This module exists to provide documentation for lenses for working with HashMap, which might otherwise be obscured by their genericity.

HashMap is an instance of At and provides at as a lens on values at keys:

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

>>> HashMap.empty & at 1 .~ Just "world"
fromList [(1,"world")]

>>> HashMap.empty & at 0 .~ Just "hello"
fromList [(0,"hello")]

We can traverse, fold over, and map over key-value pairs in a HashMap, thanks to indexed traversals, folds and setters.

>>> iover imapped const $ HashMap.fromList [(1, "Venus")]
fromList [(1,1)]

>>> ifoldMapOf ifolded (\i _ -> Sum i) $ HashMap.fromList [(2, "Earth"), (3, "Mars")]
Sum {getSum = 5}

>>> itraverseOf_ ifolded (curry print) $ HashMap.fromList [(4, "Jupiter")]
(4,"Jupiter")

>>> itoListOf ifolded $ HashMap.fromList [(5, "Saturn")]
[(5,"Saturn")]

A related class, Ixed, allows us to use ix to traverse a value at a particular key.

>>> HashMap.fromList [(2, "Earth")] & ix 2 %~ ("New " ++)
fromList [(2,"New Earth")]

>>> preview (ix 8) HashMap.empty
Nothing