lens-4.12.3: Lenses, Folds and Traversals

Copyright(C) 2012-15 Edward Kmett
LicenseBSD-style (see the file LICENSE)
MaintainerEdward Kmett <ekmett@gmail.com>
Stabilityexperimental
Portabilitynon-portable
Safe HaskellTrustworthy
LanguageHaskell98

Control.Lens.Cons

Contents

Description

 

Synopsis

Cons

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

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

Minimal complete definition

Nothing

Methods

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

_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 
(~) * a b => Cons (NonEmpty a) (NonEmpty 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 infixr 5 Source

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 infixr 5 Source

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) Source

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 Source

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 Source

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 Source

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

Minimal complete definition

Nothing

Methods

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

_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 
(~) * a b => Snoc (NonEmpty a) (NonEmpty 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 infixl 5 Source

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 infixl 5 Source

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) Source

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 Source

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 Source

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') == Vector.fromList "abcdQ"
True
_last :: Traversal' [a] a
_last :: Traversal' (Seq a) a
_last :: Traversal' (Vector a) a