Data.List.Lens

Traversals for manipulating parts of a list.

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 Control.Lens.Prism.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 Control.Lens.Prism.Prism for accessing the end of a list. Note that this Control.Lens.Prism.Prism always will need to traverse the whole list.

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

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

An instance of Plated allows for finding locations in the list where a traversal matches.

>>> [Nothing, Just 7, Just 3, Nothing] & deep (ix 0 . _Just) +~ 10[Nothing,Just 17,Just 3,Nothing]

An instance of Control.Lens.Iso.Reversing provides an Control.Lens.Iso.Iso between a list and its reverse.

>>> "live" & reversed %~ ('d':)"lived"

It's possible to work under a prefix or suffix of a list using Prefixed and Suffixed.

>>> "preview" ^? prefixed "pre"Just "view"

>>> suffixed ".o" # "hello""hello.o"

At present, Data.List.Lens re-exports Prefixed and Suffixed for backwards compatibility, as prefixed and suffixed used to be top-level functions defined in this module. This may change in a future major release of lens.

Finally, it's possible to traverse, fold over, and map over index-value pairs thanks to instances of TraversableWithIndex, FoldableWithIndex, and 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')