Data.List.Lens

Traversals for manipulating parts of a list.

Partial Lenses

_head :: SimpleIndexedTraversal Int [a] a

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

>>> [1,2,3]^?!_head
1

>>> []^?_head
Nothing

>>> [1,2]^?_head
Just 1

>>> [] & _head .~ 1
[]

>>> [0] & _head .~ 2
[2]

>>> [0,1] & _head .~ 2
[2,1]

_tail :: Simple Traversal [a] [a]

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

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

>>> [] & _tail .~ [1,2]
[]

>>> [1,2,3]^?_tail
Just [2,3]

>>> [1,2]^?!_tail
[2]

>>> "hello"^._tail
"ello"

>>> ""^._tail
""

_last :: SimpleIndexedTraversal Int [a] a

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

>>> [1,2,3]^?!_last
3

>>> []^?_last
Nothing

>>> [1,2]^?_last
Just 2

>>> [] & _last .~ 1
[]

>>> [0] & _last .~ 2
[2]

>>> [0,1] & _last .~ 2
[0,2]

_init :: Simple Traversal [a] [a]

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

>>> [1,2,3,4]^?!_init
[1,2,3]

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

>>> [] & _init .~ [1,2]
[]

>>> [1,2,3]^?_init
Just [1,2]

>>> [1,2]^?!_init
[1]

>>> "hello"^._init
"hell"

>>> ""^._init
""