Just package:universum

Specialized version of for_ for Maybe. It's used for code readability. Also helps to avoid space leaks: Foldable.mapM_ space leak.

>>> whenJust Nothing $ \b -> print (not b)

>>> whenJust (Just True) $ \b -> print (not b)
False

Monadic version of whenJust.

The isJust function returns True iff its argument is of the form Just _.

Examples

Basic usage:

>>> isJust (Just 3)
True

>>> isJust (Just ())
True

>>> isJust Nothing
False

Only the outer constructor is taken into consideration:

>>> isJust (Just Nothing)
True

The fromJust function extracts the element out of a Just and throws an error if its argument is Nothing.

Examples

Basic usage:

>>> fromJust (Just 1)
1

>>> 2 * (fromJust (Just 10))
20

>>> 2 * (fromJust Nothing)
*** Exception: Maybe.fromJust: Nothing
...

WARNING: This function is partial. You can use case-matching instead.