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

Delete the value associated with a key in a Map-like container

sans k = at k .~ Nothing

An indexed version of at.

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

>>> iat 1 %@~ (\i x -> if odd i then Just "hello" else Nothing) $ Map.empty
fromList [(1,"hello")]

>>> iat 2 %@~ (\i x -> if odd i then Just "hello" else Nothing) $ Map.empty
fromList []

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

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.

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

An indexed version of ix.

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

>>> Seq.fromList [a,b,c,d] & iix 2 .@~ h
fromList [a,b,h 2,d]

>>> Seq.fromList [a,b,c,d] ^@? iix 2
Just (2,c)

>>> Seq.fromList [] ^@? iix 2
Nothing

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

An indexed version of contains.

>>> IntSet.fromList [1,2,3,4] ^@. icontains 3
(3,True)

>>> IntSet.fromList [1,2,3,4] ^@. icontains 5
(5,False)

>>> IntSet.fromList [1,2,3,4] & icontains 3 %@~ \i x -> if odd i then not x else x
fromList [1,2,4]

>>> IntSet.fromList [1,2,3,4] & icontains 3 %@~ \i x -> if even i then not x else x
fromList [1,2,3,4]