Module

Control.MonadPlus

Package
purescript-control
Repository
purescript/purescript-control

#MonadPlus Source

class (MonadZero m) <= MonadPlus m 

The MonadPlus type class has no members of its own but extends MonadZero with an additional law:

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

Instances

Re-exports from Control.Alt

#Alt Source

class (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.

Members

  • alt :: forall a. f a -> f a -> f a

Instances

#(<|>) Source

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

Re-exports from Control.Alternative

#Alternative Source

class (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

Re-exports from Control.Applicative

#Applicative Source

class (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

#when Source

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

Perform an applicative action when a condition is true.

#unless Source

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

Perform an applicative action unless a condition is true.

#liftA1 Source

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

Re-exports from Control.Apply

#Apply Source

class (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.

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

#(<*>) Source

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

#(<*) Source

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

#(*>) Source

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

Re-exports from Control.Bind

#Bind Source

class (Apply m) <= Bind m  where

The Bind type class extends the Apply type class with a "bind" operation (>>=) which composes computations in sequence, using the return value of one computation to determine the next computation.

The >>= operator can also be expressed using do notation, as follows:

x >>= f = do y <- x
             f y

where the function argument of f is given the name y.

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

  • Associativity: (x >>= f) >>= g = x >>= (\k -> f k >>= g)

Associativity tells us that we can regroup operations which use do notation so that we can unambiguously write, for example:

do x <- m1
   y <- m2 x
   m3 x y

Members

  • bind :: forall a b. m a -> (a -> m b) -> m b

Instances

#join Source

join :: forall a m. Bind m => m (m a) -> m a

Collapse two applications of a monadic type constructor into one.

#ifM Source

ifM :: forall a m. Bind m => m Boolean -> m a -> m a -> m a

Execute a monadic action if a condition holds.

For example:

main = ifM ((< 0.5) <$> random)
         (trace "Heads")
         (trace "Tails")

#(>>=) Source

Operator alias for Control.Bind.bind (left-associative / precedence 1)

#(>=>) Source

Operator alias for Control.Bind.composeKleisli (right-associative / precedence 1)

#(=<<) Source

Operator alias for Control.Bind.bindFlipped (right-associative / precedence 1)

#(<=<) Source

Operator alias for Control.Bind.composeKleisliFlipped (right-associative / precedence 1)

Re-exports from Control.Monad

#Monad Source

class (Applicative m, Bind m) <= Monad m 

The Monad type class combines the operations of the Bind and Applicative type classes. Therefore, Monad instances represent type constructors which support sequential composition, and also lifting of functions of arbitrary arity.

Instances must satisfy the following laws in addition to the Applicative and Bind laws:

  • Left Identity: pure x >>= f = f x
  • Right Identity: x >>= pure = x
  • Applicative Superclass: apply = ap

Instances

#liftM1 Source

liftM1 :: forall m a b. Monad m => (a -> b) -> m a -> m b

liftM1 provides a default implementation of (<$>) for any Monad, without using (<$>) as provided by the Functor-Monad superclass relationship.

liftM1 can therefore be used to write Functor instances as follows:

instance functorF :: Functor F where
  map = liftM1

#ap Source

ap :: forall m a b. Monad m => m (a -> b) -> m a -> m b

ap provides a default implementation of (<*>) for any Monad, without using (<*>) as provided by the Apply-Monad superclass relationship.

ap can therefore be used to write Apply instances as follows:

instance applyF :: Apply F where
  apply = ap

Re-exports from Control.MonadZero

#MonadZero Source

class (Monad m, Alternative m) <= MonadZero m 

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

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

  • Annihilation: empty >>= f = empty

Instances

#guard Source

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

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

For example:

import Prelude
import Control.Monad (bind)
import Control.MonadZero (guard)
import Data.Array ((..))

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

Re-exports from Control.Plus

#Plus Source

class (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

Instances

Re-exports from Data.Functor

#Functor Source

class 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

#void Source

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)

#(<$>) Source

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

#(<$) Source

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

#(<#>) Source

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

#($>) Source

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