these-0.2: An either-or-both data type, with corresponding hybrid error/writer monad transformer.

Data.These

Contents

Description

The `These` type and associated operations. Now enhanced with `Control.Lens` magic!

Synopsis

# Documentation

data These a b Source

The `These` type represents values with two non-exclusive possibilities.

This can be useful to represent combinations of two values, where the combination is defined if either input is. Algebraically, the type `These A B` represents `(A + B + AB)`, which doesn't factor easily into sums and products--a type like `Either A (B, Maybe A)` is unclear and awkward to use.

`These` has straightforward instances of `Functor`, `Monad`, &c., and behaves like a hybrid error/writer monad, as would be expected.

Constructors

 This a That b These a b

Instances

 Bitraversable1 These Bitraversable These Bifunctor These Bifoldable1 These Bifoldable These Bicrosswalk These (Monad (These c), Monoid c) => MonadChronicle c (These c) Monoid a => Monad (These a) Functor (These a) (Functor (These a), Monoid a) => Applicative (These a) Foldable (These a) (Functor (These a), Foldable (These a)) => Traversable (These a) (Functor (These a), Monoid a) => Apply (These a) (Apply (These a), Monoid a) => Bind (These a) (Functor (These a), Foldable (These a)) => Crosswalk (These a) (Eq a, Eq b) => Eq (These a b) (Eq (These a b), Ord a, Ord b) => Ord (These a b) (Read a, Read b) => Read (These a b) (Show a, Show b) => Show (These a b) (Semigroup a, Semigroup b) => Semigroup (These a b)

# Functions to get rid of `These`

these :: (a -> c) -> (b -> c) -> (a -> b -> c) -> These a b -> cSource

Case analysis for the `These` type.

fromThese :: a -> b -> These a b -> (a, b)Source

Takes two default values and produces a tuple.

mergeThese :: (a -> a -> a) -> These a a -> aSource

Coalesce with the provided operation.

# Traversals

here :: Applicative f => (a -> f b) -> These a t -> f (These b t)Source

A `Traversal` of the first half of a `These`, suitable for use with `Control.Lens`.

there :: Applicative f => (a -> f b) -> These t a -> f (These t b)Source

A `Traversal` of the second half of a `These`, suitable for use with `Control.Lens`.

# Prisms

_This :: (Choice p, Applicative f) => p a (f a) -> p (These a b) (f (These a b))Source

A `Prism` selecting the `This` constructor.

_That :: (Choice p, Applicative f) => p b (f b) -> p (These a b) (f (These a b))Source

A `Prism` selecting the `That` constructor.

_These :: (Choice p, Applicative f) => p (a, b) (f (a, b)) -> p (These a b) (f (These a b))Source

A `Prism` selecting the `These` constructor. `These` names are ridiculous!

# Case selections

justThis :: These a b -> Maybe aSource

``justThis` = preview `_This``

justThat :: These a b -> Maybe bSource

``justThat` = preview `_That``

justThese :: These a b -> Maybe (a, b)Source

``justThese` = preview `_These``

catThis :: [These a b] -> [a]Source

Select all `This` constructors from a list.

catThat :: [These a b] -> [b]Source

Select all `That` constructors from a list.

catThese :: [These a b] -> [(a, b)]Source

Select all `These` constructors from a list.

partitionThese :: [These a b] -> ([(a, b)], ([a], [b]))Source

Select each constructor and partition them into separate lists.

# Case predicates

isThis :: These a b -> BoolSource

``isThis` = `isJust` . `justThis``

isThat :: These a b -> BoolSource

``isThat` = `isJust` . `justThat``

isThese :: These a b -> BoolSource

``isThese` = `isJust` . `justThese``

# Map operations

mapThese :: (a -> c) -> (b -> d) -> These a b -> These c dSource

`Bifunctor` map.

mapThis :: (a -> c) -> These a b -> These c bSource

``mapThis` = over `here``

mapThat :: (b -> d) -> These a b -> These a dSource

``mapThat` = over `there``

For zipping and unzipping of structures with `These` values, see Data.Align.