Control.Alternative

class Alternative :: (Type -> Type) -> Constraintclass (Applicative f, Plus f) <= Alternative f 

The Alternative type class has no members of its own; it just specifies that the type constructor has both Applicative and Plus instances.

Types which have Alternative instances should also satisfy the following laws:

• Distributivity: (f <|> g) <*> x == (f <*> x) <|> (g <*> x)
• Annihilation: empty <*> f = empty

Instances

• Alternative Array

guard :: forall m. Alternative m => Boolean -> m Unit

Fail using Plus if a condition does not hold, or succeed using Applicative if it does.

For example:

import Prelude
import Control.Alternative (guard)
import Data.Array ((..))

factors :: Int -> Array Int
factors n = do
  a <- 1..n
  b <- 1..n
  guard $ a * b == n
  pure a

class Alt :: (Type -> Type) -> Constraintclass (Functor f) <= Alt f  where

The Alt type class identifies an associative operation on a type constructor. It is similar to Semigroup, except that it applies to types of kind * -> *, like Array or List, rather than concrete types String or Number.

Alt instances are required to satisfy the following laws:

• Associativity: (x <|> y) <|> z == x <|> (y <|> z)
• Distributivity: f <$> (x <|> y) == (f <$> x) <|> (f <$> y)

For example, the Array ([]) type is an instance of Alt, where (<|>) is defined to be concatenation.

A common use case is to select the first "valid" item, or, if all items are "invalid", the last "invalid" item.

For example:

import Control.Alt ((<|>))
import Data.Maybe (Maybe(..)
import Data.Either (Either(..))

Nothing <|> Just 1 <|> Just 2 == Just 1
Left "err" <|> Right 1 <|> Right 2 == Right 1
Left "err 1" <|> Left "err 2" <|> Left "err 3" == Left "err 3"

Members

• alt :: forall a. f a -> f a -> f a

Instances

• Alt Array

Operator alias for Control.Alt.alt (right-associative / precedence 3)

class Applicative :: (Type -> Type) -> Constraintclass (Apply f) <= Applicative f  where

The Applicative type class extends the Apply type class with a pure function, which can be used to create values of type f a from values of type a.

Where Apply provides the ability to lift functions of two or more arguments to functions whose arguments are wrapped using f, and Functor provides the ability to lift functions of one argument, pure can be seen as the function which lifts functions of zero arguments. That is, Applicative functors support a lifting operation for any number of function arguments.

Instances must satisfy the following laws in addition to the Apply laws:

• Identity: (pure identity) <*> v = v
• Composition: pure (<<<) <*> f <*> g <*> h = f <*> (g <*> h)
• Homomorphism: (pure f) <*> (pure x) = pure (f x)
• Interchange: u <*> (pure y) = (pure (_ $ y)) <*> u

Members

• pure :: forall a. a -> f a

Instances

• Applicative (Function r)
• Applicative Array
• Applicative Proxy

when :: forall m. Applicative m => Boolean -> m Unit -> m Unit

Perform an applicative action when a condition is true.

unless :: forall m. Applicative m => Boolean -> m Unit -> m Unit

Perform an applicative action unless a condition is true.

liftA1 :: forall f a b. Applicative f => (a -> b) -> f a -> f b

liftA1 provides a default implementation of (<$>) for any Applicative functor, without using (<$>) as provided by the Functor-Applicative superclass relationship.

liftA1 can therefore be used to write Functor instances as follows:

instance functorF :: Functor F where
  map = liftA1

class Apply :: (Type -> Type) -> Constraintclass (Functor f) <= Apply f  where

The Apply class provides the (<*>) which is used to apply a function to an argument under a type constructor.

Apply can be used to lift functions of two or more arguments to work on values wrapped with the type constructor f. It might also be understood in terms of the lift2 function:

lift2 :: forall f a b c. Apply f => (a -> b -> c) -> f a -> f b -> f c
lift2 f a b = f <$> a <*> b

(<*>) is recovered from lift2 as lift2 ($). That is, (<*>) lifts the function application operator ($) to arguments wrapped with the type constructor f.

Put differently...

foo =
  functionTakingNArguments <$> computationProducingArg1
                           <*> computationProducingArg2
                           <*> ...
                           <*> computationProducingArgN

Instances must satisfy the following law in addition to the Functor laws:

• Associative composition: (<<<) <$> f <*> g <*> h = f <*> (g <*> h)

Formally, Apply represents a strong lax semi-monoidal endofunctor.

Members

• apply :: forall a b. f (a -> b) -> f a -> f b

Instances

• Apply (Function r)
• Apply Array
• Apply Proxy

Operator alias for Control.Apply.apply (left-associative / precedence 4)

Operator alias for Control.Apply.applyFirst (left-associative / precedence 4)

Operator alias for Control.Apply.applySecond (left-associative / precedence 4)

class Plus :: (Type -> Type) -> Constraintclass (Alt f) <= Plus f  where

The Plus type class extends the Alt type class with a value that should be the left and right identity for (<|>).

It is similar to Monoid, except that it applies to types of kind * -> *, like Array or List, rather than concrete types like String or Number.

Plus instances should satisfy the following laws:

• Left identity: empty <|> x == x
• Right identity: x <|> empty == x
• Annihilation: f <$> empty == empty

Members

• empty :: forall a. f a

Instances

• Plus Array

class Functor :: (Type -> Type) -> Constraintclass Functor f  where

A Functor is a type constructor which supports a mapping operation map.

map can be used to turn functions a -> b into functions f a -> f b whose argument and return types use the type constructor f to represent some computational context.

Instances must satisfy the following laws:

• Identity: map identity = identity
• Composition: map (f <<< g) = map f <<< map g

Members

• map :: forall a b. (a -> b) -> f a -> f b

Instances

• Functor (Function r)
• Functor Array
• Functor Proxy

void :: forall f a. Functor f => f a -> f Unit

The void function is used to ignore the type wrapped by a Functor, replacing it with Unit and keeping only the type information provided by the type constructor itself.

void is often useful when using do notation to change the return type of a monadic computation:

main = forE 1 10 \n -> void do
  print n
  print (n * n)

Operator alias for Data.Functor.map (left-associative / precedence 4)

Operator alias for Data.Functor.voidRight (left-associative / precedence 4)

Operator alias for Data.Functor.mapFlipped (left-associative / precedence 1)

Operator alias for Data.Functor.voidLeft (left-associative / precedence 4)