Data.List.Lens

Traversals for manipulating parts of a list.

Documentation

_head :: Simple Lens [a] a

A lens reading and writing to the head of a non-empty list

>>> [1,2,3]^._head
1

_tail :: Simple Lens [a] [a]

A lens reading and writing to the tail of a non-empty list

>>> _tail .~ [3,4,5] $ [1,2]
[1,3,4,5]

_last :: Simple Lens [a] a

A lens reading and writing to the last element of a non-empty list

>>> [1,2]^._last
2

_init :: Simple Lens [a] [a]

A lens reading and replacing all but the a last element of a non-empty list

>>> [1,2,3,4]^._init
[1,2,3]

interspersed :: a -> Getter [a] [a]

Obtain a version of the list with the supplied value interspersed.

>>> "abcde"^.interspersed ','
"a,b,c,d,e"

 xs^.interspersed a = intersperse a xs

intercalated :: [a] -> Getter [[a]] [a]

Obtain a version of the list with the supplied value intercalated

Traversals

traverseList :: IndexedTraversal Int [a] [b] a b

Indexed traversal of a list. The position in the list is available as the index.

traverseHead :: SimpleIndexedTraversal Int [a] a

The traversal for reading and writing to the head of a list

The position of the head in the original list (0) is available as the index.

>>> traverseHead +~ 1 $ [1,2,3]
[2,2,3]

 traverseHead :: Applicative f => (a -> f a) -> [a] -> f [a]

traverseTail :: SimpleIndexedTraversal Int [a] a

Traversal for editing the tail of a list.

The position of each element in the original list is available as the index.

>>> traverseTail +~ 1 $ [1,2,3]
[1,3,4]

 traverseTail :: Applicative f => (a -> f a) -> [a] -> f [a]

traverseInit :: SimpleIndexedTraversal Int [a] a

Traverse all but the last element of a list

The position of each element is available as the index.

>>> traverseInit +~ 1 $ [1,2,3]
[2,3,3]

 traverseInit :: Applicative f => (a -> f a) -> [a] -> f [a]

traverseLast :: SimpleIndexedTraversal Int [a] a

Traverse the last element in a list.

The position of the last element in the original list is available as the index.

>>> traverseLast +~ 1 $ [1,2,3]
[1,2,4]

 traverseLast :: Applicative f => (a -> f a) -> [a] -> f [a]