:: Either a b -> a package:either

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

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