MaybeT package:universum

The parameterizable maybe monad, obtained by composing an arbitrary monad with the Maybe monad. Computations are actions that may produce a value or exit. The return function yields a computation that produces that value, while >>= sequences two subcomputations, exiting if either computation does.

Maps Maybe to Either wrapping default value into Right.

>>> maybeToLeft True (Just "aba")
Left "aba"

>>> maybeToLeft True Nothing
Right True

Maps Maybe to Either wrapping default value into Left.

>>> maybeToRight True (Just "aba")
Right "aba"

>>> maybeToRight True Nothing
Left True

Convert a MaybeT computation to ExceptT, with a default exception value.

The maybeToList function returns an empty list when given Nothing or a singleton list when given Just.

Examples

Basic usage:

>>> maybeToList (Just 7)
[7]

>>> maybeToList Nothing
[]

One can use maybeToList to avoid pattern matching when combined with a function that (safely) works on lists:

>>> import Text.Read ( readMaybe )

>>> sum $ maybeToList (readMaybe "3")
3

>>> sum $ maybeToList (readMaybe "")
0

Extracts Monoid value from Maybe returning mempty if Nothing.

>>> maybeToMonoid (Just [1,2,3] :: Maybe [Int])
[1,2,3]

>>> maybeToMonoid (Nothing :: Maybe [Int])
[]

Convert a ExceptT computation to MaybeT, discarding the value of any exception.