elm/exercises/list-ops/ListOps.example

module ListOps (..) where


length : List a -> Int
length list =
  foldl (\_ acc -> 1 + acc) 0 list


reverse : List a -> List a
reverse list =
  foldl (::) [] list


foldl : (a -> b -> b) -> b -> List a -> b
foldl f acc list =
  case list of
    [] ->
      acc

    head :: tail ->
      foldl f (f head acc) tail


foldr : (a -> b -> b) -> b -> List a -> b
foldr f acc list =
  case list of
    [] ->
      acc

    head :: tail ->
      f head (foldr f acc tail)


map : (a -> b) -> List a -> List b
map f list =
  foldr (\x acc -> f x :: acc) [] list


filter : (a -> Bool) -> List a -> List a
filter f list =
  foldr
    (\x acc ->
      if f x then
        x :: acc
      else
        acc
    )
    []
    list


append : List a -> List a -> List a
append xs ys =
  foldr (::) ys xs


concat : List (List a) -> List a
concat list =
  foldr append [] list