Thinking in Scala vs Erlang

Caoyuan Blog - - December 16, 2008

Keeping Erlang in mind, I’ve coded two months in Scala, I’m thinking something called “Scala vs Erlang”, I wrote some benchmark code to prove me (the code and result may be available someday), and I’d like to do some gradually summary on it in practical aspect. These opinions may be or not be correct currently due to lacking of deep experience and understanding, but, anyway, I need to record them now and correct myself with more experiences and understanding got on both Scala and Erlang.

Part I. Syntax

List comprehension

Erlang:

Lst = [1,2,3,4],
[X + 1 || X <- Lst],
lists:map(fun(X) -> X + 1 end, Lst)

Scala:

val lst = List(1,2,3,4) for (x <- lst) yield x + 1
lst.map{x => x + 1}
lst.map{_ + 1} // or place holder

Pattern match

Erlang:

case X of {A, B} when is_integer(A), A > 1 -> ok; _ -> error
end, {ok, [{A, B} = H|T]} = my_function(X)

Scala:

x match { case (a:Int, b:_) if a > 1 => OK // can match type case _ => ERROR
} val ("ok", (h@(a, b)) :: t) = my_function(x)

List, Tuple, Array, Map, Binary, Bit

Erlang:

Lst = [1, 2, 3] %% List
[0 | Lst] %% List concat
{1, 2, 3} %% Tuple
<<1, 2, “abcâ€�?>> %% Binary
%% no Array, Map syntax

Scala:

val lst = List(1, 2, 3) // List
0 :: lst // List concat
(1, 2, 3) // Tuple
Array(1, 2, 3) // Array
Map(“aâ€�? -> 1, “bâ€�? -> 2) // Map
// no Binary, Bit syntax

Process, Actor

Erlang:

the_actor(X) -> receive ok -> io:format(“~p~nâ€�?, [X]); I -> the_actor(X + I) %% needs to explicitly continue loop end.
P = spawn(mymodule, the_actor, [0])
P ! 1
P ! ok

Scala I:

class TheActor(x:Int) extends Actor { def act = loop { react { case “okâ€�? => println(x); exit // needs to explicitly exit loop case i:Int => x += i } }
}
val a = new TheActor(0)
a ! 1
a ! “okâ€�?

Scala II:

val a = actor { def loop(x:Int) = { react { case "ok" => println(x) case i:Int => loop(x + i) } } loop(0)
}
a ! 1
a ! "ok"

Part II. Processes vs Actors

Something I

Erlang:

• Lightweight processes
• You can always (almost) create a new process for each new comer
• Scheduler treats all processes fairly
• Share nothing between processes
• Lightweight context switch between processes
• IO has been carefully delegated to independent processes

Scala:

• Active actor is delegated to JVM thread, actor /= thread
• You can create a new actor for each new comer
• But the amount of real workers (threads) is dynamically adjusted according to the processing time
• The later comers may be in wait list for further processing until a spare thread is available
• Share nothing or share something upon you decision
• Heavy context switch between working threads
• IO block is still pain unless good NIO framework (Grizzly?)

Something II

Erlang:

• Try to service everyone simultaneously
• But may loss service quality when the work is heavy, may time out (out of service)
• Ideal when processing cost is comparable to context switching cost
• Ideal for small message processing in soft real-time
• Bad for massive data processing, and cpu-heavy work

Scala:

• Try to service limited number of customers best first
• If can not service all, the later comers will be put in waiting list and may time out (out of service)
• It’s difficult for soft real-time on all coming concurrent customers
• Ideal when processing cost is far more than context switching cost (context switch time is in ns on modern JVM)
• When will there be perfect NIO + Actor library?

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