-- | 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 'Control.Lens.At.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 -- 'Data.Map.Map', thanks to its 'Control.Lens.Indexed.TraversableWithIndex', -- 'Control.Lens.Indexed.FoldableWithIndex', and -- 'Control.Lens.Indexed.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, 'Control.Lens.At.Ixed', allows us to use -- 'Control.Lens.At.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, 'Data.Map.Map' has 'Control.Lens.Traversal.TraverseMin' and -- 'Control.Lens.Traversal.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" module Data.Map.Lens () where -- $setup -- >>> import Control.Lens -- >>> import Data.Monoid -- >>> import qualified Data.Map as Map -- >>> :set -XNoOverloadedStrings