recursion.elm

import Graphics.Element exposing (show)
import Graphics.Collage exposing (collage, filled, move, ngon)
import Color exposing (green, purple)
import List exposing (filter)

-- list length(http://elm-lang.org/examples/length)
-- main =
--   show (length [1..9]) -- 9

length : List a -> Int
length list =
  case list of
    [] ->
      0
    first :: rest ->
      1 + length rest

-- zip(http://elm-lang.org/examples/zip)

-- main =
--   show (zip ["Tom", "Sue", "Bob"] [45, 31, 26]) -- [("Tom",45),("Sue",31),("Bob",26)]

zip : List a -> List b -> List (a, b)
zip xs xy =
  case (xs, xy) of
    (x :: xs', y :: ys') ->
      (x, y) :: zip xs' ys'

    (_, _) ->
      []

-- quick-sort(http://elm-lang.org/examples/quick-sort)

-- main =
--   show (quicksort [5, 3, 8, 1, 9, 4, 7]) -- [1,3,4,5,7,8,9]

quicksort : List comparable -> List comparable
quicksort list =
  case list of
    [] ->
      []
    pivot :: rest ->
      let
        lower = filter (\n -> n <= pivot) rest
        higher = filter (\n -> n > pivot) rest
      in
        quicksort lower ++ [pivot] ++ quicksort higher


-- merge-sort(http://elm-lang.org/examples/merge-sort)

main =
  show (mergesort[5, 3, 8, 1, 9, 4, 7]) -- [1,3,4,5,7,8,9]

mergesort : List comparable -> List comparable
mergesort list =
  case list of
    [] ->
      list
    [_] ->
      list
    _ ->
      let
        (xs, ys) = split list
      in
        merge (mergesort xs) (mergesort ys)

split : List a -> (List a, List a)
split list =
  case list of
    [] ->
      ([], [])

    x :: rest ->
      let
        (xs, ys) = split rest
      in
        (ys, x :: xs)

merge : List comparable -> List comparable -> List comparable
merge xs ys =
  case (xs, ys) of
    (x :: xs', y :: ys') ->
      if x < y
        then x :: merge xs' ys
        else y :: merge xs ys'
    ([], rest) ->
      rest

    (rest, []) ->
      rest


-- main =
--   show (isEven 2, filter isEven [1..9]) -- (True,[2,4,6,8])


isEven : Int -> Bool
isEven int =
  int % 2 == 0