Modulelens-5.3.5Haskell2010

Data.Map.Lens

One of most commonly-asked questions about this package is whether it provides lenses for working with Data.Map.Map. It does, but their uses are perhaps obscured by their genericity. This module exists to provide documentation for them.

Data.Map.Map is an instance of At, so we have a lenses on values at keys:

Example1 expression

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

Example1 expression

>>> at 1 .~ Just "world" $ Map.emptyfromList [(1,"world")]

Example1 expression

>>> at 0 ?~ "hello" $ Map.emptyfromList [(0,"hello")]

We can traverse, fold over, and map over key-value pairs in a Data.Map.Map, thanks to its TraversableWithIndex, FoldableWithIndex, and FunctorWithIndex instances.

Example1 expression

>>> imap const $ Map.fromList [(1, "Venus")]fromList [(1,1)]

Example1 expression

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

Example1 expression

>>> itraverse_ (curry print) $ Map.fromList [(4, "Jupiter")](4,"Jupiter")

Example1 expression

>>> itoList $ Map.fromList [(5, "Saturn")][(5,"Saturn")]

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

Example1 expression

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

Example1 expression

>>> preview (ix 8) $ Map.emptyNothing

Additionally, Data.Map.Map has TraverseMin and TraverseMax instances, which let us traverse over the value at the least and greatest keys, respectively.

Example1 expression

>>> preview traverseMin $ Map.fromList [(5, "Saturn"), (6, "Uranus")]Just "Saturn"

Example1 expression

>>> preview traverseMax $ Map.fromList [(5, "Saturn"), (6, "Uranus")]Just "Uranus"

1 value

Packagelens-5.3.5
Exports1
LanguageHaskell2010
LicenceBSD-2-Clause
SourceLens.hs

valuetoMapOf :: IndexedGetting i (Map i a) s a -> s -> Map i a

Construct a map from a IndexedGetter, Control.Lens.Fold.IndexedFold, Control.Lens.Traversal.IndexedTraversal or Control.Lens.Lens.IndexedLens

The construction is left-biased (see Data.Map.Lazy.union), i.e. the first occurrences of keys in the fold or traversal order are preferred.

Example1 expression

>>> toMapOf folded ["hello", "world"]fromList [(0,"hello"),(1,"world")]

Example1 expression

>>> toMapOf (folded . ifolded) [('a',"alpha"),('b', "beta")]fromList [('a',"alpha"),('b',"beta")]

Example1 expression

>>> toMapOf (folded <.> folded) ["foo", "bar"]fromList [((0,0),'f'),((0,1),'o'),((0,2),'o'),((1,0),'b'),((1,1),'a'),((1,2),'r')]

Example1 expression

>>> toMapOf ifolded $ Map.fromList [('a', "hello"), ('b', "world")]fromList [('a',"hello"),('b',"world")]

toMapOf ::          IndexedGetter i s a     -> s -> Map i a
toMapOf :: Ord i => IndexedFold i s a       -> s -> Map i a
toMapOf ::          IndexedLens' i s a      -> s -> Map i a
toMapOf :: Ord i => IndexedTraversal' i s a -> s -> Map i a