sweet two bugs :D

example.erl

-module(example).

-export(sort/1).
-export(spawnedsort/3, merge/3).        % publishing internal funs
                                        % only for debugging


sort(X) ->
  ChunkLen = length(X) div 2,
  {L1, L2} = lists:split(ChunkLen, X),  % pattern matching example 1

  MainProcessPID = self(),

  PidA = spawn(?MODULE, spawnedsort, [L1, MainProcessPID, 100]),
  PidB = spawn(?MODULE, spawnedsort, [L2, MainProcessPID, 1]),

  receive {SortedA, PidA} ->            % pattern matching example 2
    receive {SortedB, PidB} ->
      merge( SortedA, SortedB, [] )
    end
  end.


spawnedsort(List, RemotePID, SleepTime) ->
  timer:sleep(SleepTime),
  ThisProcessPID = self(),
  RemotePID ! { lists:sort(List), ThisProcessPID }.


% there is lists:merge/2 but for a sake of example let's implement this too!
merge( [], [], Acc ) ->
  lists:reversed( Acc );                % common idiom: temporary data
                                        % structure is reversed

merge( [ Head1 | Tail1 ],
       [ Head2 | Tail2 ],
       Acc
     ) when Head1 > Head2 ->            % guard with additional constrains
  merge( Tail1, Tail2, [Head1, Head2] ++ Acc );
                                        % functional idiom: don't loop, but
                                        % rather recurse

merge( [ Head1 | Tail1 ],
       [ Head2 | Tail2 ],
       Acc
     ) ->                               % no other option, yes?
  merge( Tail1, Tail2, [Head2, Head1] ++ Acc ).