map -package:amazonka-core -package:basement -package:base-prelude package:foldl

Fold pairs into a map.

Converts an effectful function to a fold. Specialized version of sink.

Given a Fold, produces a HashMap which applies that fold to each a separated by key k.

>>> List.sort (HashMap.toList (fold (foldByKeyHashMap Control.Foldl.sum) [("a",1), ("b",2), ("b",20), ("a",10)]))
[("a",11),("b",22)]

Given a Fold, produces a Map which applies that fold to each a separated by key k.

>>> fold (foldByKeyMap Control.Foldl.sum) [("a",1), ("b",2), ("b",20), ("a",10)]
fromList [("a",11),("b",22)]

Convert a "foldMap" to a Fold

Fold pairs into a hash-map.

(postmapM f folder) returns a new FoldM where f is applied to the final value.

postmapM return = id

postmapM (f >=> g) = postmapM g . postmapM f

postmapM k (pure r) = k r

(premap f folder) returns a new Fold where f is applied at each step

fold (premap f folder) list = fold folder (List.map f list)

>>> fold (premap Sum Foldl.mconcat) [1..10]
Sum {getSum = 55}

>>> fold Foldl.mconcat (List.map Sum [1..10])
Sum {getSum = 55}

premap id = id

premap (f . g) = premap g . premap f

premap k (pure r) = pure r

premap k (f <*> x) = premap k f <*> premap k x

(premapM f folder) returns a new FoldM where f is applied to each input element

premapM return = id

premapM (f <=< g) = premap g . premap f

premapM k (pure r) = pure r

premapM k (f <*> x) = premapM k f <*> premapM k x

(premap f scaner) returns a new Scan where f is applied at each step

scan (premap f scaner) list = scan scaner (map f list)

premap id = id

premap (f . g) = premap g . premap f

premap k (pure r) = pure r

premap k (f <*> x) = premap k f <*> premap k x

(premapM f scaner) returns a new ScanM where f is applied to each input element

premapM return = id

premapM (f <=< g) = premap g . premap f

premapM k (pure r) = pure r

premapM k (f <*> x) = premapM k f <*> premapM k x