This post originated from an RSS feed registered with Scala Buzz by Caoyuan Deng. Original Post: An Example Syntax in Haskell, Erlang and Scala Feed Title: Blogs about Scala from Caoyuan Feed URL: http://blogtrader.org/page/dcaoyuan/feed/entries/atom?tags=scala Feed Description: Blogs about Scala from Caoyuan Deng Latest Scala Buzz Posts Latest Scala Buzz Posts by Caoyuan Deng Latest Posts From Blogs about Scala from Caoyuan

Now it's time to write some real world Scala code, I choose to port Paul R. Brown's perpubplat blog engine, which is written in Haskell. And I have also some curiosities on how the syntax looks in Erlang, so I tried some Erlang code too.

Here's some code piece of entry module in Haskell, Erlang and Scala:

Original Haskell code piece

empty :: Model
empty = Model M.empty M.empty M.empty [] 0

build_model :: [Item] -> Model
build_model [] = empty
build_model items = Model (map_by permatitle sorted_items)
                    bid                    
                    (build_child_map sorted_items)
                    (sorted_items)
                    (n+1)
    where
      sorted_items = sort_by_created_reverse items
      bid = (map_by internal_id sorted_items)
      n = fst . M.findMax $ bid

build_child_map :: [Item] -> M.Map Int [Int]
build_child_map i = build_child_map_ (M.fromList $ (map (\x -> (internal_id x,[])) i)) i

-- Constructed to take advantage of the input being in sorted order.
build_child_map_ :: M.Map Int [Int] -> [Item] -> M.Map Int [Int]
build_child_map_ m [] = m
build_child_map_ m (i:is) = if (parent i == Nothing) then
                                build_child_map_ m is
                            else
                                build_child_map_ (M.insertWith (++) (unwrap $ parent i) [internal_id i] m) is

sort_by_created_reverse :: [Item] -> [Item]
sort_by_created_reverse = sortBy created_sort_reverse

created_sort_reverse :: Item -> Item -> Ordering
created_sort_reverse a b = compare (created b) (created a)

In Erlang:

% @spec empty :: Model
empty() -> #model{}.

% @spec build_model :: [Item] -> Model
build_model([]) -> empty();
build_model(Is) -> 
    SortedIs = sort_by_created_reverse(Is),
    Bid = dict:from_list([{I#item.internal_id, I} || I <- SortedIs]),
    N = lists:max(dict:fetch_keys(Bid)),
    
    #model{by_permatitle = dict:from_list([{X#item.permatitle, X} || X <- SortedIs]),
           by_int_id = Bid,               
           child_map = build_child_map(SortedIs),
           all_items = SortedIs,
           next_id = N + 1}.


% @spec build_child_map :: [Item] -> M.Map Int [Int]
build_child_map(Is) -> build_child_map_(dict:from_list(lists:map(fun (X) -> {X#item.internal_id, []} end), Is), Is).

%% Constructed to take advantage of the input being in sorted order.
% @spec build_child_map_ :: M.Map Int [Int] -> [Item] -> M.Map Int [Int]
build_child_map_(D, []) -> D;
build_child_map_(D, [I|Is]) -> 
    case I#item.parent of 
        undefined ->                
            build_child_map_(D, Is);
        P_Id ->
            build_child_map_(dict:append(unwrap(P_Id), I#item.internal_id, D), Is)
    end.

% @spec sort_by_created_reverse :: [Item] -> [Item]
sort_by_created_reverse(Is) -> lists:sort(fun created_sort_reverse/2, Is).

% @spec created_sort_reverse :: Item -> Item -> Ordering
created_sort_reverse(A, B) -> compare(B#item.created, A#item.created).

In Scala

object Entry {
    def empty = new Model()

    def build_model(items:List[Item]) = items match {
        case Nil => empty
        case _ =>
            val sortedItems = sortByCreatedReverse(items)
            val bid = Map() ++ sortedItems.map(item => (item.internalId -> item))
            val n = bid.keys.toList.sort((A, B) => A > B).head // max

            new Model(Map() ++ sortedItems.map(item => (item.permatitle -> item)),
                      bid,
                      buildChildMap(sortedItems),
                      sortedItems,
                      n + 1)
    }

    def buildChildMap(items:List[Item]) = buildChildMap_(Map() ++ items.map(item => (item.internalId -> Nil)), items)

    private
    def buildChildMap_(map:Map[Int, List[Int]], items:List[Item]) = {
        for (item <- items if item.parent.isDefined; pid = item.parent.get) {
            map + (pid -> (map.getOrElse(pid, Nil) + item.internalId))
        }
        map
    }

    def sortByCreatedReverse(items:List[Item]) = items.sort((A, B) => A.created before B.created)
}

I use ErlyBird for Erlang coding, and Scala for NetBeans for Scala coding. The experience seems that IDE is much aware of Scala, and I can get the typing a bit faster than writing Erlang code.

If you are not familiar with all these 3 languages, which one looks more understandable?