Optic.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

Minimal complete definition

at

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

Defined in Control.Lens.At

Methods

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

At (Maybe a)

Defined in Control.Lens.At

Methods

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

At (IntMap a)

Defined in Control.Lens.At

Methods

at :: Index (IntMap a) -> Lens' (IntMap a) (Maybe (IxValue (IntMap a)))

Ord k => At (Set k)

Defined in Control.Lens.At

Methods

at :: Index (Set k) -> Lens' (Set k) (Maybe (IxValue (Set k)))

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

Defined in Control.Lens.At

Methods

at :: Index (HashSet k) -> Lens' (HashSet k) (Maybe (IxValue (HashSet k)))

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

Defined in Control.Lens.At

Methods

at :: Index (HashMap k a) -> Lens' (HashMap k a) (Maybe (IxValue (HashMap k a)))

Ord k => At (Map k a)

Defined in Control.Lens.At

Methods

at :: Index (Map k a) -> Lens' (Map k a) (Maybe (IxValue (Map k a)))

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

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

sans k = at k .~ Nothing

Re-exports

module Optic.Traversal.Ixed