lens-3.10: Lenses, Folds and Traversals

Portabilitynon-portable
Stabilityexperimental
MaintainerEdward Kmett <ekmett@gmail.com>
Safe HaskellTrustworthy

Control.Lens.Cons

Contents

Description

 

Synopsis

Cons

class (Profunctor p, Functor f) => Cons p f s t a b | s -> a, t -> b, s b -> t, t a -> s whereSource

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

Methods

_Cons :: Overloaded p f s t (a, s) (b, t)Source

Most of the time this is a Prism. 

 _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)

However, by including p and f in the class you can write instances that only permit uncons or which only permit cons, or where _head and _tail are lenses and not traversals.

Instances

(Choice p, Applicative f) => Cons p f Text Text Char Char 
(Choice p, Applicative f) => Cons p f Text Text Char Char 
(Choice p, Applicative f) => Cons p f ByteString ByteString Word8 Word8 
(Choice p, Applicative f) => Cons p f ByteString ByteString Word8 Word8 
(Choice p, Applicative f, Unbox a, Unbox b) => Cons p f (Vector a) (Vector b) a b 
(Choice p, Applicative f, Storable a, Storable b) => Cons p f (Vector a) (Vector b) a b 
(Choice p, Applicative f, Prim a, Prim b) => Cons p f (Vector a) (Vector b) a b 
(Choice p, Applicative f) => Cons p f (Vector a) (Vector b) a b 
(Choice p, Applicative f) => Cons p f (Seq a) (Seq b) a b 
(Choice p, Applicative f) => Cons p f [a] [b] a b 
(Choice p, Applicative f) => Cons p f (Deque a) (Deque b) a b 

(<|) :: Cons Reviewed Identity s s a a => a -> s -> sSource

cons an element onto a container.

This is an infix alias for cons.

cons :: Cons Reviewed Identity s s a a => a -> s -> sSource

cons an element onto a container.

uncons :: Cons (->) (Accessor (First (a, s))) 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.

_head :: Cons (->) f s s a a => LensLike' f s aSource

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 (->) f s s a a => LensLike' f s sSource

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 (Profunctor p, Functor f) => Snoc p f s t a b | s -> a, t -> b, s b -> t, t a -> s whereSource

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

Methods

_Snoc :: Overloaded p f s t (s, a) (t, b)Source

Most of the time this is a Prism. 

 _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)

However, by including p and f in the class you can write instances that only permit unsnoc or which only permit snoc or where _init and _last are lenses and not traversals.

Instances

(Choice p, Applicative f) => Snoc p f Text Text Char Char 
(Choice p, Applicative f) => Snoc p f Text Text Char Char 
(Choice p, Applicative f) => Snoc p f ByteString ByteString Word8 Word8 
(Choice p, Applicative f) => Snoc p f ByteString ByteString Word8 Word8 
(Choice p, Applicative f, Unbox a, Unbox b) => Snoc p f (Vector a) (Vector b) a b 
(Choice p, Applicative f, Storable a, Storable b) => Snoc p f (Vector a) (Vector b) a b 
(Choice p, Applicative f, Prim a, Prim b) => Snoc p f (Vector a) (Vector b) a b 
(Choice p, Applicative f) => Snoc p f (Vector a) (Vector b) a b 
(Choice p, Applicative f) => Snoc p f (Seq a) (Seq b) a b 
(Choice p, Applicative f) => Snoc p f [a] [b] a b 
(Choice p, Applicative f) => Snoc p f (Deque a) (Deque b) a b 

(|>) :: Snoc Reviewed Identity s s a a => s -> a -> sSource

snoc an element onto the end of a container.

This is an infix alias for snoc.

snoc :: Snoc Reviewed Identity s s a a => s -> a -> sSource

snoc an element onto the end of a container.

unsnoc :: Snoc (->) (Accessor (First (s, a))) 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.

_init :: Snoc (->) f s s a a => LensLike' f s sSource

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 (->) f s s a a => LensLike' f s aSource

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