This library is an experiment in porting Gabriella Gonzalez' foldl library (in Haskell) to Elm.

A Fold is a "left fold", the arguments to List.foldl plus an extraction function bottled up as a data type. You can either construct a Fold from other Folds using combinators (map, andMap, etc.) or manually from the unfoldFold which takes an initial state, an update function, and the extraction function to produce output from state (similar to init, update, view in Elm—TEA and Fold are Moore machines). Then pass it and an input list to foldList to run it and produce a single output, or scanList to produce output for each step through the list.

import Fold exposing (..)

foldList (List.range 0 10) sum
--> 55

avg : Fold Float Float
avg = map2 (/) sum length

range : Fold comparable (Maybe ( comparable, comparable ))
range = map2 (Maybe.map2 Tuple.pair) minimum maximum

map2 (\a r -> { avg = a, range = r }) avg range
    |> foldList [ 5, 1, 9, 99 ]
--> { avg = 28.5, range = Just ( 1, 99 ) }

No, sadly, it is relatively slow. There's a trick with an existential state s type to make a more efficient encoding (forall s. (a -> s -> s) s (s -> b)) in languages that support it, which would defer computing s -> b as a last step in foldList, but in the current implementation we are producing a Fold at each step through the input list.