Control.Applicative

base-4.3.1.0: Basic libraries

Portability | portable
Stability | experimental
Maintainer | libraries@haskell.org

Control.Applicative

Contents

- Applicative functors
- Alternatives
- Instances
- Utility functions

Description

This module describes a structure intermediate between a functor and a monad: it provides pure expressions and sequencing, but no binding. (Technically, a strong lax monoidal functor.) For more details, see Applicative Programming with Effects, by Conor McBride and Ross Paterson, online at http://www.soi.city.ac.uk/~ross/papers/Applicative.html.

This interface was introduced for parsers by Niklas Röjemo, because it admits more sharing than the monadic interface. The names here are mostly based on recent parsing work by Doaitse Swierstra.

This class is also useful with instances of the `Data.Traversable.Traversable` class.

Synopsis

- class Functor f => Applicative f where
  - pure :: a -> f a
  - (<*>) :: f (a -> b) -> f a -> f b
  - (*>) :: f a -> f b -> f b
  - (<*) :: f a -> f b -> f a
- class Applicative f => Alternative f where
  - empty :: f a
  - (<|>) :: f a -> f a -> f a
  - some :: f a -> f [a]
  - many :: f a -> f [a]
- newtype Const a b = Const {
  - getConst :: a
  }
- newtype WrappedMonad m a = WrapMonad {
  - unwrapMonad :: m a
  }
- newtype WrappedArrow a b c = WrapArrow {
  - unwrapArrow :: a b c
  }
- newtype ZipList a = ZipList {
  - getZipList :: [a]
  }
- (<$>) :: Functor f => (a -> b) -> f a -> f b
- (<$) :: Functor f => a -> f b -> f a
- (<**>) :: Applicative f => f a -> f (a -> b) -> f b
- liftA :: Applicative f => (a -> b) -> f a -> f b
- liftA2 :: Applicative f => (a -> b -> c) -> f a -> f b -> f c
- liftA3 :: Applicative f => (a -> b -> c -> d) -> f a -> f b -> f c -> f d
- optional :: Alternative f => f a -> f (Maybe a)

# Applicative functors

class Functor f => Applicative f where

A functor with application.

Instances should satisfy the following laws:

identity
  ``pure` `id` `<*>` v = v`
composition
  ``pure` (.) `<*>` u `<*>` v `<*>` w = u `<*>` (v `<*>` w)`
homomorphism
  ``pure` f `<*>` `pure` x = `pure` (f x)`
interchange
  `u `<*>` `pure` y = `pure` (`$` y) `<*>` u`
ignore left value
  `u `*>` v = `pure` (`const` `id`) `<*>` u `<*>` v`
ignore right value
  `u `<*` v = `pure` `const` `<*>` u `<*>` v`

The `Functor` instance should satisfy

          fmap f x = pure f <*> x

If `f` is also a `Monad`, define ``pure` = `return`` and `(`<*>`) = `ap``.

Minimal complete definition: `pure` and `<*>`.

Methods

pure :: a -> f a

Lift a value.

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

Sequential application.

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

Sequence actions, discarding the value of the first argument.

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

Sequence actions, discarding the value of the second argument.

Instances

Applicative [] |
Applicative IO |
Applicative Maybe |
Applicative STM |
Applicative ZipList |
Applicative Id |
Applicative ((->) a) |
Applicative (Either e) |
Monoid a => Applicative ((,) a) |
Monad m => Applicative (WrappedMonad m) |
Monoid m => Applicative (Const m) |
Applicative (StateR s) |
Applicative (StateL s) |
Arrow a => Applicative (WrappedArrow a b) |

# Alternatives

class Applicative f => Alternative f where

A monoid on applicative functors.

Minimal complete definition: `empty` and `<|>`.

`some` and `many` should be the least solutions of the equations:

-     some v = (:) <$> v <*> many v

-     many v = some v <|> pure []

Methods

empty :: f a

The identity of `<|>`

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

An associative binary operation

some :: f a -> f [a]

One or more.

many :: f a -> f [a]

Zero or more.

Instances

Alternative [] |
Alternative Maybe |
Alternative STM |
MonadPlus m => Alternative (WrappedMonad m) |
(ArrowZero a, ArrowPlus a) => Alternative (WrappedArrow a b) |

# Instances

newtype Const a b

Constructors

Const |
Fields getConst :: a

Instances

Functor (Const m) |
Monoid m => Applicative (Const m) |

newtype WrappedMonad m a

Constructors

WrapMonad |
Fields unwrapMonad :: m a

Instances

Monad m => Functor (WrappedMonad m) |
Monad m => Applicative (WrappedMonad m) |
MonadPlus m => Alternative (WrappedMonad m) |

newtype WrappedArrow a b c

Constructors

WrapArrow |
Fields unwrapArrow :: a b c

Instances

Arrow a => Functor (WrappedArrow a b) |
Arrow a => Applicative (WrappedArrow a b) |
(ArrowZero a, ArrowPlus a) => Alternative (WrappedArrow a b) |

newtype ZipList a

Lists, but with an `Applicative` functor based on zipping, so that

    f <$> ZipList xs1 <*> ... <*> ZipList xsn = ZipList (zipWithn f xs1 ... xsn)

Constructors

ZipList |
Fields getZipList :: [a]

Instances

Functor ZipList |
Applicative ZipList |

# Utility functions

(<$>) :: Functor f => (a -> b) -> f a -> f b

An infix synonym for `fmap`.

(<$) :: Functor f => a -> f b -> f a

Replace all locations in the input with the same value. The default definition is ``fmap` . `const``, but this may be overridden with a more efficient version.

(<**>) :: Applicative f => f a -> f (a -> b) -> f b

A variant of `<*>` with the arguments reversed.

liftA :: Applicative f => (a -> b) -> f a -> f b

Lift a function to actions. This function may be used as a value for `fmap` in a `Functor` instance.

liftA2 :: Applicative f => (a -> b -> c) -> f a -> f b -> f c

Lift a binary function to actions.

liftA3 :: Applicative f => (a -> b -> c -> d) -> f a -> f b -> f c -> f d

Lift a ternary function to actions.

optional :: Alternative f => f a -> f (Maybe a)

One or none.