Control.Lens.Cons

Cons

class Cons s t a b | s -> a, t -> b, s b -> t, t a -> s where

This class provides a way to attach or detach elements on the left side of a structure in a flexible manner.

Methods

_Cons :: Prism s t (a, s) (b, t)

 _Cons :: Prism [a] [b] (a, [a]) (b, [b])
 _Cons :: Prism (Seq a) (Seq b) (a, Seq a) (b, Seq b)
 _Cons :: Prism (Vector a) (Vector b) (a, Vector a) (b, Vector b)
 _Cons :: Prism' String (Char, String)
 _Cons :: Prism' Text (Char, Text)
 _Cons :: Prism' ByteString (Word8, ByteString)

Instances

Cons ByteString ByteString Word8 Word8

Cons ByteString ByteString Word8 Word8

Cons Text Text Char Char

Cons Text Text Char Char

Cons [a] [b] a b

Cons (Seq a) (Seq b) a b

Cons (Vector a) (Vector b) a b

(Unbox a, Unbox b) => Cons (Vector a) (Vector b) a b

(Storable a, Storable b) => Cons (Vector a) (Vector b) a b

(Prim a, Prim b) => Cons (Vector a) (Vector b) a b

Cons (Deque a) (Deque b) a b

(<|) :: Cons s s a a => a -> s -> s

cons an element onto a container.

This is an infix alias for cons.

>>> a <| []
[a]

>>> a <| [b, c]
[a,b,c]

>>> a <| Seq.fromList []
fromList [a]

>>> a <| Seq.fromList [b, c]
fromList [a,b,c]

cons :: Cons s s a a => a -> s -> s

cons an element onto a container.

>>> cons a []
[a]

>>> cons a [b, c]
[a,b,c]

>>> cons a (Seq.fromList [])
fromList [a]

>>> cons a (Seq.fromList [b, c])
fromList [a,b,c]

uncons :: Cons s s a a => s -> Maybe (a, s)

Attempt to extract the left-most element from a container, and a version of the container without that element.

>>> uncons []
Nothing

>>> uncons [a, b, c]
Just (a,[b,c])

_head :: Cons s s a a => Traversal' s a

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

_tail :: Cons s s a a => Traversal' s s

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

>>> [a,b] & _tail .~ [c,d,e]
[a,c,d,e]

>>> [] & _tail .~ [a,b]
[]

>>> [a,b,c,d,e] & _tail.traverse %~ f
[a,f b,f c,f d,f e]

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

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

>>> [a,b,c]^?_tail
Just [b,c]

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

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

>>> ""^._tail
""

This isn't limited to lists. For instance you can also traverse the tail of a Seq.

>>> Seq.fromList [a,b] & _tail .~ Seq.fromList [c,d,e]
fromList [a,c,d,e]

>>> Seq.fromList [a,b,c] ^? _tail
Just (fromList [b,c])

>>> Seq.fromList [] ^? _tail
Nothing

 _tail :: Traversal' [a] [a]
 _tail :: Traversal' (Seq a) (Seq a)
 _tail :: Traversal' (Vector a) (Vector a)

Snoc

class Snoc s t a b | s -> a, t -> b, s b -> t, t a -> s where

This class provides a way to attach or detach elements on the right side of a structure in a flexible manner.

Methods

_Snoc :: Prism s t (s, a) (t, b)

 _Snoc :: Prism [a] [b] ([a], a) ([b], b)
 _Snoc :: Prism (Seq a) (Seq b) (Seq a, a) (Seq b, b)
 _Snoc :: Prism (Vector a) (Vector b) (Vector a, a) (Vector b, b)
 _Snoc :: Prism' String (String, Char)
 _Snoc :: Prism' Text (Text, Char)
 _Snoc :: Prism' ByteString (ByteString, Word8)

Instances

Snoc ByteString ByteString Word8 Word8

Snoc ByteString ByteString Word8 Word8

Snoc Text Text Char Char

Snoc Text Text Char Char

Snoc [a] [b] a b

Snoc (Seq a) (Seq b) a b

Snoc (Vector a) (Vector b) a b

(Unbox a, Unbox b) => Snoc (Vector a) (Vector b) a b

(Storable a, Storable b) => Snoc (Vector a) (Vector b) a b

(Prim a, Prim b) => Snoc (Vector a) (Vector b) a b

Snoc (Deque a) (Deque b) a b

(|>) :: Snoc s s a a => s -> a -> s

snoc an element onto the end of a container.

This is an infix alias for snoc.

>>> Seq.fromList [] |> a
fromList [a]

>>> Seq.fromList [b, c] |> a
fromList [b,c,a]

>>> LazyT.pack "hello" |> '!'
"hello!"

snoc :: Snoc s s a a => s -> a -> s

snoc an element onto the end of a container.

>>> snoc (Seq.fromList []) a
fromList [a]

>>> snoc (Seq.fromList [b, c]) a
fromList [b,c,a]

>>> snoc (LazyT.pack "hello") '!'
"hello!"

unsnoc :: Snoc s s a a => s -> Maybe (s, a)

Attempt to extract the right-most element from a container, and a version of the container without that element.

>>> unsnoc (LazyT.pack "hello!")
Just ("hello",'!')

>>> unsnoc (LazyT.pack "")
Nothing

>>> unsnoc (Seq.fromList [b,c,a])
Just (fromList [b,c],a)

>>> unsnoc (Seq.fromList [])
Nothing

_init :: Snoc s s a a => Traversal' s s

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

>>> [a,b,c,d]^?_init
Just [a,b,c]

>>> []^?_init
Nothing

>>> [a,b] & _init .~ [c,d,e]
[c,d,e,b]

>>> [] & _init .~ [a,b]
[]

>>> [a,b,c,d] & _init.traverse %~ f
[f a,f b,f c,d]

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

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

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

>>> ""^._init
""

 _init :: Traversal' [a] [a]
 _init :: Traversal' (Seq a) (Seq a)
 _init :: Traversal' (Vector a) (Vector a)

_last :: Snoc s s a a => Traversal' s a

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

>>> [a,b,c]^?!_last
c

>>> []^?_last
Nothing

>>> [a,b,c] & _last %~ f
[a,b,f c]

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

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

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

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

This Traversal is not limited to lists, however. We can also work with other containers, such as a Vector.

>>> Vector.fromList "abcde" ^? _last
Just 'e'

>>> Vector.empty ^? _last
Nothing

>>> Vector.fromList "abcde" & _last .~ 'Q'
fromList "abcdQ"

 _last :: Traversal' [a] a
 _last :: Traversal' (Seq a) a
 _last :: Traversal' (Vector a) a