Copyright | (c) The University of Glasgow 2001 |
---|---|

License | BSD-style (see the file libraries/base/LICENSE) |

Maintainer | libraries@haskell.org |

Stability | provisional |

Portability | portable |

Safe Haskell | Trustworthy |

Language | Haskell2010 |

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.

Functor [] | Since: base-2.1 |

Functor Maybe | Since: base-2.1 |

Functor IO | Since: base-2.1 |

Functor Par1 | Since: base-4.9.0.0 |

Functor NonEmpty | Since: base-4.9.0.0 |

Functor ReadP | Since: base-2.1 |

Functor ReadPrec | Since: base-2.1 |

Functor Down | Since: base-4.11.0.0 |

Functor Product | Since: base-4.8.0.0 |

Functor Sum | Since: base-4.8.0.0 |

Functor Dual | Since: base-4.8.0.0 |

Functor Last | Since: base-4.8.0.0 |

Functor First | Since: base-4.8.0.0 |

Functor STM | Since: base-4.3.0.0 |

Functor Handler | Since: base-4.6.0.0 |

Functor Identity | Since: base-4.8.0.0 |

Functor ZipList | Since: base-2.1 |

Functor ArgDescr | Since: base-4.6.0.0 |

Functor OptDescr | Since: base-4.6.0.0 |

Functor ArgOrder | Since: base-4.6.0.0 |

Functor Option | Since: base-4.9.0.0 |

Functor Last | Since: base-4.9.0.0 |

Functor First | Since: base-4.9.0.0 |

Functor Max | Since: base-4.9.0.0 |

Functor Min | Since: base-4.9.0.0 |

Functor Complex | Since: base-4.9.0.0 |

Functor (Either a) | Since: base-3.0 |

Functor (V1 :: Type -> Type) | Since: base-4.9.0.0 |

Functor (U1 :: Type -> Type) | Since: base-4.9.0.0 |

Functor ((,) a) | Since: base-2.1 |

Functor (ST s) | Since: base-2.1 |

Functor (Proxy :: Type -> Type) | Since: base-4.7.0.0 |

Arrow a => Functor (ArrowMonad a) | Since: base-4.6.0.0 |

Defined in Control.Arrow ## Methodsfmap :: (a0 -> b) -> ArrowMonad a a0 -> ArrowMonad a b Source (<$) :: a0 -> ArrowMonad a b -> ArrowMonad a a0 Source | |

Monad m => Functor (WrappedMonad m) | Since: base-2.1 |

Defined in Control.Applicative ## Methodsfmap :: (a -> b) -> WrappedMonad m a -> WrappedMonad m b Source (<$) :: a -> WrappedMonad m b -> WrappedMonad m a Source | |

Functor (ST s) | Since: base-2.1 |

Functor (Arg a) | Since: base-4.9.0.0 |

Functor f => Functor (Rec1 f) | Since: base-4.9.0.0 |

Functor (URec Char :: Type -> Type) | Since: base-4.9.0.0 |

Functor (URec Double :: Type -> Type) | Since: base-4.9.0.0 |

Functor (URec Float :: Type -> Type) | Since: base-4.9.0.0 |

Functor (URec Int :: Type -> Type) | Since: base-4.9.0.0 |

Functor (URec Word :: Type -> Type) | Since: base-4.9.0.0 |

Functor (URec (Ptr ()) :: Type -> Type) | Since: base-4.9.0.0 |

Functor f => Functor (Alt f) | Since: base-4.8.0.0 |

Functor f => Functor (Ap f) | Since: base-4.12.0.0 |

Functor (Const m :: Type -> Type) | Since: base-2.1 |

Arrow a => Functor (WrappedArrow a b) | Since: base-2.1 |

Defined in Control.Applicative ## Methodsfmap :: (a0 -> b0) -> WrappedArrow a b a0 -> WrappedArrow a b b0 Source (<$) :: a0 -> WrappedArrow a b b0 -> WrappedArrow a b a0 Source | |

Functor ((->) r :: Type -> Type) | Since: base-2.1 |

Functor (K1 i c :: Type -> Type) | Since: base-4.9.0.0 |

(Functor f, Functor g) => Functor (f :+: g) | Since: base-4.9.0.0 |

(Functor f, Functor g) => Functor (f :*: g) | Since: base-4.9.0.0 |

(Functor f, Functor g) => Functor (Sum f g) | Since: base-4.9.0.0 |

(Functor f, Functor g) => Functor (Product f g) | Since: base-4.9.0.0 |

Functor f => Functor (M1 i c f) | Since: base-4.9.0.0 |

(Functor f, Functor g) => Functor (f :.: g) | Since: base-4.9.0.0 |

(Functor f, Functor g) => Functor (Compose f g) | Since: base-4.9.0.0 |

class Applicative m => Monad m where Source

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.

(>>=) :: forall a b. m a -> (a -> m b) -> m b infixl 1 Source

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 Source

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

Inject a value into the monadic type.

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.

Monad [] | Since: base-2.1 |

Monad Maybe | Since: base-2.1 |

Monad IO | Since: base-2.1 |

Monad Par1 | Since: base-4.9.0.0 |

Monad NonEmpty | Since: base-4.9.0.0 |

Monad ReadP | Since: base-2.1 |

Monad ReadPrec | Since: base-2.1 |

Monad Down | Since: base-4.11.0.0 |

Monad Product | Since: base-4.8.0.0 |

Monad Sum | Since: base-4.8.0.0 |

Monad Dual | Since: base-4.8.0.0 |

Monad Last | Since: base-4.8.0.0 |

Monad First | Since: base-4.8.0.0 |

Monad STM | Since: base-4.3.0.0 |

Monad Identity | Since: base-4.8.0.0 |

Monad Option | Since: base-4.9.0.0 |

Monad Last | Since: base-4.9.0.0 |

Monad First | Since: base-4.9.0.0 |

Monad Max | Since: base-4.9.0.0 |

Monad Min | Since: base-4.9.0.0 |

Monad Complex | Since: base-4.9.0.0 |

Monad (Either e) | Since: base-4.4.0.0 |

Monad (U1 :: Type -> Type) | Since: base-4.9.0.0 |

Monoid a => Monad ((,) a) | Since: base-4.9.0.0 |

Monad (ST s) | Since: base-2.1 |

Monad (Proxy :: Type -> Type) | Since: base-4.7.0.0 |

ArrowApply a => Monad (ArrowMonad a) | Since: base-2.1 |

Defined in Control.Arrow ## Methods(>>=) :: ArrowMonad a a0 -> (a0 -> ArrowMonad a b) -> ArrowMonad a b Source (>>) :: ArrowMonad a a0 -> ArrowMonad a b -> ArrowMonad a b Source return :: a0 -> ArrowMonad a a0 Source fail :: String -> ArrowMonad a a0 Source | |

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 Source (>>) :: WrappedMonad m a -> WrappedMonad m b -> WrappedMonad m b Source return :: a -> WrappedMonad m a Source fail :: String -> WrappedMonad m a Source | |

Monad (ST s) | Since: base-2.1 |

Monad f => Monad (Rec1 f) | Since: base-4.9.0.0 |

Monad f => Monad (Alt f) | Since: base-4.8.0.0 |

Monad f => Monad (Ap f) | Since: base-4.12.0.0 |

Monad ((->) r :: Type -> Type) | Since: base-2.1 |

(Monad f, Monad g) => Monad (f :*: g) | Since: base-4.9.0.0 |

(Monad f, Monad g) => Monad (Product f g) | Since: base-4.9.0.0 |

Monad f => Monad (M1 i c f) | Since: base-4.9.0.0 |

class (Alternative m, Monad m) => MonadPlus m where Source

Monads that also support choice and failure.

Nothing

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 Source

An associative operation. The default definition is

mplus = (<|>)

MonadPlus [] | Since: base-2.1 |

MonadPlus Maybe | Since: base-2.1 |

MonadPlus IO | Since: base-4.9.0.0 |

MonadPlus ReadP | Since: base-2.1 |

MonadPlus ReadPrec | Since: base-2.1 |

MonadPlus STM | Since: base-4.3.0.0 |

MonadPlus Option | Since: base-4.9.0.0 |

MonadPlus (U1 :: Type -> Type) | Since: base-4.9.0.0 |

MonadPlus (Proxy :: Type -> Type) | Since: base-4.9.0.0 |

(ArrowApply a, ArrowPlus a) => MonadPlus (ArrowMonad a) | Since: base-4.6.0.0 |

Defined in Control.Arrow ## Methodsmzero :: ArrowMonad a a0 Source mplus :: ArrowMonad a a0 -> ArrowMonad a a0 -> ArrowMonad a a0 Source | |

MonadPlus f => MonadPlus (Rec1 f) | Since: base-4.9.0.0 |

MonadPlus f => MonadPlus (Alt f) | Since: base-4.8.0.0 |

MonadPlus f => MonadPlus (Ap f) | Since: base-4.12.0.0 |

(MonadPlus f, MonadPlus g) => MonadPlus (f :*: g) | Since: base-4.9.0.0 |

(MonadPlus f, MonadPlus g) => MonadPlus (Product f g) | Since: base-4.9.0.0 |

MonadPlus f => MonadPlus (M1 i c f) | Since: base-4.9.0.0 |

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:

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

`Monad`

functionsmapM :: (Traversable t, Monad m) => (a -> m b) -> t a -> m (t b) Source

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 () Source

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) Source

`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 () Source

`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) Source

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 () Source

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 Source

Same as `>>=`

, but with the arguments interchanged.

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

Left-to-right composition of Kleisli arrows.

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

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

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

Repeat an action indefinitely.

A common use of `forever`

is to process input from network sockets, `Handle`

s, 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

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

`void value`

discards or ignores the result of evaluation, such as the return value of an `IO`

action.

Replace the contents of a `Maybe Int`

with unit:

>>>void NothingNothing >>>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

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

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.

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

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

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 Source

Direct `MonadPlus`

equivalent of `filter`

.

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

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

This generalizes the list-based `filter`

function.

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

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] Source

The `zipWithM`

function generalizes `zipWith`

to arbitrary applicative functors.

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

`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 Source

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 () Source

Like `foldM`

, but discards the result.

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

`replicateM n act`

performs the action `n`

times, gathering the results.

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

Like `replicateM`

, but discards the result.

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

Conditional failure of `Alternative`

computations. Defined by

guard True = pure () guard False = empty

Common uses of `guard`

include conditionally signaling an error in an error monad and conditionally rejecting the current choice in an `Alternative`

-based parser.

As an example of signaling an error in the error monad `Maybe`

, consider a safe division function `safeDiv x y`

that returns `Nothing`

when the denominator `y`

is zero and ```
Just (x `div`
y)
```

otherwise. For example:

>>> safeDiv 4 0 Nothing >>> safeDiv 4 2 Just 2

A definition of `safeDiv`

using guards, but not `guard`

:

safeDiv :: Int -> Int -> Maybe Int safeDiv x y | y /= 0 = Just (x `div` y) | otherwise = Nothing

A definition of `safeDiv`

using `guard`

and `Monad`

`do`

-notation:

safeDiv :: Int -> Int -> Maybe Int safeDiv x y = do guard (y /= 0) return (x `div` y)

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

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 () Source

The reverse of `when`

.

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

Promote a function to a monad.

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

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 Source

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 Source

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 Source

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 Source

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

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

Strict version of `<$>`

.

Since: base-4.8.0.0

© The University of Glasgow and others

Licensed under a BSD-style license (see top of the page).

https://downloads.haskell.org/~ghc/8.6.1/docs/html/libraries/base-4.12.0.0/Control-Monad.html