A
Traversal reading and writing to the
head of a
non-empty container.
>>> [a,b,c]^? _head
Just a
>>> [a,b,c] & _head .~ d
[d,b,c]
>>> [a,b,c] & _head %~ f
[f a,b,c]
>>> [] & _head %~ f
[]
>>> [1,2,3]^?!_head
1
>>> []^?_head
Nothing
>>> [1,2]^?_head
Just 1
>>> [] & _head .~ 1
[]
>>> [0] & _head .~ 2
[2]
>>> [0,1] & _head .~ 2
[2,1]
This isn't limited to lists.
For instance you can also
traverse the head of a
Seq:
>>> Seq.fromList [a,b,c,d] & _head %~ f
fromList [f a,b,c,d]
>>> Seq.fromList [] ^? _head
Nothing
>>> Seq.fromList [a,b,c,d] ^? _head
Just a
_head :: Traversal' [a] a
_head :: Traversal' (Seq a) a
_head :: Traversal' (Vector a) a