option package:base

option x p will either parse p or return x without consuming any input.

One or none. It is useful for modelling any computation that is allowed to fail.

Examples

Using the Alternative instance of Control.Monad.Except, the following functions:

>>> import Control.Monad.Except

>>> canFail = throwError "it failed" :: Except String Int

>>> final = return 42                :: Except String Int

Can be combined by allowing the first function to fail:

>>> runExcept $ canFail *> final
Left "it failed"

>>> runExcept $ optional canFail *> final
Right 42

optional p optionally parses p and always returns ().