Data.List.Optics

Additional optics for manipulating lists are present more generically in this package.

The Ixed class allows traversing the element at a specific list index.

>>> [0..10] ^? ix 4Just 4

>>> [0..5] & ix 4 .~ 2[0,1,2,3,2,5]

>>> [0..10] ^? ix 14Nothing

>>> [0..5] & ix 14 .~ 2[0,1,2,3,4,5]

The Cons and AsEmpty classes provide Prisms for list constructors.

>>> [1..10] ^? _ConsJust (1,[2,3,4,5,6,7,8,9,10])

>>> [] ^? _ConsNothing

>>> [] ^? _EmptyJust ()

>>> _Cons # (1, _Empty # ()) :: [Int][1]

Additionally, Snoc provides a Prism for accessing the end of a list. Note that this Prism always will need to traverse the whole list.

>>> [1..5] ^? _SnocJust ([1,2,3,4],5)

>>> _Snoc # ([1,2],5)[1,2,5]

Finally, it's possible to traverse, fold over, and map over index-value pairs thanks to instances of Optics.Indexed.Core.TraversableWithIndex, Optics.Indexed.Core.FoldableWithIndex, and Optics.Indexed.Core.FunctorWithIndex.

>>> imap (,) "Hello"[(0,'H'),(1,'e'),(2,'l'),(3,'l'),(4,'o')]

>>> ifoldMap replicate "Hello""ellllloooo"

>>> itraverse_ (curry print) "Hello"(0,'H')(1,'e')(2,'l')(3,'l')(4,'o')