Safe Haskell | None |
---|---|

Language | Haskell98 |

The `These`

type and associated operations. Now enhanced with `Control.Lens`

magic!

- data These a b
- these :: (a -> c) -> (b -> c) -> (a -> b -> c) -> These a b -> c
- fromThese :: a -> b -> These a b -> (a, b)
- mergeThese :: (a -> a -> a) -> These a a -> a
- mergeTheseWith :: (a -> c) -> (b -> c) -> (c -> c -> c) -> These a b -> c
- here :: Applicative f => (a -> f b) -> These a t -> f (These b t)
- there :: Applicative f => (a -> f b) -> These t a -> f (These t b)
- _This :: (Choice p, Applicative f) => p a (f a) -> p (These a b) (f (These a b))
- _That :: (Choice p, Applicative f) => p b (f b) -> p (These a b) (f (These a b))
- _These :: (Choice p, Applicative f) => p (a, b) (f (a, b)) -> p (These a b) (f (These a b))
- justThis :: These a b -> Maybe a
- justThat :: These a b -> Maybe b
- justThese :: These a b -> Maybe (a, b)
- catThis :: [These a b] -> [a]
- catThat :: [These a b] -> [b]
- catThese :: [These a b] -> [(a, b)]
- partitionThese :: [These a b] -> ([(a, b)], ([a], [b]))
- isThis :: These a b -> Bool
- isThat :: These a b -> Bool
- isThese :: These a b -> Bool
- mapThese :: (a -> c) -> (b -> d) -> These a b -> These c d
- mapThis :: (a -> c) -> These a b -> These c b
- mapThat :: (b -> d) -> These a b -> These a d

# Documentation

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.

# Functions to get rid of `These`

these :: (a -> c) -> (b -> c) -> (a -> b -> c) -> These a b -> c Source #

Case analysis for the `These`

type.

mergeThese :: (a -> a -> a) -> These a a -> a Source #

Coalesce with the provided operation.

mergeTheseWith :: (a -> c) -> (b -> c) -> (c -> c -> c) -> These a b -> c Source #

BiMap and coalesce results 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 #

# Case selections

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

Select each constructor and partition them into separate lists.

# Case predicates

# Map operations

For zipping and unzipping of structures with `These`

values, see
Data.Align.