Documentation

The isLeft function returns True iff its argument is of the form Left _.

Using Control.Lens:

 isLeft ≡ has _Left

>>> isLeft (Left 12)
True

>>> isLeft (Right 12)
False

The isRight function returns True iff its argument is of the form Right _.

Using Control.Lens:

 isRight ≡ has _Right

>>> isRight (Left 12)
False

>>> isRight (Right 12)
True

Extract the left value or a default.

 fromLeft ≡ either id

>>> fromLeft "hello" (Right 42)
"hello"

>>> fromLeft "hello" (Left "world")
"world"

Extract the right value or a default.

 fromRight b ≡ either b id

>>> fromRight "hello" (Right "world")
"world"

>>> fromRight "hello" (Left 42)
"hello"

Extracts the element out of a Left and throws an error if its argument take the form Right _.

Using Control.Lens:

 fromLeft' x ≡ x^?!_Left

>>> fromLeft' (Left 12)
12

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

The mapBoth function takes two functions and applies the first if iff the value takes the form Left _ and the second if the value takes the form Right _.

Using Data.Bifunctor:

 mapBoth = bimap

Using Control.Arrow:

 mapBoth = (+++)

>>> mapBoth (*2) (*3) (Left 4)
Left 8

>>> mapBoth (*2) (*3) (Right 4)
Right 12

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"

The mapRight function takes a function and applies it to an Either value iff the value takes the form Right _.

Using Data.Bifunctor:

 mapRight = second

Using Control.Arrow:

 mapRight = (right)

Using Control.Lens:

 mapRight = over _Right

>>> mapRight (*2) (Left "hello")
Left "hello"

>>> mapRight (*2) (Right 4)
Right 8

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 Left _, otherwise it does nothing.

Using Control.Lens:

 whenLeft ≡ forOf_ _Left

>>> whenLeft (Left 12) print
12

The whenRight 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

A synonym of whenRight.

A synonym of whenLeft.

Maybe get the Left side of an Either.

 leftToMaybe ≡ either Just (const Nothing)

Using Control.Lens:

 leftToMaybe ≡ preview _Left
 leftToMaybe x ≡ x^?_Left

>>> leftToMaybe (Left 12)
Just 12

>>> leftToMaybe (Right 12)
Nothing

Maybe get the Right side of an Either.

 rightToMaybe ≡ either (const Nothing) Just

Using Control.Lens:

 rightToMaybe ≡ preview _Right
 rightToMaybe x ≡ x^?_Right

>>> rightToMaybe (Left 12)
Nothing

>>> rightToMaybe (Right 12)
Just 12