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 [] | |

Functor IO | |

Functor Maybe | |

Functor ReadP | |

Functor ReadPrec | |

Functor Last | |

Functor First | |

Functor STM | |

Functor Handler | |

Functor ZipList | |

Functor Identity | |

Functor ArgDescr | |

Functor OptDescr | |

Functor ArgOrder | |

Functor ((->) r) | |

Functor (Either a) | |

Functor ((,) a) | |

Functor (ST s) | |

Functor (Proxy *) | |

Arrow a => Functor (ArrowMonad a) | |

Monad m => Functor (WrappedMonad m) | |

Functor (Const m) | |

Functor (ST s) | |

Functor f => Functor (Alt * f) | |

Arrow a => Functor (WrappedArrow a b) |

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.

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

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

an associative operation

The functions in this library use the following naming conventions:

- A postfix '
`M`

' always stands for a function in the Kleisli category: The monad type constructor`m`

is added to function results (modulo currying) and nowhere else. So, for example,

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

- A postfix '
`_`

' changes the result type from`(m a)`

to`(m ())`

. Thus, for example:

sequence :: Monad m => [m a] -> m [a] sequence_ :: Monad m => [m a] -> m ()

- A prefix '
`m`

' generalizes an existing function to a monadic form. Thus, for example:

sum :: Num a => [a] -> a msum :: MonadPlus m => [m a] -> m a

`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 Kleisli composition of monads.

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

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

, with the arguments flipped

forever :: Monad m => m a -> m b Source

`forever act`

repeats the action infinitely.

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:

`>>>`

Nothing`void Nothing`

`>>>`

Just ()`void (Just 3)`

Replace the contents of an `Either Int Int`

with unit, resulting in an `Either Int '()'`

:

`>>>`

Left 8675309`void (Left 8675309)`

`>>>`

Right ()`void (Right 8675309)`

Replace every element of a list with unit:

`>>>`

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

Replace the second element of a pair with unit:

`>>>`

(1,())`void (1,2)`

Discard the result of an `IO`

action:

`>>>`

1 2 [(),()]`mapM print [1,2]`

`>>>`

1 2`void $ mapM print [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.

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`

`filter`

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

applicable to any `MonadPlus`

, for example `mfilter odd (Just 1) == Just 1`

`mfilter odd (Just 2) == Nothing`

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

This generalizes the list-based `filter`

function.

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

The `zipWithM`

function generalizes `zipWith`

to arbitrary monads.

zipWithM_ :: Monad 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 :: Monad m => Int -> m a -> m [a] Source

`replicateM n act`

performs the action `n`

times, gathering the results.

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

Like `replicateM`

, but discards the result.

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

`guard b`

is `pure ()`

if `b`

is `True`

, and `empty`

if `b`

is `False`

.

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: 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/7.10.3/docs/html/libraries/base-4.8.2.0/Control-Monad.html