Copyright	Conor McBride and Ross Paterson 2005
License	Conor McBride and Ross Paterson 2005
Maintainer	libraries@haskell.org
Stability	experimental
Portability	portable
Safe Haskell	Trustworthy

Control.Applicative

Contents

Applicative functors
Alternatives
Instances
Utility functions

Description

This module describes a structure intermediate between a functor and a monad (technically, a strong lax monoidal functor). Compared with monads, this interface lacks the full power of the binding operation >>=, but

it has more instances.
it is sufficient for many uses, e.g. context-free parsing, or the Traversable class.
instances can perform analysis of computations before they are executed, and thus produce shared optimizations.

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 parsing work by Doaitse Swierstra.

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.

Synopsis

Applicative functors

class Functor f => Applicative f whereSource

A functor with application, providing operations to

embed pure expressions (pure), and
sequence computations and combine their results (<*>).

A minimal complete definition must include implementations of these functions satisfying 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

The other methods have the following default definitions, which may be overridden with equivalent specialized implementations:

      u *> v = pure (const id) <*> u <*> v
      u <* v = pure const <*> u <*> v

As a consequence of these laws, the Functor instance for f will satisfy

      fmap f x = pure f <*> x

If f is also a Monad, it should satisfy pure = return and (<*>) = ap (which implies that pure and <*> satisfy the applicative functor laws).

Methods

pure :: a -> f aSource

Lift a value.

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

Sequential application.

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

Sequence actions, discarding the value of the first argument.

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

Sequence actions, discarding the value of the second argument.

Instances

Applicative []
Applicative IO
Applicative Maybe
Applicative ReadP
Applicative ReadPrec
Applicative STM
Applicative ZipList
Applicative ((->) a)
Applicative (Either e)
Monoid a => Applicative ((,) a)
Applicative (ST s)
Applicative (Proxy *)
Arrow a => Applicative (ArrowMonad a)
Applicative (ST s)
Monad m => Applicative (WrappedMonad m)
Monoid m => Applicative (Const m)
Arrow a => Applicative (WrappedArrow a b)

Alternatives

class Applicative f => Alternative f whereSource

A monoid on applicative functors.

Minimal complete definition: empty and <|>.

If defined, some and many should be the least solutions of the equations:

```
some v = (:) <$> v <*> many v
```
```
many v = some v <|> pure []
```

Methods

empty :: f aSource

The identity of <|>

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

An associative binary operation

some :: f a -> f [a]Source

One or more.

many :: f a -> f [a]Source

Zero or more.

Instances

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

Instances

newtype Const a b Source

Constructors

Const
Fields getConst :: a

Instances

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

newtype WrappedMonad m a Source

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 Source

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 Source

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
Eq a => Eq (ZipList a)
Ord a => Ord (ZipList a)
Read a => Read (ZipList a)
Show a => Show (ZipList a)

Utility functions

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

An infix synonym for fmap.

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

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 bSource

A variant of <*> with the arguments reversed.

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

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 cSource

Lift a binary function to actions.

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

Lift a ternary function to actions.

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

One or none.