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Reminders

Reminders. 1 st Erlang assignment due Friday, Apr. 3 2 nd mid-term in-class Monday, Apr. 6. Questions about Assignment. Interpreting error messages: Error in process <0.38.0> with exit value: {function_clause,[{ring,waitSend,[<0.39.0>]}]}

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Reminders

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  1. Reminders • 1st Erlang assignment due Friday, Apr. 3 • 2nd mid-term in-class Monday, Apr. 6

  2. Questions about Assignment • Interpreting error messages: Error in process <0.38.0> with exit value: {function_clause,[{ring,waitSend,[<0.39.0>]}]} Exit values are documented in the Erlang manual online. This one means: died trying to call ring:waitSend with argument <0.39.0> (a Pid) because there was no matching function clause. Problem: the argument name began with a lowercase letter.

  3. Objects simulated in C • In C, objects can be emulated by using structs typedef obj = struct {m1type *m1; m2type *m2; d1type d1… } obj *o1 = oFactory(…init values); o1->m1(o1, …); • C function pointers refer only to function code; not function closures (code together with lookup environment) so all data needed to specialize the code has to be included in the struct • Conceptually not hard, but requires explicit code to do it

  4. Objects simulated in functional style • Objects can be simulated using records that contain function closures • First, for only a single method we don’t need a record: newFoo(InitState) -> (fun(NewState)-> (InitState, newFoo(NewState)) end). F0 = newFoo(3). {V1, F1} = F0(hi). {V2, F2} = F1(6).

  5. Using a tuple (easily extend to record so methods have names) newFoo(InitState) -> { fun (Args1) -> {f11(InitState, Args1), newFoo(f12(InitState, Args1))} end, fun (Args2) -> {f21(InitState, Args2), newFoo(f22(InitState, Args2))} end, … }.

  6. Using Processes its clearer and easier newServer(InitState) -> Server = spawn(server), {fun(Args1) -> rpc(Server, {f1, Args1}), fun(Args2) -> rpc(Server, {f2, Args2}), … } end. • rpc here is just the same rpc function we’ve seen before • f1 and f2 are atoms in this context

  7. The server server(State) -> receive {From, {f1, Args1}} -> From ! f11(State, Args1), server(f12(State, Args1)); {From, {f2, Args2}} -> From ! f21(State, Args2), server(f22(State, Args2)) … end. • f11, f12, f21, and f22 are the same functions as in the purely sequential simulation

  8. The client Server1 = newServer(SomeState), {F1, F2} = Server, V1 = F1(Args1), V2 = F2(Args2), % note V2 depends on both Args1 and Args2 as well as SomeState Server2 = newServer(SomeOtherState), V3 = F1(OtherArgs1), V4 = F2(OtherArgs2), % note V4 depends on OtherArgs1 and 2 and SomeOtherState but not on SomeState, Args1 and Args2

  9. The behavior is not quite the same • If the sequential simulation is used from multiple processes each gets its own “fork” of the object history • Because the way it is used results in a new object every time a method is called • The process simulation involves only a single object that, oh by the way, has synchronized access • A process, inherently, does only one thing at a time

  10. A complete, simple example -module(tester). -export([start/1]). start(Id) -> Server = spawn(fun () -> serverloop(Id, []) end), fun(Item) -> rpc( Server, Item ) end . serverloop(Id, History) -> receive { From, Something } -> io:format( "Server ~p: Got ~p from ~p with history ~p~n", [Id, Something, From, History] ), From ! { self(), ok }, serverloop(Id, [Something|History]) end.

  11. Exam Review • Material covered: • Thread pools – pool sizing; thread-starvation; policies • GUI issues – why single threaded; model-view-controller; idea of listeners; when and how to use concurrency • Deadlocks – wait-for graph; lock ordering; livelock; starvation; lock performance; processor atomic operations • Testing issues – not covered • Erlang • Sequential – single assignment variables; lists; pattern matching • Concurrent – mailboxes, send and receive primitives, pattern matching on the mailbox, linked processes, system processes • Function closures and their use • You may use your notes on your computer; also any compiler. Turn off/disable the network!

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