Documentation

List index (subscript) operator, starting from 0. Returns Nothing if the index is out of bounds

>>> ['a', 'b', 'c'] !? 0
Just 'a'
>>> ['a', 'b', 'c'] !? 2
Just 'c'
>>> ['a', 'b', 'c'] !? 3
Nothing
>>> ['a', 'b', 'c'] !? (-1)
Nothing

This is the total variant of the partial !! operator.

WARNING: This function takes linear time in the index.

\(\mathcal{O}(n)\). Decompose a list into init and last.

• If the list is empty, returns Nothing.
• If the list is non-empty, returns Just (xs, x), where xs is the initial part of the list and x is its last element.

Since: 4.19.0.0

>>> unsnoc []
Nothing
>>> unsnoc [1]
Just ([],1)
>>> unsnoc [1, 2, 3]
Just ([1,2],3)

Laziness:

>>> fst <$> unsnoc [undefined]
Just []
>>> head . fst <$> unsnoc (1 : undefined)
Just *** Exception: Prelude.undefined
>>> head . fst <$> unsnoc (1 : 2 : undefined)
Just 1

unsnoc is dual to uncons: for a finite list xs

unsnoc xs = (\(hd, tl) -> (reverse tl, hd)) <$> uncons (reverse xs)