Safe Haskell | Safe-Inferred |
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One of most commonly-asked questions about this package is whether
it provides lenses for working with Map
. It does, but their uses
are perhaps obscured by their genericity. This module exists to provide
documentation for them.
Map
is an instance of At
, so we have a lenses
on values at keys:
>>>
Map.fromList [(1, "world")] ^.at 1
Just "world"
>>>
at 1 .~ Just "world" $ Map.empty
fromList [(1,"world")]
>>>
at 0 ?~ "hello" $ Map.empty
fromList [(0,"hello")]
We can traverse, fold over, and map over key-value pairs in a
Map
, thanks to its TraversableWithIndex
,
FoldableWithIndex
, and
FunctorWithIndex
instances.
>>>
imap const $ Map.fromList [(1, "Venus")]
fromList [(1,1)]
>>>
ifoldMap (\i _ -> Sum i) $ Map.fromList [(2, "Earth"), (3, "Mars")]
Sum {getSum = 5}
>>>
itraverse_ (curry print) $ Map.fromList [(4, "Jupiter")]
(4,"Jupiter")
>>>
itoList $ Map.fromList [(5, "Saturn")]
[(5,"Saturn")]
A related class, Ixed
, allows us to use
ix
to traverse a value at a particular key.
>>>
ix 2 %~ ("New " ++) $ Map.fromList [(2, "Earth")]
fromList [(2,"New Earth")]
>>>
preview (ix 8) $ Map.empty
Nothing
Additionally, Map
has TraverseMin
and
TraverseMax
instances, which let us traverse over
the value at the least and greatest keys, respectively.
>>>
preview traverseMin $ Map.fromList [(5, "Saturn"), (6, "Uranus")]
Just "Saturn"
>>>
preview traverseMax $ Map.fromList [(5, "Saturn"), (6, "Uranus")]
Just "Uranus"