Control.Lens.At

At

class Ixed m => At m where

At provides a Lens that can be used to read, write or delete the value associated with a key in a Map-like container on an ad hoc basis.

An instance of At should satisfy:

 ix k ≡ at k . traverse

Methods

at :: Index m -> Lens' m (Maybe (IxValue m))

>>> Map.fromList [(1,"world")] ^.at 1
Just "world"

>>> at 1 ?~ "hello" $ Map.empty
fromList [(1,"hello")]

Note: Map-like containers form a reasonable instance, but not Array-like ones, where you cannot satisfy the Lens laws.

Instances

At IntSet

At (Maybe a)

At (IntMap a)

Ord k => At (Set k)

(Eq k, Hashable k) => At (HashSet k)

Ord k => At (Map k a)

(Eq k, Hashable k) => At (HashMap k a)

sans :: At m => Index m -> m -> m

Ixed

type family Index s :: *

type family IxValue m :: *

This provides a common notion of a value at an index that is shared by both Ixed and At.

class Ixed m where

This simple Traversal lets you traverse the value at a given key in a Map or element at an ordinal position in a list or Seq.

Methods

ix :: Index m -> Traversal' m (IxValue m)

This simple Traversal lets you traverse the value at a given key in a Map or element at an ordinal position in a list or Seq.

NB: Setting the value of this Traversal will only set the value in at if it is already present.

If you want to be able to insert missing values, you want at.

>>> Seq.fromList [a,b,c,d] & ix 2 %~ f
fromList [a,b,f c,d]

>>> Seq.fromList [a,b,c,d] & ix 2 .~ e
fromList [a,b,e,d]

>>> Seq.fromList [a,b,c,d] ^? ix 2
Just c

>>> Seq.fromList [] ^? ix 2
Nothing

Instances

Ixed ByteString

Ixed ByteString

Ixed IntSet

Ixed Text

Ixed Text

Ixed [a]

Ixed (Maybe a)

Ixed (Identity a)

Ixed (Tree a)

Ixed (Seq a)

Ixed (IntMap a)

Ord k => Ixed (Set k)

Ixed (NonEmpty a)

(Eq k, Hashable k) => Ixed (HashSet k)

Ixed (Vector a)

Unbox a => Ixed (Vector a)

Storable a => Ixed (Vector a)

Prim a => Ixed (Vector a)

Eq e => Ixed (e -> a)

~ * a b => Ixed (a, b)

(IArray UArray e, Ix i) => Ixed (UArray i e)

 arr ! i ≡ arr ^. ix i
 arr // [(i,e)] ≡ ix i .~ e $ arr

Ix i => Ixed (Array i e)

 arr ! i ≡ arr ^. ix i
 arr // [(i,e)] ≡ ix i .~ e $ arr

Ord k => Ixed (Map k a)

(Eq k, Hashable k) => Ixed (HashMap k a)

(~ * a b, ~ * a c) => Ixed (a, b, c)

(~ * a b, ~ * a c, ~ * a d) => Ixed (a, b, c, d)

(~ * a b, ~ * a c, ~ * a d, ~ * a e) => Ixed (a, b, c, d, e)

(~ * a b, ~ * a c, ~ * a d, ~ * a e, ~ * a f) => Ixed (a, b, c, d, e, f)

(~ * a b, ~ * a c, ~ * a d, ~ * a e, ~ * a f, ~ * a g) => Ixed (a, b, c, d, e, f, g)

(~ * a b, ~ * a c, ~ * a d, ~ * a e, ~ * a f, ~ * a g, ~ * a h) => Ixed (a, b, c, d, e, f, g, h)

(~ * a b, ~ * a c, ~ * a d, ~ * a e, ~ * a f, ~ * a g, ~ * a h, ~ * a i) => Ixed (a, b, c, d, e, f, g, h, i)

ixAt :: At m => Index m -> Traversal' m (IxValue m)

A definition of ix for types with an At instance. This is the default if you don't specify a definition for ix.

Contains

class Contains m where

This class provides a simple IndexedFold (or IndexedTraversal) that lets you view (and modify) information about whether or not a container contains a given Index.

Methods

contains :: Index m -> Lens' m Bool

>>> IntSet.fromList [1,2,3,4] ^. contains 3
True

>>> IntSet.fromList [1,2,3,4] ^. contains 5
False

>>> IntSet.fromList [1,2,3,4] & contains 3 .~ False
fromList [1,2,4]

Instances

Contains IntSet

Ord a => Contains (Set a)

(Eq a, Hashable a) => Contains (HashSet a)