods

ask :: StateT s m r

local :: (r -> r) -> StateT s m a -> StateT s m a

reader :: (r -> a) -> StateT s m a

Monad m => MonadReader r (ReaderT r m)

Defined in Control.Monad.Reader.Class

Methods

ask :: ReaderT r m r

local :: (r -> r) -> ReaderT r m a -> ReaderT r m a

reader :: (r -> a) -> ReaderT r m a

MonadReader r m => MonadReader r (IdentityT m)

Defined in Control.Monad.Reader.Class

Methods

ask :: IdentityT m r

local :: (r -> r) -> IdentityT m a -> IdentityT m a

reader :: (r -> a) -> IdentityT m a

MonadReader r m => MonadReader r (ExceptT e m)

Since: mtl-2.2

Defined in Control.Monad.Reader.Class

Methods

ask :: ExceptT e m r

local :: (r -> r) -> ExceptT e m a -> ExceptT e m a

reader :: (r -> a) -> ExceptT e m a

(Error e, MonadReader r m) => MonadReader r (ErrorT e m)

Defined in Control.Monad.Reader.Class

Methods

ask :: ErrorT e m r

local :: (r -> r) -> ErrorT e m a -> ErrorT e m a

reader :: (r -> a) -> ErrorT e m a

MonadReader r ((->) r)

Defined in Control.Monad.Reader.Class

Methods

ask :: r -> r

local :: (r -> r) -> (r -> a) -> r -> a

reader :: (r -> a) -> r -> a

MonadReader r' m => MonadReader r' (ContT r m)

Defined in Control.Monad.Reader.Class

Methods

ask :: ContT r m r'

local :: (r' -> r') -> ContT r m a -> ContT r m a

reader :: (r' -> a) -> ContT r m a

(Monad m, Monoid w) => MonadReader r (RWST r w s m)

Defined in Control.Monad.Reader.Class

Methods

ask :: RWST r w s m r

local :: (r -> r) -> RWST r w s m a -> RWST r w s m a

reader :: (r -> a) -> RWST r w s m a

(Monad m, Monoid w) => MonadReader r (RWST r w s m)

Defined in Control.Monad.Reader.Class

Methods

ask :: RWST r w s m r

local :: (r -> r) -> RWST r w s m a -> RWST r w s m a

reader :: (r -> a) -> RWST r w s m a

newtype ReaderT r (m :: Type -> Type) a

The reader monad transformer, which adds a read-only environment to the given monad.

The return function ignores the environment, while >>= passes the inherited environment to both subcomputations.

Constructors

ReaderT

Fields

• runReaderT :: r -> m a

Instances

MonadState s m => MonadState s (ReaderT r m)

Defined in Control.Monad.State.Class

Methods

get :: ReaderT r m s

put :: s -> ReaderT r m ()

state :: (s -> (a, s)) -> ReaderT r m a

Monad m => MonadReader r (ReaderT r m)

Defined in Control.Monad.Reader.Class

Methods

ask :: ReaderT r m r

local :: (r -> r) -> ReaderT r m a -> ReaderT r m a

reader :: (r -> a) -> ReaderT r m a

MonadTrans (ReaderT r)

Defined in Control.Monad.Trans.Reader

Methods

lift :: Monad m => m a -> ReaderT r m a

Monad m => Monad (ReaderT r m)

Defined in Control.Monad.Trans.Reader

Methods

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

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

return :: a -> ReaderT r m a

Functor m => Functor (ReaderT r m)

Defined in Control.Monad.Trans.Reader

Methods

fmap :: (a -> b) -> ReaderT r m a -> ReaderT r m b

(<$) :: a -> ReaderT r m b -> ReaderT r m a

MonadFix m => MonadFix (ReaderT r m)

Defined in Control.Monad.Trans.Reader

Methods

mfix :: (a -> ReaderT r m a) -> ReaderT r m a

MonadFail m => MonadFail (ReaderT r m)

Defined in Control.Monad.Trans.Reader

Methods

fail :: String -> ReaderT r m a

Applicative m => Applicative (ReaderT r m)

Defined in Control.Monad.Trans.Reader

Methods

pure :: a -> ReaderT r m a

(<*>) :: ReaderT r m (a -> b) -> ReaderT r m a -> ReaderT r m b

liftA2 :: (a -> b -> c) -> ReaderT r m a -> ReaderT r m b -> ReaderT r m c

(*>) :: ReaderT r m a -> ReaderT r m b -> ReaderT r m b

(<*) :: ReaderT r m a -> ReaderT r m b -> ReaderT r m a

Contravariant m => Contravariant (ReaderT r m)

Defined in Control.Monad.Trans.Reader

Methods

contramap :: (a' -> a) -> ReaderT r m a -> ReaderT r m a'

(>$) :: b -> ReaderT r m b -> ReaderT r m a

MonadZip m => MonadZip (ReaderT r m)

Defined in Control.Monad.Trans.Reader

Methods

mzip :: ReaderT r m a -> ReaderT r m b -> ReaderT r m (a, b)

mzipWith :: (a -> b -> c) -> ReaderT r m a -> ReaderT r m b -> ReaderT r m c

munzip :: ReaderT r m (a, b) -> (ReaderT r m a, ReaderT r m b)

MonadIO m => MonadIO (ReaderT r m)

Defined in Control.Monad.Trans.Reader

Methods

liftIO :: IO a -> ReaderT r m a

Alternative m => Alternative (ReaderT r m)

Defined in Control.Monad.Trans.Reader

Methods

empty :: ReaderT r m a

(<|>) :: ReaderT r m a -> ReaderT r m a -> ReaderT r m a

some :: ReaderT r m a -> ReaderT r m [a]

many :: ReaderT r m a -> ReaderT r m [a]

MonadPlus m => MonadPlus (ReaderT r m)

Defined in Control.Monad.Trans.Reader

Methods

mzero :: ReaderT r m a

mplus :: ReaderT r m a -> ReaderT r m a -> ReaderT r m a

type Reader r = ReaderT r Identity

The parameterizable reader monad.

Computations are functions of a shared environment.

The return function ignores the environment, while >>= passes the inherited environment to both subcomputations.

runReader

Arguments

:: Reader r a

A Reader to run.

-> r

An initial environment.

-> a

Runs a Reader and extracts the final value from it. (The inverse of reader.)

withReader

Arguments

:: (r' -> r)

The function to modify the environment.

-> Reader r a

Computation to run in the modified environment.

-> Reader r' a

Execute a computation in a modified environment (a specialization of withReaderT).

• runReader (withReader f m) = runReader m . f

withReaderT

Arguments

:: forall r' r (m :: Type -> Type) a. (r' -> r)

The function to modify the environment.

-> ReaderT r m a

Computation to run in the modified environment.

-> ReaderT r' m a

Execute a computation in a modified environment (a more general version of local).

• runReaderT (withReaderT f m) = runReaderT m . f

gets :: MonadState s m => (s -> a) -> m a

Gets specific component of the state, using a projection function supplied.

modify' :: MonadState s m => (s -> s) -> m ()

A variant of modify in which the computation is strict in the new state.

Since: mtl-2.2

modify :: MonadState s m => (s -> s) -> m ()

Monadic state transformer.

Maps an old state to a new state inside a state monad. The old state is thrown away.

     Main> :t modify ((+1) :: Int -> Int)
     modify (...) :: (MonadState Int a) => a ()

This says that modify (+1) acts over any Monad that is a member of the MonadState class, with an Int state.

class Monad m => MonadState s (m :: Type -> Type) | m -> s where

Minimal definition is either both of get and put or just state

Minimal complete definition

state | get, put

Methods

get :: m s

Return the state from the internals of the monad.

put :: s -> m ()

Replace the state inside the monad.

state :: (s -> (a, s)) -> m a

Embed a simple state action into the monad.

Instances

MonadState s m => MonadState s (MaybeT m)

Defined in Control.Monad.State.Class

Methods

get :: MaybeT m s

put :: s -> MaybeT m ()

state :: (s -> (a, s)) -> MaybeT m a

MonadState s m => MonadState s (ListT m)

Defined in Control.Monad.State.Class

Methods

get :: ListT m s

put :: s -> ListT m ()

state :: (s -> (a, s)) -> ListT m a

(Monoid w, MonadState s m) => MonadState s (WriterT w m)

Defined in Control.Monad.State.Class

Methods

get :: WriterT w m s

put :: s -> WriterT w m ()

state :: (s -> (a, s)) -> WriterT w m a

(Monoid w, MonadState s m) => MonadState s (WriterT w m)

Defined in Control.Monad.State.Class

Methods

get :: WriterT w m s

put :: s -> WriterT w m ()

state :: (s -> (a, s)) -> WriterT w m a

Monad m => MonadState s (StateT s m)

Defined in Control.Monad.State.Class

Methods

get :: StateT s m s

put :: s -> StateT s m ()

state :: (s -> (a, s)) -> StateT s m a

Monad m => MonadState s (StateT s m)

Defined in Control.Monad.State.Class

Methods

get :: StateT s m s

put :: s -> StateT s m ()

state :: (s -> (a, s)) -> StateT s m a

MonadState s m => MonadState s (ReaderT r m)

Defined in Control.Monad.State.Class

Methods

get :: ReaderT r m s

put :: s -> ReaderT r m ()

state :: (s -> (a, s)) -> ReaderT r m a

MonadState s m => MonadState s (IdentityT m)

Defined in Control.Monad.State.Class

Methods

get :: IdentityT m s

put :: s -> IdentityT m ()

state :: (s -> (a, s)) -> IdentityT m a

MonadState s m => MonadState s (ExceptT e m)

Since: mtl-2.2

Defined in Control.Monad.State.Class

Methods

get :: ExceptT e m s

put :: s -> ExceptT e m ()

state :: (s -> (a, s)) -> ExceptT e m a

(Error e, MonadState s m) => MonadState s (ErrorT e m)

Defined in Control.Monad.State.Class

Methods

get :: ErrorT e m s

put :: s -> ErrorT e m ()

state :: (s -> (a, s)) -> ErrorT e m a

MonadState s m => MonadState s (ContT r m)

Defined in Control.Monad.State.Class

Methods

get :: ContT r m s

put :: s -> ContT r m ()

state :: (s -> (a, s)) -> ContT r m a

(Monad m, Monoid w) => MonadState s (RWST r w s m)

Defined in Control.Monad.State.Class

Methods

get :: RWST r w s m s

put :: s -> RWST r w s m ()

state :: (s -> (a, s)) -> RWST r w s m a

(Monad m, Monoid w) => MonadState s (RWST r w s m)

Defined in Control.Monad.State.Class

Methods

get :: RWST r w s m s

put :: s -> RWST r w s m ()

state :: (s -> (a, s)) -> RWST r w s m a

newtype StateT s (m :: Type -> Type) a

A state transformer monad parameterized by:

• s - The state.
• m - The inner monad.

The return function leaves the state unchanged, while >>= uses the final state of the first computation as the initial state of the second.

Constructors

StateT

Fields

• runStateT :: s -> m (a, s)

Instances

Monad m => MonadState s (StateT s m)

Defined in Control.Monad.State.Class

Methods

get :: StateT s m s

put :: s -> StateT s m ()

state :: (s -> (a, s)) -> StateT s m a

MonadReader r m => MonadReader r (StateT s m)

Defined in Control.Monad.Reader.Class

Methods

ask :: StateT s m r

local :: (r -> r) -> StateT s m a -> StateT s m a

reader :: (r -> a) -> StateT s m a

MonadTrans (StateT s)

Defined in Control.Monad.Trans.State.Strict

Methods

lift :: Monad m => m a -> StateT s m a

Monad m => Monad (StateT s m)

Defined in Control.Monad.Trans.State.Strict

Methods

(>>=) :: StateT s m a -> (a -> StateT s m b) -> StateT s m b

(>>) :: StateT s m a -> StateT s m b -> StateT s m b

return :: a -> StateT s m a

Functor m => Functor (StateT s m)

Defined in Control.Monad.Trans.State.Strict

Methods

fmap :: (a -> b) -> StateT s m a -> StateT s m b

(<$) :: a -> StateT s m b -> StateT s m a

MonadFix m => MonadFix (StateT s m)

Defined in Control.Monad.Trans.State.Strict

Methods

mfix :: (a -> StateT s m a) -> StateT s m a

MonadFail m => MonadFail (StateT s m)

Defined in Control.Monad.Trans.State.Strict

Methods

fail :: String -> StateT s m a

(Functor m, Monad m) => Applicative (StateT s m)

Defined in Control.Monad.Trans.State.Strict

Methods

pure :: a -> StateT s m a

(<*>) :: StateT s m (a -> b) -> StateT s m a -> StateT s m b

liftA2 :: (a -> b -> c) -> StateT s m a -> StateT s m b -> StateT s m c

(*>) :: StateT s m a -> StateT s m b -> StateT s m b

(<*) :: StateT s m a -> StateT s m b -> StateT s m a

Contravariant m => Contravariant (StateT s m)

Defined in Control.Monad.Trans.State.Strict

Methods

contramap :: (a' -> a) -> StateT s m a -> StateT s m a'

(>$) :: b -> StateT s m b -> StateT s m a

MonadIO m => MonadIO (StateT s m)

Defined in Control.Monad.Trans.State.Strict

Methods

liftIO :: IO a -> StateT s m a

(Functor m, MonadPlus m) => Alternative (StateT s m)

Defined in Control.Monad.Trans.State.Strict

Methods

empty :: StateT s m a

(<|>) :: StateT s m a -> StateT s m a -> StateT s m a

some :: StateT s m a -> StateT s m [a]

many :: StateT s m a -> StateT s m [a]

MonadPlus m => MonadPlus (StateT s m)

Defined in Control.Monad.Trans.State.Strict

Methods

mzero :: StateT s m a

mplus :: StateT s m a -> StateT s m a -> StateT s m a

type State s = StateT s Identity

A state monad parameterized by the type s of the state to carry.

The return function leaves the state unchanged, while >>= uses the final state of the first computation as the initial state of the second.

runState

Arguments

:: State s a

state-passing computation to execute

-> s

initial state

-> (a, s)

return value and final state

Unwrap a state monad computation as a function. (The inverse of state.)

evalState

Arguments

:: State s a

state-passing computation to execute

-> s

initial value

-> a

return value of the state computation

Evaluate a state computation with the given initial state and return the final value, discarding the final state.

• evalState m s = fst (runState m s)

execState

Arguments

:: State s a

state-passing computation to execute

-> s

initial value

-> s

final state

Evaluate a state computation with the given initial state and return the final state, discarding the final value.

• execState m s = snd (runState m s)

withState :: (s -> s) -> State s a -> State s a

withState f m executes action m on a state modified by applying f.

• withState f m = modify f >> m

evalStateT :: Monad m => StateT s m a -> s -> m a

Evaluate a state computation with the given initial state and return the final value, discarding the final state.

• evalStateT m s = liftM fst (runStateT m s)

execStateT :: Monad m => StateT s m a -> s -> m s

Evaluate a state computation with the given initial state and return the final state, discarding the final value.

• execStateT m s = liftM snd (runStateT m s)

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

Monads in which IO computations may be embedded. Any monad built by applying a sequence of monad transformers to the IO monad will be an instance of this class.

Instances should satisfy the following laws, which state that liftIO is a transformer of monads:

• liftIO . return = return

• liftIO (m >>= f) = liftIO m >>= (liftIO . f)

Methods

liftIO :: IO a -> m a

Lift a computation from the IO monad. This allows us to run IO computations in any monadic stack, so long as it supports these kinds of operations (i.e. IO is the base monad for the stack).

Example

import Control.Monad.Trans.State -- from the "transformers" library

printState :: Show s => StateT s IO ()
printState = do
  state <- get
  liftIO $ print state

Had we omitted liftIO, we would have ended up with this error:

• Couldn't match type ‘IO’ with ‘StateT s IO’
 Expected type: StateT s IO ()
   Actual type: IO ()

The important part here is the mismatch between StateT s IO () and IO ().

Luckily, we know of a function that takes an IO a and returns an (m a): liftIO, enabling us to run the program and see the expected results:

> evalStateT printState "hello"
"hello"

> evalStateT printState 3
3

Instances

MonadIO IO

Since: base-4.9.0.0

Defined in Control.Monad.IO.Class

Methods

liftIO :: IO a -> IO a

MonadIO Q

Defined in Language.Haskell.TH.Syntax

Methods

liftIO :: IO a -> Q a

MonadIO m => MonadIO (MaybeT m)

Defined in Control.Monad.Trans.Maybe

Methods

liftIO :: IO a -> MaybeT m a

MonadIO m => MonadIO (IdentityT m)

Defined in Control.Monad.Trans.Identity

Methods

liftIO :: IO a -> IdentityT m a

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

Defined in Control.Monad.Trans.Error

Methods

liftIO :: IO a -> ErrorT e m a

MonadIO m => MonadIO (ExceptT e m)

Defined in Control.Monad.Trans.Except

Methods

liftIO :: IO a -> ExceptT e m a

MonadIO m => MonadIO (ReaderT r m)

Defined in Control.Monad.Trans.Reader

Methods

liftIO :: IO a -> ReaderT r m a

MonadIO m => MonadIO (StateT s m)

Defined in Control.Monad.Trans.State.Strict

Methods

liftIO :: IO a -> StateT s m a

class MonadTrans (t :: (Type -> Type) -> Type -> Type) where

The class of monad transformers. Instances should satisfy the following laws, which state that lift is a monad transformation:

• lift . return = return

• lift (m >>= f) = lift m >>= (lift . f)

Methods

lift :: Monad m => m a -> t m a

Lift a computation from the argument monad to the constructed monad.

Instances

MonadTrans MaybeT

Defined in Control.Monad.Trans.Maybe

Methods

lift :: Monad m => m a -> MaybeT m a

MonadTrans (IdentityT :: (Type -> Type) -> Type -> Type)

Defined in Control.Monad.Trans.Identity

Methods

lift :: Monad m => m a -> IdentityT m a

MonadTrans (ErrorT e)

Defined in Control.Monad.Trans.Error

Methods

lift :: Monad m => m a -> ErrorT e m a

MonadTrans (ExceptT e)

Defined in Control.Monad.Trans.Except

Methods

lift :: Monad m => m a -> ExceptT e m a

MonadTrans (ReaderT r)

Defined in Control.Monad.Trans.Reader

Methods

lift :: Monad m => m a -> ReaderT r m a

MonadTrans (StateT s)

Defined in Control.Monad.Trans.State.Strict

Methods

lift :: Monad m => m a -> StateT s m a

data IdentityT (f :: k -> Type) (a :: k)

The trivial monad transformer, which maps a monad to an equivalent monad.

Instances

MonadState s m => MonadState s (IdentityT m)

Defined in Control.Monad.State.Class

Methods

get :: IdentityT m s

put :: s -> IdentityT m ()

state :: (s -> (a, s)) -> IdentityT m a

MonadReader r m => MonadReader r (IdentityT m)

Defined in Control.Monad.Reader.Class

Methods

ask :: IdentityT m r

local :: (r -> r) -> IdentityT m a -> IdentityT m a

reader :: (r -> a) -> IdentityT m a

MonadTrans (IdentityT :: (Type -> Type) -> Type -> Type)

Defined in Control.Monad.Trans.Identity

Methods

lift :: Monad m => m a -> IdentityT m 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

Functor m => Functor (IdentityT m)

Defined in Control.Monad.Trans.Identity

Methods

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

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

MonadFix m => MonadFix (IdentityT m)

Defined in Control.Monad.Trans.Identity

Methods

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

MonadFail m => MonadFail (IdentityT m)

Defined in Control.Monad.Trans.Identity

Methods

fail :: String -> IdentityT m a

Applicative m => Applicative (IdentityT m)

Defined in Control.Monad.Trans.Identity

Methods

pure :: a -> IdentityT m a

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

liftA2 :: (a -> b -> c) -> IdentityT m a -> IdentityT m b -> IdentityT m c

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

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

Foldable f => Foldable (IdentityT f)

Defined in Control.Monad.Trans.Identity

Methods

fold :: Monoid m => IdentityT f m -> m

foldMap :: Monoid m => (a -> m) -> IdentityT f a -> m

foldMap' :: Monoid m => (a -> m) -> IdentityT f a -> m

foldr :: (a -> b -> b) -> b -> IdentityT f a -> b

foldr' :: (a -> b -> b) -> b -> IdentityT f a -> b

foldl :: (b -> a -> b) -> b -> IdentityT f a -> b

foldl' :: (b -> a -> b) -> b -> IdentityT f a -> b

foldr1 :: (a -> a -> a) -> IdentityT f a -> a

foldl1 :: (a -> a -> a) -> IdentityT f a -> a

toList :: IdentityT f a -> [a]

null :: IdentityT f a -> Bool

length :: IdentityT f a -> Int

elem :: Eq a => a -> IdentityT f a -> Bool

maximum :: Ord a => IdentityT f a -> a

minimum :: Ord a => IdentityT f a -> a

sum :: Num a => IdentityT f a -> a

product :: Num a => IdentityT f a -> a

Traversable f => Traversable (IdentityT f)

Defined in Control.Monad.Trans.Identity

Methods

traverse :: Applicative f0 => (a -> f0 b) -> IdentityT f a -> f0 (IdentityT f b)

sequenceA :: Applicative f0 => IdentityT f (f0 a) -> f0 (IdentityT f a)

mapM :: Monad m => (a -> m b) -> IdentityT f a -> m (IdentityT f b)

sequence :: Monad m => IdentityT f (m a) -> m (IdentityT f a)

Contravariant f => Contravariant (IdentityT f)

Defined in Control.Monad.Trans.Identity

Methods

contramap :: (a' -> a) -> IdentityT f a -> IdentityT f a'

(>$) :: b -> IdentityT f b -> IdentityT f a

Eq1 f => Eq1 (IdentityT f)

Defined in Control.Monad.Trans.Identity

Methods

liftEq :: (a -> b -> Bool) -> IdentityT f a -> IdentityT f b -> Bool

Ord1 f => Ord1 (IdentityT f)

Defined in Control.Monad.Trans.Identity

Methods

liftCompare :: (a -> b -> Ordering) -> IdentityT f a -> IdentityT f b -> Ordering

Read1 f => Read1 (IdentityT f)

Defined in Control.Monad.Trans.Identity

Methods

liftReadsPrec :: (Int -> ReadS a) -> ReadS [a] -> Int -> ReadS (IdentityT f a)

liftReadList :: (Int -> ReadS a) -> ReadS [a] -> ReadS [IdentityT f a]

liftReadPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec (IdentityT f a)

liftReadListPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec [IdentityT f a]

Show1 f => Show1 (IdentityT f)

Defined in Control.Monad.Trans.Identity

Methods

liftShowsPrec :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> Int -> IdentityT f a -> ShowS

liftShowList :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> [IdentityT f a] -> ShowS

MonadZip m => MonadZip (IdentityT m)

Defined in Control.Monad.Trans.Identity

Methods

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

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

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

MonadIO m => MonadIO (IdentityT m)

Defined in Control.Monad.Trans.Identity

Methods

liftIO :: IO a -> IdentityT m a

Alternative m => Alternative (IdentityT m)

Defined in Control.Monad.Trans.Identity

Methods

empty :: IdentityT m a

(<|>) :: IdentityT m a -> IdentityT m a -> IdentityT m a

some :: IdentityT m a -> IdentityT m [a]

many :: IdentityT m a -> IdentityT m [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

(Eq1 f, Eq a) => Eq (IdentityT f a)

Defined in Control.Monad.Trans.Identity

Methods

(==) :: IdentityT f a -> IdentityT f a -> Bool

(/=) :: IdentityT f a -> IdentityT f a -> Bool

(Ord1 f, Ord a) => Ord (IdentityT f a)

Defined in Control.Monad.Trans.Identity

Methods

compare :: IdentityT f a -> IdentityT f a -> Ordering

(<) :: IdentityT f a -> IdentityT f a -> Bool

(<=) :: IdentityT f a -> IdentityT f a -> Bool

(>) :: IdentityT f a -> IdentityT f a -> Bool

(>=) :: IdentityT f a -> IdentityT f a -> Bool

max :: IdentityT f a -> IdentityT f a -> IdentityT f a

min :: IdentityT f a -> IdentityT f a -> IdentityT f a

(Read1 f, Read a) => Read (IdentityT f a)

Defined in Control.Monad.Trans.Identity

Methods

readsPrec :: Int -> ReadS (IdentityT f a)

readList :: ReadS [IdentityT f a]

readPrec :: ReadPrec (IdentityT f a)

readListPrec :: ReadPrec [IdentityT f a]

(Show1 f, Show a) => Show (IdentityT f a)

Defined in Control.Monad.Trans.Identity

Methods

showsPrec :: Int -> IdentityT f a -> ShowS

show :: IdentityT f a -> String

showList :: [IdentityT f a] -> ShowS

exceptToMaybeT :: forall (m :: Type -> Type) e a. Functor m => ExceptT e m a -> MaybeT m a

Convert a ExceptT computation to MaybeT, discarding the value of any exception.

maybeToExceptT :: forall (m :: Type -> Type) e a. Functor m => e -> MaybeT m a -> ExceptT e m a

Convert a MaybeT computation to ExceptT, with a default exception value.

newtype MaybeT (m :: Type -> Type) a

The parameterizable maybe monad, obtained by composing an arbitrary monad with the Maybe monad.

Computations are actions that may produce a value or exit.

The return function yields a computation that produces that value, while >>= sequences two subcomputations, exiting if either computation does.

Constructors

MaybeT

Fields

• runMaybeT :: m (Maybe a)

Instances

MonadTrans MaybeT

Defined in Control.Monad.Trans.Maybe

Methods

lift :: Monad m => m a -> MaybeT m a

MonadState s m => MonadState s (MaybeT m)

Defined in Control.Monad.State.Class

Methods

get :: MaybeT m s

put :: s -> MaybeT m ()

state :: (s -> (a, s)) -> MaybeT m a

MonadReader r m => MonadReader r (MaybeT m)

Defined in Control.Monad.Reader.Class

Methods

ask :: MaybeT m r

local :: (r -> r) -> MaybeT m a -> MaybeT m a

reader :: (r -> a) -> MaybeT m 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

Functor m => Functor (MaybeT m)

Defined in Control.Monad.Trans.Maybe

Methods

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

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

MonadFix m => MonadFix (MaybeT m)

Defined in Control.Monad.Trans.Maybe

Methods

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

Monad m => MonadFail (MaybeT m)

Defined in Control.Monad.Trans.Maybe

Methods

fail :: String -> MaybeT m a

(Functor m, Monad m) => Applicative (MaybeT m)

Defined in Control.Monad.Trans.Maybe

Methods

pure :: a -> MaybeT m a

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

liftA2 :: (a -> b -> c) -> MaybeT m a -> MaybeT m b -> MaybeT m c

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

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

Foldable f => Foldable (MaybeT f)

Defined in Control.Monad.Trans.Maybe

Methods

fold :: Monoid m => MaybeT f m -> m

foldMap :: Monoid m => (a -> m) -> MaybeT f a -> m

foldMap' :: Monoid m => (a -> m) -> MaybeT f a -> m

foldr :: (a -> b -> b) -> b -> MaybeT f a -> b

foldr' :: (a -> b -> b) -> b -> MaybeT f a -> b

foldl :: (b -> a -> b) -> b -> MaybeT f a -> b

foldl' :: (b -> a -> b) -> b -> MaybeT f a -> b

foldr1 :: (a -> a -> a) -> MaybeT f a -> a

foldl1 :: (a -> a -> a) -> MaybeT f a -> a

toList :: MaybeT f a -> [a]

null :: MaybeT f a -> Bool

length :: MaybeT f a -> Int

elem :: Eq a => a -> MaybeT f a -> Bool

maximum :: Ord a => MaybeT f a -> a

minimum :: Ord a => MaybeT f a -> a

sum :: Num a => MaybeT f a -> a

product :: Num a => MaybeT f a -> a

Traversable f => Traversable (MaybeT f)

Defined in Control.Monad.Trans.Maybe

Methods

traverse :: Applicative f0 => (a -> f0 b) -> MaybeT f a -> f0 (MaybeT f b)

sequenceA :: Applicative f0 => MaybeT f (f0 a) -> f0 (MaybeT f a)

mapM :: Monad m => (a -> m b) -> MaybeT f a -> m (MaybeT f b)

sequence :: Monad m => MaybeT f (m a) -> m (MaybeT f a)

Contravariant m => Contravariant (MaybeT m)

Defined in Control.Monad.Trans.Maybe

Methods

contramap :: (a' -> a) -> MaybeT m a -> MaybeT m a'

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

Eq1 m => Eq1 (MaybeT m)

Defined in Control.Monad.Trans.Maybe

Methods

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

Ord1 m => Ord1 (MaybeT m)

Defined in Control.Monad.Trans.Maybe

Methods

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

Read1 m => Read1 (MaybeT m)

Defined in Control.Monad.Trans.Maybe

Methods

liftReadsPrec :: (Int -> ReadS a) -> ReadS [a] -> Int -> ReadS (MaybeT m a)

liftReadList :: (Int -> ReadS a) -> ReadS [a] -> ReadS [MaybeT m a]

liftReadPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec (MaybeT m a)

liftReadListPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec [MaybeT m a]

Show1 m => Show1 (MaybeT m)

Defined in Control.Monad.Trans.Maybe

Methods

liftShowsPrec :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> Int -> MaybeT m a -> ShowS

liftShowList :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> [MaybeT m a] -> ShowS

MonadZip m => MonadZip (MaybeT m)

Defined in Control.Monad.Trans.Maybe

Methods

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

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

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

MonadIO m => MonadIO (MaybeT m)

Defined in Control.Monad.Trans.Maybe

Methods

liftIO :: IO a -> MaybeT m a

(Functor m, Monad m) => Alternative (MaybeT m)

Defined in Control.Monad.Trans.Maybe

Methods

empty :: MaybeT m a

(<|>) :: MaybeT m a -> MaybeT m a -> MaybeT m a

some :: MaybeT m a -> MaybeT m [a]

many :: MaybeT m a -> MaybeT m [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

(Eq1 m, Eq a) => Eq (MaybeT m a)

Defined in Control.Monad.Trans.Maybe

Methods

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

(/=) :: MaybeT m a -> MaybeT m a -> Bool

(Ord1 m, Ord a) => Ord (MaybeT m a)

Defined in Control.Monad.Trans.Maybe

Methods

compare :: MaybeT m a -> MaybeT m a -> Ordering

(<) :: MaybeT m a -> MaybeT m a -> Bool

(<=) :: MaybeT m a -> MaybeT m a -> Bool

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

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

max :: MaybeT m a -> MaybeT m a -> MaybeT m a

min :: MaybeT m a -> MaybeT m a -> MaybeT m a

(Read1 m, Read a) => Read (MaybeT m a)

Defined in Control.Monad.Trans.Maybe

Methods

readsPrec :: Int -> ReadS (MaybeT m a)

readList :: ReadS [MaybeT m a]

readPrec :: ReadPrec (MaybeT m a)

readListPrec :: ReadPrec [MaybeT m a]

(Show1 m, Show a) => Show (MaybeT m a)

Defined in Control.Monad.Trans.Maybe

Methods

showsPrec :: Int -> MaybeT m a -> ShowS

show :: MaybeT m a -> String

showList :: [MaybeT m a] -> ShowS

Reexport monadic 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.

'join bss' can be understood as the do expression

do bs <- bss
   bs

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:

Left identity

return a >>= k = k a

Right identity

m >>= return = m

Associativity

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

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

• pure = return

• m1 <*> m2 = m1 >>= (x1 -> m2 >>= (x2 -> return (x1 x2)))

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.

'as >>= bs' can be understood as the do expression

do a <- as
   bs a

(>>) :: 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.

'as >> bs' can be understood as the do expression

do as
   bs

return :: a -> m a

Inject a value into the monadic type.

Instances

Monad []

Since: base-2.1

Defined in GHC.Base

Methods

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

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

return :: a -> [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

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

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

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

Monad Solo

Since: base-4.15

Defined in GHC.Base

Methods

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

(>>) :: Solo a -> Solo b -> Solo b

return :: a -> Solo 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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Monad Tree

Defined in Data.Tree

Methods

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

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

return :: a -> 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

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

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

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

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)

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

ArrowApply a => Monad (ArrowMonad a)

Since: base-2.1

Defined in Control.Arrow

Methods

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

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

return :: a0 -> ArrowMonad a a0

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

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

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

(Monoid a, Monoid b) => Monad ((,,) a b)

Since: base-4.14.0.0

Defined in GHC.Base

Methods

(>>=) :: (a, b, a0) -> (a0 -> (a, b, b0)) -> (a, b, b0)

(>>) :: (a, b, a0) -> (a, b, b0) -> (a, b, b0)

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

Monad m => Monad (Kleisli m a)

Since: base-4.14.0.0

Defined in Control.Arrow

Methods

(>>=) :: Kleisli m a a0 -> (a0 -> Kleisli m a b) -> Kleisli m a b

(>>) :: Kleisli m a a0 -> Kleisli m a b -> Kleisli m a b

return :: a0 -> Kleisli m a a0

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

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

(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

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

(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

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

Monad m => Monad (ReaderT r m)

Defined in Control.Monad.Trans.Reader

Methods

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

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

return :: a -> ReaderT r m a

Monad m => Monad (StateT s m)

Defined in Control.Monad.Trans.State.Strict

Methods

(>>=) :: StateT s m a -> (a -> StateT s m b) -> StateT s m b

(>>) :: StateT s m a -> StateT s m b -> StateT s m b

return :: a -> StateT s m 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

Monad ((->) r)

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

(Monoid a, Monoid b, Monoid c) => Monad ((,,,) a b c)

Since: base-4.14.0.0

Defined in GHC.Base

Methods

(>>=) :: (a, b, c, a0) -> (a0 -> (a, b, c, b0)) -> (a, b, c, b0)

(>>) :: (a, b, c, a0) -> (a, b, c, b0) -> (a, b, c, b0)

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

(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

(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

(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

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

(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

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

Direct MonadPlus equivalent of filter.

Examples

The filter function is just mfilter specialized to the list monad:

filter = ( mfilter :: (a -> Bool) -> [a] -> [a] )

An example using mfilter with the Maybe monad:

>>> mfilter odd (Just 1)
Just 1
>>> mfilter odd (Just 2)
Nothing

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

Strict version of <$>.

Since: base-4.8.0.0

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

Like replicateM, but discards the result.

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

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

Using ApplicativeDo: 'replicateM 5 as' can be understood as the do expression

do a1 <- as
   a2 <- as
   a3 <- as
   a4 <- as
   a5 <- as
   pure [a1,a2,a3,a4,a5]

Note the Applicative constraint.

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.

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 monad.

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

Repeat an action indefinitely.

Using ApplicativeDo: 'forever as' can be understood as the pseudo-do expression

do as
   as
   ..

with as repeating.

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.

'(bs >=> cs) a' can be understood as the do expression

do b <- bs a
   cs b

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

This generalizes the list-based filter function.

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

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

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 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

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

Since: base-4.6.0.0

Defined in Control.Arrow

Methods

mzero :: ArrowMonad a a0

mplus :: ArrowMonad a a0 -> ArrowMonad a a0 -> ArrowMonad a a0

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

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 m => MonadPlus (Kleisli m a)

Since: base-4.14.0.0

Defined in Control.Arrow

Methods

mzero :: Kleisli m a a0

mplus :: Kleisli m a a0 -> Kleisli m a a0 -> Kleisli m a a0

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, 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

(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

MonadPlus m => MonadPlus (ReaderT r m)

Defined in Control.Monad.Trans.Reader

Methods

mzero :: ReaderT r m a

mplus :: ReaderT r m a -> ReaderT r m a -> ReaderT r m a

MonadPlus m => MonadPlus (StateT s m)

Defined in Control.Monad.Trans.State.Strict

Methods

mzero :: StateT s m a

mplus :: StateT s m a -> StateT s m a -> StateT s 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

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

When a value is bound in do-notation, the pattern on the left hand side of <- might not match. In this case, this class provides a function to recover.

A Monad without a MonadFail instance may only be used in conjunction with pattern that always match, such as newtypes, tuples, data types with only a single data constructor, and irrefutable patterns (~pat).

Instances of MonadFail should satisfy the following law: fail s should be a left zero for >>=,

fail s >>= f  =  fail s

If your Monad is also MonadPlus, a popular definition is

fail _ = mzero

Since: base-4.9.0.0

Methods

fail :: String -> m a

Instances

MonadFail []

Since: base-4.9.0.0

Defined in Control.Monad.Fail

Methods

fail :: String -> [a]

MonadFail Maybe

Since: base-4.9.0.0

Defined in Control.Monad.Fail

Methods

fail :: String -> Maybe a

MonadFail IO

Since: base-4.9.0.0

Defined in Control.Monad.Fail

Methods

fail :: String -> IO a

MonadFail Q

Defined in Language.Haskell.TH.Syntax

Methods

fail :: String -> Q a

MonadFail ReadPrec

Since: base-4.9.0.0

Defined in Text.ParserCombinators.ReadPrec

Methods

fail :: String -> ReadPrec a

MonadFail ReadP

Since: base-4.9.0.0

Defined in Text.ParserCombinators.ReadP

Methods

fail :: String -> ReadP a

MonadFail P

Since: base-4.9.0.0

Defined in Text.ParserCombinators.ReadP

Methods

fail :: String -> P a

IsString str => MonadFail (Either str)

For convenient work with MonadFail.

Since: 0.1.0

Defined in Relude.Monad.Either

Methods

fail :: String -> Either str a

Monad m => MonadFail (MaybeT m)

Defined in Control.Monad.Trans.Maybe

Methods

fail :: String -> MaybeT m a

MonadFail f => MonadFail (Ap f)

Since: base-4.12.0.0

Defined in Data.Monoid

Methods

fail :: String -> Ap f a

MonadFail m => MonadFail (IdentityT m)

Defined in Control.Monad.Trans.Identity

Methods

fail :: String -> IdentityT m a

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

Defined in Control.Monad.Trans.Error

Methods

fail :: String -> ErrorT e m a

MonadFail m => MonadFail (ExceptT e m)

Defined in Control.Monad.Trans.Except

Methods

fail :: String -> ExceptT e m a

MonadFail m => MonadFail (ReaderT r m)

Defined in Control.Monad.Trans.Reader

Methods

fail :: String -> ReaderT r m a

MonadFail m => MonadFail (StateT s m)

Defined in Control.Monad.Trans.State.Strict

Methods

fail :: String -> StateT s m a

Reexport Maybe

data Maybe a

The Maybe type encapsulates an optional value. A value of type Maybe a either contains a value of type a (represented as Just a), or it is empty (represented as Nothing). Using Maybe is a good way to deal with errors or exceptional cases without resorting to drastic measures such as error.

The Maybe type is also a monad. It is a simple kind of error monad, where all errors are represented by Nothing. A richer error monad can be built using the Either type.

Constructors

Nothing

Just a

Instances

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

Functor Maybe

Since: base-2.1

Defined in GHC.Base

Methods

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

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

MonadFail Maybe

Since: base-4.9.0.0

Defined in Control.Monad.Fail

Methods

fail :: String -> Maybe a

Applicative Maybe

Since: base-2.1

Defined in GHC.Base

Methods

pure :: a -> Maybe a

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

liftA2 :: (a -> b -> c) -> Maybe a -> Maybe b -> Maybe c

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

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

Foldable Maybe

Since: base-2.1

Defined in Data.Foldable

Methods

fold :: Monoid m => Maybe m -> m

foldMap :: Monoid m => (a -> m) -> Maybe a -> m

foldMap' :: Monoid m => (a -> m) -> Maybe a -> m

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

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

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

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

foldr1 :: (a -> a -> a) -> Maybe a -> a

foldl1 :: (a -> a -> a) -> Maybe a -> a

toList :: Maybe a -> [a]

null :: Maybe a -> Bool

length :: Maybe a -> Int

elem :: Eq a => a -> Maybe a -> Bool

maximum :: Ord a => Maybe a -> a

minimum :: Ord a => Maybe a -> a

sum :: Num a => Maybe a -> a

product :: Num a => Maybe a -> a

Traversable Maybe

Since: base-2.1

Defined in Data.Traversable

Methods

traverse :: Applicative f => (a -> f b) -> Maybe a -> f (Maybe b)

sequenceA :: Applicative f => Maybe (f a) -> f (Maybe a)

mapM :: Monad m => (a -> m b) -> Maybe a -> m (Maybe b)

sequence :: Monad m => Maybe (m a) -> m (Maybe a)

Eq1 Maybe

Since: base-4.9.0.0

Defined in Data.Functor.Classes

Methods

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

Ord1 Maybe

Since: base-4.9.0.0

Defined in Data.Functor.Classes

Methods

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

Read1 Maybe

Since: base-4.9.0.0

Defined in Data.Functor.Classes

Methods

liftReadsPrec :: (Int -> ReadS a) -> ReadS [a] -> Int -> ReadS (Maybe a)

liftReadList :: (Int -> ReadS a) -> ReadS [a] -> ReadS [Maybe a]

liftReadPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec (Maybe a)

liftReadListPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec [Maybe a]

Show1 Maybe

Since: base-4.9.0.0

Defined in Data.Functor.Classes

Methods

liftShowsPrec :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> Int -> Maybe a -> ShowS

liftShowList :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> [Maybe a] -> ShowS

Alternative Maybe

Since: base-2.1

Defined in GHC.Base

Methods

empty :: Maybe a

(<|>) :: Maybe a -> Maybe a -> Maybe a

some :: Maybe a -> Maybe [a]

many :: Maybe a -> Maybe [a]

MonadPlus Maybe

Since: base-2.1

Defined in GHC.Base

Methods

mzero :: Maybe a

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

NFData1 Maybe

Since: deepseq-1.4.3.0

Defined in Control.DeepSeq

Methods

liftRnf :: (a -> ()) -> Maybe a -> ()

Hashable1 Maybe

Defined in Data.Hashable.Class

Methods

liftHashWithSalt :: (Int -> a -> Int) -> Int -> Maybe a -> Int

Lift a => Lift (Maybe a :: Type)

Defined in Language.Haskell.TH.Syntax

Methods

lift :: Quote m => Maybe a -> m Exp

liftTyped :: forall (m :: Type -> Type). Quote m => Maybe a -> Code m (Maybe a)

Eq a => Eq (Maybe a)

Since: base-2.1

Defined in GHC.Maybe

Methods

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

(/=) :: Maybe a -> Maybe a -> Bool

Data a => Data (Maybe a)

Since: base-4.0.0.0

Defined in Data.Data

Methods

gfoldl :: (forall d b. Data d => c (d -> b) -> d -> c b) -> (forall g. g -> c g) -> Maybe a -> c (Maybe a)

gunfold :: (forall b r. Data b => c (b -> r) -> c r) -> (forall r. r -> c r) -> Constr -> c (Maybe a)

toConstr :: Maybe a -> Constr

dataTypeOf :: Maybe a -> DataType

dataCast1 :: Typeable t => (forall d. Data d => c (t d)) -> Maybe (c (Maybe a))

dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c (t d e)) -> Maybe (c (Maybe a))

gmapT :: (forall b. Data b => b -> b) -> Maybe a -> Maybe a

gmapQl :: (r -> r' -> r) -> r -> (forall d. Data d => d -> r') -> Maybe a -> r

gmapQr :: forall r r'. (r' -> r -> r) -> r -> (forall d. Data d => d -> r') -> Maybe a -> r

gmapQ :: (forall d. Data d => d -> u) -> Maybe a -> [u]

gmapQi :: Int -> (forall d. Data d => d -> u) -> Maybe a -> u

gmapM :: Monad m => (forall d. Data d => d -> m d) -> Maybe a -> m (Maybe a)

gmapMp :: MonadPlus m => (forall d. Data d => d -> m d) -> Maybe a -> m (Maybe a)

gmapMo :: MonadPlus m => (forall d. Data d => d -> m d) -> Maybe a -> m (Maybe a)

Ord a => Ord (Maybe a)

Since: base-2.1

Defined in GHC.Maybe

Methods

compare :: Maybe a -> Maybe a -> Ordering

(<) :: Maybe a -> Maybe a -> Bool

(<=) :: Maybe a -> Maybe a -> Bool

(>) :: Maybe a -> Maybe a -> Bool

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

max :: Maybe a -> Maybe a -> Maybe a

min :: Maybe a -> Maybe a -> Maybe a

Read a => Read (Maybe a)

Since: base-2.1

Defined in GHC.Read

Methods

readsPrec :: Int -> ReadS (Maybe a)

readList :: ReadS [Maybe a]

readPrec :: ReadPrec (Maybe a)

readListPrec :: ReadPrec [Maybe a]

Show a => Show (Maybe a)

Since: base-2.1

Defined in GHC.Show

Methods

showsPrec :: Int -> Maybe a -> ShowS

show :: Maybe a -> String

showList :: [Maybe a] -> ShowS

Generic (Maybe a)

Since: base-4.6.0.0

Defined in GHC.Generics

Associated Types

type Rep (Maybe a) :: Type -> Type

Methods

from :: Maybe a -> Rep (Maybe a) x

to :: Rep (Maybe a) x -> Maybe a

Semigroup a => Semigroup (Maybe a)

Since: base-4.9.0.0

Defined in GHC.Base

Methods

(<>) :: Maybe a -> Maybe a -> Maybe a

sconcat :: NonEmpty (Maybe a) -> Maybe a

stimes :: Integral b => b -> Maybe a -> Maybe a

Semigroup a => Monoid (Maybe a)

Lift a semigroup into Maybe forming a Monoid according to http://en.wikipedia.org/wiki/Monoid: "Any semigroup S may be turned into a monoid simply by adjoining an element e not in S and defining e*e = e and e*s = s = s*e for all s ∈ S."

Since 4.11.0: constraint on inner a value generalised from Monoid to Semigroup.

Since: base-2.1

Defined in GHC.Base

Methods

mempty :: Maybe a

mappend :: Maybe a -> Maybe a -> Maybe a

mconcat :: [Maybe a] -> Maybe a

NFData a => NFData (Maybe a)

Defined in Control.DeepSeq

Methods

rnf :: Maybe a -> ()

Hashable a => Hashable (Maybe a)

Defined in Data.Hashable.Class

Methods

hashWithSalt :: Int -> Maybe a -> Int

hash :: Maybe a -> Int

SingKind a => SingKind (Maybe a)

Since: base-4.9.0.0

Defined in GHC.Generics

Associated Types

type DemoteRep (Maybe a)

Methods

fromSing :: forall (a0 :: Maybe a). Sing a0 -> DemoteRep (Maybe a)

Generic1 Maybe

Since: base-4.6.0.0

Defined in GHC.Generics

Associated Types

type Rep1 Maybe :: k -> Type

Methods

from1 :: forall (a :: k). Maybe a -> Rep1 Maybe a

to1 :: forall (a :: k). Rep1 Maybe a -> Maybe a

SingI ('Nothing :: Maybe a)

Since: base-4.9.0.0

Defined in GHC.Generics

Methods

sing :: Sing 'Nothing

SingI a2 => SingI ('Just a2 :: Maybe a1)

Since: base-4.9.0.0

Defined in GHC.Generics

Methods

sing :: Sing ('Just a2)

type Rep (Maybe a)

Defined in GHC.Generics

type Rep (Maybe a) = D1 ('MetaData "Maybe" "GHC.Maybe" "base" 'False) (C1 ('MetaCons "Nothing" 'PrefixI 'False) (U1 :: Type -> Type) :+: C1 ('MetaCons "Just" 'PrefixI 'False) (S1 ('MetaSel ('Nothing :: Maybe Symbol) 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 a)))

type DemoteRep (Maybe a)

Defined in GHC.Generics

type DemoteRep (Maybe a) = Maybe (DemoteRep a)

data Sing (b :: Maybe a)

Defined in GHC.Generics

data Sing (b :: Maybe a) where

• SNothing :: forall {a}. Sing ('Nothing :: Maybe a)
• SJust :: forall {a} (a1 :: a). Sing a1 -> Sing ('Just a1)

type Rep1 Maybe

Defined in GHC.Generics

type Rep1 Maybe = D1 ('MetaData "Maybe" "GHC.Maybe" "base" 'False) (C1 ('MetaCons "Nothing" 'PrefixI 'False) (U1 :: Type -> Type) :+: C1 ('MetaCons "Just" 'PrefixI 'False) (S1 ('MetaSel ('Nothing :: Maybe Symbol) 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) Par1))

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

The mapMaybe function is a version of map which can throw out elements. In particular, the functional argument returns something of type Maybe b. If this is Nothing, no element is added on to the result list. If it is Just b, then b is included in the result list.

Examples

Using mapMaybe f x is a shortcut for catMaybes $ map f x in most cases:

>>> import Text.Read ( readMaybe )
>>> let readMaybeInt = readMaybe :: String -> Maybe Int
>>> mapMaybe readMaybeInt ["1", "Foo", "3"]
[1,3]
>>> catMaybes $ map readMaybeInt ["1", "Foo", "3"]
[1,3]

If we map the Just constructor, the entire list should be returned:

>>> mapMaybe Just [1,2,3]
[1,2,3]

catMaybes :: [Maybe a] -> [a]

The catMaybes function takes a list of Maybes and returns a list of all the Just values.

Examples

Basic usage:

>>> catMaybes [Just 1, Nothing, Just 3]
[1,3]

When constructing a list of Maybe values, catMaybes can be used to return all of the "success" results (if the list is the result of a map, then mapMaybe would be more appropriate):

>>> import Text.Read ( readMaybe )
>>> [readMaybe x :: Maybe Int | x <- ["1", "Foo", "3"] ]
[Just 1,Nothing,Just 3]
>>> catMaybes $ [readMaybe x :: Maybe Int | x <- ["1", "Foo", "3"] ]
[1,3]

listToMaybe :: [a] -> Maybe a

The listToMaybe function returns Nothing on an empty list or Just a where a is the first element of the list.

Examples

Basic usage:

>>> listToMaybe []
Nothing

>>> listToMaybe [9]
Just 9

>>> listToMaybe [1,2,3]
Just 1

Composing maybeToList with listToMaybe should be the identity on singleton/empty lists:

>>> maybeToList $ listToMaybe [5]
[5]
>>> maybeToList $ listToMaybe []
[]

But not on lists with more than one element:

>>> maybeToList $ listToMaybe [1,2,3]
[1]

maybeToList :: Maybe a -> [a]

The maybeToList function returns an empty list when given Nothing or a singleton list when given Just.

Examples

Basic usage:

>>> maybeToList (Just 7)
[7]

>>> maybeToList Nothing
[]

One can use maybeToList to avoid pattern matching when combined with a function that (safely) works on lists:

>>> import Text.Read ( readMaybe )
>>> sum $ maybeToList (readMaybe "3")
3
>>> sum $ maybeToList (readMaybe "")
0

fromMaybe :: a -> Maybe a -> a

The fromMaybe function takes a default value and a Maybe value. If the Maybe is Nothing, it returns the default value; otherwise, it returns the value contained in the Maybe.

Examples

Basic usage:

>>> fromMaybe "" (Just "Hello, World!")
"Hello, World!"

>>> fromMaybe "" Nothing
""

Read an integer from a string using readMaybe. If we fail to parse an integer, we want to return 0 by default:

>>> import Text.Read ( readMaybe )
>>> fromMaybe 0 (readMaybe "5")
5
>>> fromMaybe 0 (readMaybe "")
0

isNothing :: Maybe a -> Bool

The isNothing function returns True iff its argument is Nothing.

Examples

Basic usage:

>>> isNothing (Just 3)
False

>>> isNothing (Just ())
False

>>> isNothing Nothing
True

Only the outer constructor is taken into consideration:

>>> isNothing (Just Nothing)
False

isJust :: Maybe a -> Bool

The isJust function returns True iff its argument is of the form Just _.

Examples

Basic usage:

>>> isJust (Just 3)
True

>>> isJust (Just ())
True

>>> isJust Nothing
False

Only the outer constructor is taken into consideration:

>>> isJust (Just Nothing)
True

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

The maybe function takes a default value, a function, and a Maybe value. If the Maybe value is Nothing, the function returns the default value. Otherwise, it applies the function to the value inside the Just and returns the result.

Examples

Basic usage:

>>> maybe False odd (Just 3)
True

>>> maybe False odd Nothing
False

Read an integer from a string using readMaybe. If we succeed, return twice the integer; that is, apply (*2) to it. If instead we fail to parse an integer, return 0 by default:

>>> import Text.Read ( readMaybe )
>>> maybe 0 (*2) (readMaybe "5")
10
>>> maybe 0 (*2) (readMaybe "")
0

Apply show to a Maybe Int. If we have Just n, we want to show the underlying Int n. But if we have Nothing, we return the empty string instead of (for example) "Nothing":

>>> maybe "" show (Just 5)
"5"
>>> maybe "" show Nothing
""

Reexport Either

data Either a b

The Either type represents values with two possibilities: a value of type Either a b is either Left a or Right b.

The Either type is sometimes used to represent a value which is either correct or an error; by convention, the Left constructor is used to hold an error value and the Right constructor is used to hold a correct value (mnemonic: "right" also means "correct").

Examples

The type Either String Int is the type of values which can be either a String or an Int. The Left constructor can be used only on Strings, and the Right constructor can be used only on Ints:

>>> let s = Left "foo" :: Either String Int
>>> s
Left "foo"
>>> let n = Right 3 :: Either String Int
>>> n
Right 3
>>> :type s
s :: Either String Int
>>> :type n
n :: Either String Int

The fmap from our Functor instance will ignore Left values, but will apply the supplied function to values contained in a Right:

>>> let s = Left "foo" :: Either String Int
>>> let n = Right 3 :: Either String Int
>>> fmap (*2) s
Left "foo"
>>> fmap (*2) n
Right 6

The Monad instance for Either allows us to chain together multiple actions which may fail, and fail overall if any of the individual steps failed. First we'll write a function that can either parse an Int from a Char, or fail.

>>> import Data.Char ( digitToInt, isDigit )
>>> :{
    let parseEither :: Char -> Either String Int
        parseEither c
          | isDigit c = Right (digitToInt c)
          | otherwise = Left "parse error"
>>> :}

The following should work, since both '1' and '2' can be parsed as Ints.

>>> :{
    let parseMultiple :: Either String Int
        parseMultiple = do
          x <- parseEither '1'
          y <- parseEither '2'
          return (x + y)
>>> :}

>>> parseMultiple
Right 3

But the following should fail overall, since the first operation where we attempt to parse 'm' as an Int will fail:

>>> :{
    let parseMultiple :: Either String Int
        parseMultiple = do
          x <- parseEither 'm'
          y <- parseEither '2'
          return (x + y)
>>> :}

>>> parseMultiple
Left "parse error"

Constructors

Left a

Right b

Instances

Bitraversable Either

Since: base-4.10.0.0

Defined in Data.Bitraversable

Methods

bitraverse :: Applicative f => (a -> f c) -> (b -> f d) -> Either a b -> f (Either c d)

Bifoldable Either

Since: base-4.10.0.0

Defined in Data.Bifoldable

Methods

bifold :: Monoid m => Either m m -> m

bifoldMap :: Monoid m => (a -> m) -> (b -> m) -> Either a b -> m

bifoldr :: (a -> c -> c) -> (b -> c -> c) -> c -> Either a b -> c

bifoldl :: (c -> a -> c) -> (c -> b -> c) -> c -> Either a b -> c

Bifunctor Either

Since: base-4.8.0.0

Defined in Data.Bifunctor

Methods

bimap :: (a -> b) -> (c -> d) -> Either a c -> Either b d

first :: (a -> b) -> Either a c -> Either b c

second :: (b -> c) -> Either a b -> Either a c

Eq2 Either

Since: base-4.9.0.0

Defined in Data.Functor.Classes

Methods

liftEq2 :: (a -> b -> Bool) -> (c -> d -> Bool) -> Either a c -> Either b d -> Bool

Ord2 Either

Since: base-4.9.0.0

Defined in Data.Functor.Classes

Methods

liftCompare2 :: (a -> b -> Ordering) -> (c -> d -> Ordering) -> Either a c -> Either b d -> Ordering

Read2 Either

Since: base-4.9.0.0

Defined in Data.Functor.Classes

Methods

liftReadsPrec2 :: (Int -> ReadS a) -> ReadS [a] -> (Int -> ReadS b) -> ReadS [b] -> Int -> ReadS (Either a b)

liftReadList2 :: (Int -> ReadS a) -> ReadS [a] -> (Int -> ReadS b) -> ReadS [b] -> ReadS [Either a b]

liftReadPrec2 :: ReadPrec a -> ReadPrec [a] -> ReadPrec b -> ReadPrec [b] -> ReadPrec (Either a b)

liftReadListPrec2 :: ReadPrec a -> ReadPrec [a] -> ReadPrec b -> ReadPrec [b] -> ReadPrec [Either a b]

Show2 Either

Since: base-4.9.0.0

Defined in Data.Functor.Classes

Methods

liftShowsPrec2 :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> (Int -> b -> ShowS) -> ([b] -> ShowS) -> Int -> Either a b -> ShowS

liftShowList2 :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> (Int -> b -> ShowS) -> ([b] -> ShowS) -> [Either a b] -> ShowS

NFData2 Either

Since: deepseq-1.4.3.0

Defined in Control.DeepSeq

Methods

liftRnf2 :: (a -> ()) -> (b -> ()) -> Either a b -> ()

Hashable2 Either

Defined in Data.Hashable.Class

Methods

liftHashWithSalt2 :: (Int -> a -> Int) -> (Int -> b -> Int) -> Int -> Either a b -> Int

(Lift a, Lift b) => Lift (Either a b :: Type)

Defined in Language.Haskell.TH.Syntax

Methods

lift :: Quote m => Either a b -> m Exp

liftTyped :: forall (m :: Type -> Type). Quote m => Either a b -> Code m (Either a b)

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

Functor (Either a)

Since: base-3.0

Defined in Data.Either

Methods

fmap :: (a0 -> b) -> Either a a0 -> Either a b

(<$) :: a0 -> Either a b -> Either a a0

IsString str => MonadFail (Either str)

For convenient work with MonadFail.

Since: 0.1.0

Defined in Relude.Monad.Either

Methods

fail :: String -> Either str a

Applicative (Either e)

Since: base-3.0

Defined in Data.Either

Methods

pure :: a -> Either e a

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

liftA2 :: (a -> b -> c) -> Either e a -> Either e b -> Either e c

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

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

Foldable (Either a)

Since: base-4.7.0.0

Defined in Data.Foldable

Methods

fold :: Monoid m => Either a m -> m

foldMap :: Monoid m => (a0 -> m) -> Either a a0 -> m

foldMap' :: Monoid m => (a0 -> m) -> Either a a0 -> m

foldr :: (a0 -> b -> b) -> b -> Either a a0 -> b

foldr' :: (a0 -> b -> b) -> b -> Either a a0 -> b

foldl :: (b -> a0 -> b) -> b -> Either a a0 -> b

foldl' :: (b -> a0 -> b) -> b -> Either a a0 -> b

foldr1 :: (a0 -> a0 -> a0) -> Either a a0 -> a0

foldl1 :: (a0 -> a0 -> a0) -> Either a a0 -> a0

toList :: Either a a0 -> [a0]

null :: Either a a0 -> Bool

length :: Either a a0 -> Int

elem :: Eq a0 => a0 -> Either a a0 -> Bool

maximum :: Ord a0 => Either a a0 -> a0

minimum :: Ord a0 => Either a a0 -> a0

sum :: Num a0 => Either a a0 -> a0

product :: Num a0 => Either a a0 -> a0

Traversable (Either a)

Since: base-4.7.0.0

Defined in Data.Traversable

Methods

traverse :: Applicative f => (a0 -> f b) -> Either a a0 -> f (Either a b)

sequenceA :: Applicative f => Either a (f a0) -> f (Either a a0)

mapM :: Monad m => (a0 -> m b) -> Either a a0 -> m (Either a b)

sequence :: Monad m => Either a (m a0) -> m (Either a a0)

Eq a => Eq1 (Either a)

Since: base-4.9.0.0

Defined in Data.Functor.Classes

Methods

liftEq :: (a0 -> b -> Bool) -> Either a a0 -> Either a b -> Bool

Ord a => Ord1 (Either a)

Since: base-4.9.0.0

Defined in Data.Functor.Classes

Methods

liftCompare :: (a0 -> b -> Ordering) -> Either a a0 -> Either a b -> Ordering

Read a => Read1 (Either a)

Since: base-4.9.0.0

Defined in Data.Functor.Classes

Methods

liftReadsPrec :: (Int -> ReadS a0) -> ReadS [a0] -> Int -> ReadS (Either a a0)

liftReadList :: (Int -> ReadS a0) -> ReadS [a0] -> ReadS [Either a a0]

liftReadPrec :: ReadPrec a0 -> ReadPrec [a0] -> ReadPrec (Either a a0)

liftReadListPrec :: ReadPrec a0 -> ReadPrec [a0] -> ReadPrec [Either a a0]

Show a => Show1 (Either a)

Since: base-4.9.0.0

Defined in Data.Functor.Classes

Methods

liftShowsPrec :: (Int -> a0 -> ShowS) -> ([a0] -> ShowS) -> Int -> Either a a0 -> ShowS

liftShowList :: (Int -> a0 -> ShowS) -> ([a0] -> ShowS) -> [Either a a0] -> ShowS

NFData a => NFData1 (Either a)

Since: deepseq-1.4.3.0

Defined in Control.DeepSeq

Methods

liftRnf :: (a0 -> ()) -> Either a a0 -> ()

Hashable a => Hashable1 (Either a)

Defined in Data.Hashable.Class

Methods

liftHashWithSalt :: (Int -> a0 -> Int) -> Int -> Either a a0 -> Int

Generic1 (Either a :: Type -> Type)

Since: base-4.6.0.0

Defined in GHC.Generics

Associated Types

type Rep1 (Either a) :: k -> Type

Methods

from1 :: forall (a0 :: k). Either a a0 -> Rep1 (Either a) a0

to1 :: forall (a0 :: k). Rep1 (Either a) a0 -> Either a a0

(Eq a, Eq b) => Eq (Either a b)

Since: base-2.1

Defined in Data.Either

Methods

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

(/=) :: Either a b -> Either a b -> Bool

(Data a, Data b) => Data (Either a b)

Since: base-4.0.0.0

Defined in Data.Data

Methods

gfoldl :: (forall d b0. Data d => c (d -> b0) -> d -> c b0) -> (forall g. g -> c g) -> Either a b -> c (Either a b)

gunfold :: (forall b0 r. Data b0 => c (b0 -> r) -> c r) -> (forall r. r -> c r) -> Constr -> c (Either a b)

toConstr :: Either a b -> Constr

dataTypeOf :: Either a b -> DataType

dataCast1 :: Typeable t => (forall d. Data d => c (t d)) -> Maybe (c (Either a b))

dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c (t d e)) -> Maybe (c (Either a b))

gmapT :: (forall b0. Data b0 => b0 -> b0) -> Either a b -> Either a b

gmapQl :: (r -> r' -> r) -> r -> (forall d. Data d => d -> r') -> Either a b -> r

gmapQr :: forall r r'. (r' -> r -> r) -> r -> (forall d. Data d => d -> r') -> Either a b -> r

gmapQ :: (forall d. Data d => d -> u) -> Either a b -> [u]

gmapQi :: Int -> (forall d. Data d => d -> u) -> Either a b -> u

gmapM :: Monad m => (forall d. Data d => d -> m d) -> Either a b -> m (Either a b)

gmapMp :: MonadPlus m => (forall d. Data d => d -> m d) -> Either a b -> m (Either a b)

gmapMo :: MonadPlus m => (forall d. Data d => d -> m d) -> Either a b -> m (Either a b)

(Ord a, Ord b) => Ord (Either a b)

Since: base-2.1

Defined in Data.Either

Methods

compare :: Either a b -> Either a b -> Ordering

(<) :: Either a b -> Either a b -> Bool

(<=) :: Either a b -> Either a b -> Bool

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

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

max :: Either a b -> Either a b -> Either a b

min :: Either a b -> Either a b -> Either a b

(Read a, Read b) => Read (Either a b)

Since: base-3.0

Defined in Data.Either

Methods

readsPrec :: Int -> ReadS (Either a b)

readList :: ReadS [Either a b]

readPrec :: ReadPrec (Either a b)

readListPrec :: ReadPrec [Either a b]

(Show a, Show b) => Show (Either a b)

Since: base-3.0

Defined in Data.Either

Methods

showsPrec :: Int -> Either a b -> ShowS

show :: Either a b -> String

showList :: [Either a b] -> ShowS

Generic (Either a b)

Since: base-4.6.0.0

Defined in GHC.Generics

Associated Types

type Rep (Either a b) :: Type -> Type

Methods

from :: Either a b -> Rep (Either a b) x

to :: Rep (Either a b) x -> Either a b

Semigroup (Either a b)

Since: base-4.9.0.0

Defined in Data.Either

Methods

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

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

stimes :: Integral b0 => b0 -> Either a b -> Either a b

(NFData a, NFData b) => NFData (Either a b)

Defined in Control.DeepSeq

Methods

rnf :: Either a b -> ()

(Hashable a, Hashable b) => Hashable (Either a b)

Defined in Data.Hashable.Class

Methods

hashWithSalt :: Int -> Either a b -> Int

hash :: Either a b -> Int

type Rep1 (Either a :: Type -> Type)

Defined in GHC.Generics

type Rep1 (Either a :: Type -> Type) = D1 ('MetaData "Either" "Data.Either" "base" 'False) (C1 ('MetaCons "Left" 'PrefixI 'False) (S1 ('MetaSel ('Nothing :: Maybe Symbol) 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 a)) :+: C1 ('MetaCons "Right" 'PrefixI 'False) (S1 ('MetaSel ('Nothing :: Maybe Symbol) 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) Par1))

type Rep (Either a b)

Defined in GHC.Generics

type Rep (Either a b) = D1 ('MetaData "Either" "Data.Either" "base" 'False) (C1 ('MetaCons "Left" 'PrefixI 'False) (S1 ('MetaSel ('Nothing :: Maybe Symbol) 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 a)) :+: C1 ('MetaCons "Right" 'PrefixI 'False) (S1 ('MetaSel ('Nothing :: Maybe Symbol) 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 b)))

isRight :: Either a b -> Bool

Return True if the given value is a Right-value, False otherwise.

Examples

Basic usage:

>>> isRight (Left "foo")
False
>>> isRight (Right 3)
True

Assuming a Left value signifies some sort of error, we can use isRight to write a very simple reporting function that only outputs "SUCCESS" when a computation has succeeded.

This example shows how isRight might be used to avoid pattern matching when one does not care about the value contained in the constructor:

>>> import Control.Monad ( when )
>>> let report e = when (isRight e) $ putStrLn "SUCCESS"
>>> report (Left "parse error")
>>> report (Right 1)
SUCCESS

Since: base-4.7.0.0

isLeft :: Either a b -> Bool

Return True if the given value is a Left-value, False otherwise.

Examples

Basic usage:

>>> isLeft (Left "foo")
True
>>> isLeft (Right 3)
False

Assuming a Left value signifies some sort of error, we can use isLeft to write a very simple error-reporting function that does absolutely nothing in the case of success, and outputs "ERROR" if any error occurred.

This example shows how isLeft might be used to avoid pattern matching when one does not care about the value contained in the constructor:

>>> import Control.Monad ( when )
>>> let report e = when (isLeft e) $ putStrLn "ERROR"
>>> report (Right 1)
>>> report (Left "parse error")
ERROR

Since: base-4.7.0.0

partitionEithers :: [Either a b] -> ([a], [b])

Partitions a list of Either into two lists. All the Left elements are extracted, in order, to the first component of the output. Similarly the Right elements are extracted to the second component of the output.

Examples

Basic usage:

>>> let list = [ Left "foo", Right 3, Left "bar", Right 7, Left "baz" ]
>>> partitionEithers list
(["foo","bar","baz"],[3,7])

The pair returned by partitionEithers x should be the same pair as (lefts x, rights x):

>>> let list = [ Left "foo", Right 3, Left "bar", Right 7, Left "baz" ]
>>> partitionEithers list == (lefts list, rights list)
True

rights :: [Either a b] -> [b]

Extracts from a list of Either all the Right elements. All the Right elements are extracted in order.

Examples

Basic usage:

>>> let list = [ Left "foo", Right 3, Left "bar", Right 7, Left "baz" ]
>>> rights list
[3,7]

lefts :: [Either a b] -> [a]

Extracts from a list of Either all the Left elements. All the Left elements are extracted in order.

Examples

Basic usage:

>>> let list = [ Left "foo", Right 3, Left "bar", Right 7, Left "baz" ]
>>> lefts list
["foo","bar","baz"]

either :: (a -> c) -> (b -> c) -> Either a b -> c

Case analysis for the Either type. If the value is Left a, apply the first function to a; if it is Right b, apply the second function to b.

Examples

We create two values of type Either String Int, one using the Left constructor and another using the Right constructor. Then we apply "either" the length function (if we have a String) or the "times-two" function (if we have an Int):

>>> let s = Left "foo" :: Either String Int
>>> let n = Right 3 :: Either String Int
>>> either length (*2) s
3
>>> either length (*2) n
6