Portability | portable |
---|---|
Stability | provisional |
Maintainer | ekmett@gmail.com |
Safe Haskell | Safe-Inferred |
Functions for probing and unwrapping values inside of Either
.
Most of these combinators are provided for pedagogical purposes and exist in more general forms in other libraries. To that end alternative definitions are supplied below.
- isLeft :: Either a b -> Bool
- isRight :: Either a b -> Bool
- fromLeft :: a -> Either a b -> a
- fromRight :: b -> Either a b -> b
- fromLeft' :: Either a b -> a
- fromRight' :: Either a b -> b
- mapBoth :: (a -> c) -> (b -> d) -> Either a b -> Either c d
- mapLeft :: (a -> c) -> Either a b -> Either c b
- mapRight :: (b -> c) -> Either a b -> Either a c
- whenLeft :: Applicative m => Either a b -> (a -> m ()) -> m ()
- whenRight :: Applicative m => Either a b -> (b -> m ()) -> m ()
- unlessLeft :: Applicative m => Either a b -> (b -> m ()) -> m ()
- unlessRight :: Applicative m => Either a b -> (a -> m ()) -> m ()
- leftToMaybe :: Either a b -> Maybe a
- rightToMaybe :: Either a b -> Maybe b
Documentation
fromRight' :: Either a b -> bSource
Extracts the element out of a Right
and
throws an error if its argument take the form
.
Left
_
Using Control.Lens
:
fromRight'
x ≡ x^?!_Right
>>>
fromRight' (Right 12)
12
mapBoth :: (a -> c) -> (b -> d) -> Either a b -> Either c dSource
The mapBoth
function takes two functions and applies the first if iff the value
takes the form
and the second if the value takes the form Left
_
.
Right
_
Using Data.Bifunctor
:
mapBoth
= bimap
Using Control.Arrow
:
mapBoth
= (+++
)
>>>
mapBoth (*2) (*3) (Left 4)
Left 8
>>>
mapBoth (*2) (*3) (Right 4)
Right 12
mapLeft :: (a -> c) -> Either a b -> Either c bSource
The mapLeft
function takes a function and applies it to an Either value
iff the value takes the form 'Left _'.
Using Data.Bifunctor
:
mapLeft
= first
Using Control.Arrow
:
mapLeft
= (left
)
Using Control.Lens
:
mapLeft
= over _Left
>>>
mapLeft (*2) (Left 4)
Left 8
>>>
mapLeft (*2) (Right "hello")
Right "hello"
mapRight :: (b -> c) -> Either a b -> Either a cSource
The mapLeft
function takes a function and applies it to an Either value
iff the value takes the form 'Left _'.
Using Data.Bifunctor
:
mapRight
= first
Using Control.Arrow
:
mapRight
= (right
)
Using Control.Lens
:
mapRight
=over
_Right
>>>
mapRight (*2) (Left "hello")
Left "hello"
>>>
mapRight (*2) (Right 4)
Right 8
whenLeft :: Applicative m => Either a b -> (a -> m ()) -> m ()Source
whenRight :: Applicative m => Either a b -> (b -> m ()) -> m ()Source
The whenLeft
function takes an Either
value and a function which returns a monad.
The monad is only executed when the given argument takes the form Right _
, otherwise
it does nothing.
Using Data.Foldable
:
whenRight
≡forM_
Using Control.Lens
:
whenRight
≡ forOf_ _Right
>>>
whenRight (Right 12) print
12
unlessLeft :: Applicative m => Either a b -> (b -> m ()) -> m ()Source
A synonym of whenRight
.
unlessRight :: Applicative m => Either a b -> (a -> m ()) -> m ()Source
A synonym of whenLeft
.
leftToMaybe :: Either a b -> Maybe aSource
Maybe get the Left
side of an Either
.
leftToMaybe
≡either
Just
(const
Nothing
)
Using Control.Lens
:
leftToMaybe
≡ preview _LeftleftToMaybe
x ≡ x^?_Left
>>>
leftToMaybe (Left 12)
Just 12
>>>
leftToMaybe (Right 12)
Nothing
rightToMaybe :: Either a b -> Maybe bSource
Maybe get the Right
side of an Either
.
rightToMaybe
≡either
(const
Nothing
)Just
Using Control.Lens
:
rightToMaybe
≡ preview _RightrightToMaybe
x ≡ x^?_Right
>>>
rightToMaybe (Left 12)
Nothing
>>>
rightToMaybe (Right 12)
Just 12