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  • Why did you decide "against" using Erlang?

    - by Zubair
    Have you actually "tried" (means programmed in, not just read an article on it) Erlang and decided against it for a project? If so, why? Also, if you have opted to go back to your old language, or to use another functional language like F#, Haskell, Clojure, Scala, or something else then this counts too, and state why.

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  • How do I modify a record in erlang?

    - by Yadira Suazo
    Hi, I need replace the same value for variables {place} and {other_place} in the record op. #op{ action = [walk, from, {place}, to, {other_place}], preconds = [[at, {place}, me], [on, floor, me], [other_place, {place}, {other_place}]], add_list = [[at, {other_place}, me]], del_list = [[at, {place}, me]] } But erlang don´t share variables. Is there any data type for that?

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  • Accessing Erlang business layer via REST

    - by Polyn
    For a college project i'm thinking of implementing the business layer in Erlang and then accessing it via multiple front-ends using REST. I would like to avail of OTP features like distributed applications, etc. My question is how do I expose gen_server calls/casts to other applications? Obviously I could make RPC calls via language specific "bridges" like OTP.net or JInterface, but I want a consistent way to access it like REST.

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  • [Erlang] Changing working directory

    - by Zbigniew
    Hello, I have Erlang installed on my WinXP machine. Becouse so, I use it by a "werl.exe". The problem is, that I would like to change a default folder that werl starts in. I cannot find option that would let me do that, although I know there must be something like that. Could anybody help ?

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  • Returning an error code in Erlang

    - by Zubair
    I'm writing some Erlang code and I'm very unsure whether I should let everything fail fast. One problem I find with fail fast is that for the developer/user the exception generated is pretty meaningless. Any idea what I should return which would not give esoteric and hard to read stack traces?

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  • Erlang: 2 database on 2 webserver ??

    - by xRobot
    I have created a blogging system with php+postgresql. Now I want to add a web chat ( in REAL TIME for Million of users simultaneously ) where every message is saved in database. I am thinking to use Erlang+Mnesia on a different webserver for this issue. Message's table will be like this: message_id, user_id, message, date user_id should be related with users table in Postgresql database in another webserver. How can I do that without lose performance ? If you have any other creative solutions tell me please ;).

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  • Erlang: starting slave node

    - by Zim
    Hi, I'm trying to start erlang slave node on cluster and I receive "bash: erl: command not found" message. Though I have alias for erl. Here is what I actually do: [user@n001 ~]$ erl -rsh ssh -sname n001 Eshell V5.7.5 (abort with ^G) (n001@n001)1> slave:start_link("user@n002", n002, "-rsh ssh"). bash: erl: command not found {error,timeout} (n001@n001)2> Maybe, there is something wrong? Thanks.

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  • Erlang list comprehension, traversing two lists and excluding values

    - by ErJab
    I need to generate a set of coordinates in Erlang. Given one coordinate, say (x,y) I need to generate (x-1, y-1), (x-1, y), (x-1, y+1), (x, y-1), (x, y+1), (x+1, y-1), (x+1, y), (x+1, y+1). Basically all surrounding coordinates EXCEPT the middle coordinate (x,y). To generate all the nine coordinates, I do this currently: [{X,Y} || X<-lists:seq(X-1,X+1), Y<-lists:seq(Y-1,Y+1)] But this generates all the values, including (X,Y). How do I exclude (X,Y) from the list using filters in the list comprehension?

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  • Tail recursion in Erlang

    - by dagda1
    Hi, I am really struggling to understand tail recursion in Erlang. I have the following eunit test: db_write_many_test() -> Db = db:new(), Db1 = db:write(francesco, london, Db), Db2 = db:write(lelle, stockholm, Db1), ?assertEqual([{lelle, stockholm},{francesco, london}], Db2). And here is my implementation: -module(db) . -include_lib("eunit/include/eunit.hrl"). -export([new/0,write/3]). new() -> []. write(Key, Value, Database) -> Record = {Key, Value}, [Record|append(Database)]. append([H|T]) -> [H|append(T)]; append([]) -> []. Is my implementation tail recursive and if not, how can I make it so? Thanks in advance

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  • Erlang code explained

    - by dagda1
    Hi, I am having a bit of trouble getting my head around the following erlang code -module(threesix). -export([quicksort/1]). quicksort(Pivot, Left, Right, []=_Src) -> {Left, Pivot, Right}; quicksort(Pivot, Left, Right, [H|T]=_Src) when H < Pivot -> quicksort(Pivot, [H|Left], Right, T); quicksort(Pivot, Left, Right, [H|T]=_Src) -> quicksort(Pivot, Left, [H|Right], T). quicksort([]) -> []; quicksort([H|T]=_List) -> {Left, Pivot, Right} = quicksort(H, [], [], T), quicksort(Left) ++ [Pivot] ++ quicksort(Right). I am specifically talking about the use of _Src and _List in the parameters. Are these simply for documentation as I cannot see why they are used? Thanks Paul

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  • bad_application error starting erlang gen_server application

    - by user313475
    I have written a simple erlang app using gen_server. When starting it with application:start(myapp), I get the following tuple... {error,{bad_application,{application,myapp ... (rest of my application config). There are no other error or warning messages. I have also tried to google examples of how to configure gen_server and also the error itself. I am surprised at how little information there is out there. I could start trying to debug OTP?? Any pointers would be appreciated.

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  • Tracing Erlang Functions - Short forms

    - by Roberto Aloi
    As you might know, it's now possible to trace Erlang functions by using the short form: dbg:tpl(Module, Function, x). Instead of the usual: dbg:tpl(Module, Function, dbg:fun2ms(fun(_) -> exception_trace() end)). I'm actually wondering if a similar short form is available for return_trace(). Something like: dbg:tpl(Module, Function, r). Instead of: dbg:tpl(Module, Function, dbg:fun2ms(fun(_) -> return_trace() end)). The source code in the dbg module seems to suggest not: new_pattern_table() -> PT = ets:new(dbg_tab, [ordered_set, public]), ets:insert(PT, {x, term_to_binary([{'_',[],[{exception_trace}]}])}), ets:insert(PT, {exception_trace, term_to_binary(x)}), PT. But I might be wrong. Do you know of any?

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  • erlang io:format, and a hanging web application

    - by williamstw
    While I'm learning a new language, I'll typically put lots of silly println's to see what values are where at specific times. It usually suffices because the languages typically have available a tostring equivalent. In trying that same approach with erlang, my webapp just "hangs" when there's a value attempted to be printed that's not a list. This happens when variable being printed is a tuple instead of a list. There's no error, exception, nothing... just doesn't respond. Now, I'm muddling through by being careful about what I'm writing out and as I learn more, things are getting better. But I wonder, is there a way to more reliably to [blindly] print a value to stdout? Thanks, --tim

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  • Erlang message loops

    - by Roger Alsing
    How does message loops in erlang work, are they sync when it comes to processing messages? As far as I understand, the loop will start by "receive"ing a message and then perform something and hit another iteration of the loop. So that has to be sync? right? If multiple clients send messages to the same message loop, then all those messages are queued and performed one after another, or? To process multiple messages in parallell, you would have to spawn multiple message loops in different processes, right? Or did I misunderstand all of it?

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  • Erlang Edoc in Emacs

    - by Roberto Aloi
    Let's say that I have an Erlang function, with spec. -spec foo(integer(), string()) -> boolean(). foo(_Integer, _String) -> true. My dream would be to generate the edoc from this information within Emacs automatically. The generated code should look like: %%-------------------------------------------------------------------- %% @doc %% Your description goes here %% @spec foo(_Integer::integer(), _String::string()) -> %%% boolean() %% @end %%-------------------------------------------------------------------- -spec foo(integer(), string()) -> boolean(). foo(_Integer, _String) -> true. Does a similar feature already exist?

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  • Accurate clock in Erlang

    - by buddhabrot
    I was thinking about how to implement a process that gives the number of discrete intervals in time that occurred since it started. Am I losing accuracy here? How do I implement this without loss of accuracy after a while and after heavy client abuse. I am kind of stumped how to do this in Erlang. -module(clock). -compile([export_all]). start(Time) -> register(clock, spawn(fun() -> tick(Time, 0) end)). stop() -> clock ! stop. tick(Time, Count) -> receive nticks -> io:format("~p ticks have passed since start~n", [Count]) after 0 -> true end, receive stop -> void after Time -> tick(Time, Count + 1) end.

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  • [Erlang - trace] How to trace for all functions in an Erlang module except for one?

    - by Dlf
    I wanted to trace for all functions in an erlang module, with dbg:tpl, but one of the internal functions took up 95% of the trace file. I then wanted to exclude only that single function and found that it was not as easy as I thought it would be. I know there are great pattern matching possibilities for arguments when tracing. Is there a similar possibility to apply pattern matching for functions? eg.: {'=/=', '$2', function_name} I am open for outside-the-box solutions as well! Thank You!

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  • Handling Erlang inets http client errors

    - by Justin
    I have an Erlang app which makes a large number of http calls to external sites using inets, using the code below case http:request(get, {Url, []}, [{autoredirect, false}], []) of {ok, {{_, Code, _}, _, Body}}-> case Code of 200 -> HandlerFn(Body); _ -> {error, io:format("~s returned HTTP ~p", [Broker, Code])} end; Response -> %% block to handle unexpected responses from inets {error, io:format("~s returned ~p", [Broker, Response])} end. There is an explicit block to handle anything strange inets might return [Response]. Despite this, I still get what look like inets error reports dumped to the console [sample below]. What am I doing wrong here ? Do I need to configure some kind of inets error handler elsewhere ? Thanks. -- =ERROR REPORT==== 24-Apr-2010::06:49:47 === ** Generic server <0.6618.0 terminating ** Last message in was {connect_and_send, {request,#Ref<0.0.0.139358,<0.6613.0,0,http, {"***",80}, "****************", [],get, {http_request_h,undefined,"keep-alive", undefined,undefined,undefined,undefined, undefined,undefined,undefined,undefined, undefined,undefined,undefined,undefined, undefined,undefined,"news.bbc.co.uk", undefined,undefined,undefined,undefined, undefined,undefined,undefined,undefined, undefined,[],undefined,undefined,undefined, undefined,"0",undefined,undefined, undefined,undefined,undefined,undefined,[]}, {[],[]}, {http_options,"HTTP/1.1",infinity,false,[], undefined,false,infinity}, "*******************", [],none,[],1272088179114,undefined,undefined}} * When Server state == {state, {request,#Ref<0.0.0.139358,<0.6613.0,0,http, {"********",80}, "***************", [],get, {http_request_h,undefined,"keep-alive", undefined,undefined,undefined,undefined, undefined,undefined,undefined,undefined, undefined,undefined,undefined,undefined, undefined,undefined,"news.bbc.co.uk", undefined,undefined,undefined,undefined, undefined,undefined,undefined,undefined, undefined,[],undefined,undefined, undefined,undefined,"0",undefined, undefined,undefined,undefined,undefined, undefined,[]}, {[],[]}, {http_options,"HTTP/1.1",infinity,false,[], undefined,false,infinity}, "**********************", [],none,[],1272088179114,undefined,undefined}, undefined,undefined,undefined,undefined,undefined, {[],[]}, {[],[]}, undefined,[],nolimit,nolimit, {options, {undefined,[]}, 0,2,5,120000,2,disabled,false,inet,default, default,[]}, {timers,[],undefined}, httpc_manager,undefined} ** Reason for termination == ** {error,{connect_failed,{#Ref<0.0.0.139358,{error,nxdomain}}}} =ERROR REPORT==== 24-Apr-2010::06:49:47 === HTTPC-MANAGER handler (<0.6618.0, started) failed to connect and/or send request #Ref<0.0.0.139358 Result: {error,{connect_failed,{#Ref<0.0.0.139358,{error,nxdomain}}}}

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  • erlang design of a card game [closed]

    - by user601836
    I would like to discuss with you a possible implementation for a card game in erlang. The only full example I found online is OpenPoker. I would like to create one myself, so here is the implementation I have in mind: A gen_server to represent a deck: when started creates a deck of cards (shuffled). And stores it in its state. provides an handle_call (draw_card) A gen_server to represent the chat room. Stores in its state the registered name of a player process (e.g. player1, player2, luke etc etc). Exports handle_cast to join the chat (executed by default when somebody joins successfully the game) and one to broadcast a chat message to all users by calling an handle_cast on the gen_server representing a player. a gen_fsm to represent a game instance. Has two states (wait_join, and turn). Exports join/1 to join the game, play_card/2 and send_msg/2. One parameter is the pid of the player process. a gen_server to represent the player. Exports only start_link/1 where the parameter is the name to use to register the process (inside the init I call join method of gen_fsm). Has different handle_calls (e.g. get_hand, draw_card) and handle_casts (e.g. play_card, deliver_msg, and send_msg) A gen_server to represent the main process. Exports (join_game/1 which calls player:start_link/1, send_msg/2 to call player's send_msg, play_card/3 to call player's play_card). What do you think of this architecture? Thanks in advance

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  • Cryptic Erlang Errors

    - by Jim
    Okay so I started learning erlang recently but am baffled by the errors it keeps returning. I made a bunch of changes but I keep getting errors. The syntax is correct as far as I can tell but clearly I'm doing something wrong. Have a look... -module(pidprint). -export([start/0]). dostuff([]) - receive begin - io:format("~p~n", [This is a Success]) end. sender([N]) - N ! begin, io:format("~p~n", [N]). start() - StuffPid = spawn(pidprint, dostuff, []), spawn(pidprint, sender, [StuffPid]). Basically I want to compile the script, call start, spawn the "dostuff" process, pass its process identifier to the "sender" process, which then prints it out. Finally I want to send a the atom "begin" to the "dostuff" process using the process identifier initially passed into sender when I spawned it. The errors I keep getting occur when I try to use c() to compile the script. Here they are.. ./pidprint.erl:6: syntax error before: '-' ./pidprint.erl:11: syntax error before: ',' What am I doing wrong?

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  • Adding International support in Erlang Web 1.4

    - by Roberto Aloi
    I'm trying to add international support for a website based on the Erlang Web 1.4. I would like to have a couple of links on every page (the notorious Country flags) that allow the user to set his language session variable. What I have right now is a link like: <li><a href="/session/language/en">English</a></li> Where, in the session controller I do: language(Args) -> LanguageId = proplists:get_value(id, Args), case language_is_supported(LanguageId) of false -> ok; true -> wpart:fset("session:lang", LanguageId) end, {redirect, "/"}. The problem is that, after setting the preferred language, I would like the user to be redirected to the page he was visiting before changing the language. In this case the "__path" variable doesn't help because it contains the language request and not the "previous" one. How could I resolve this situation? I'm probably using the wrong approach but I cannot thing to anything else right now.

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  • Using native MySQL driver in Erlang

    - by Mickey Shine
    I am using native MySQL driver (http://code.google.com/p/erlang-mysql-driver/) with mochiweb. When I tried that MySQL driver in shell mode, all woked fine. But when I write some code with Mochiweb, it reported me the following error: =CRASH REPORT==== 4-Jul-2009::04:44:29 === crasher: initial call: mochiweb_socket_server:acceptor_loop/1 pid: <0.61.0> registered_name: [] exception error: no function clause matching mysql:fetch(p1,<<"SELECT * FROM cdb_forums LIMIT 10">>) in function perly_web:loop/2 in call from mochiweb_http:headers/5 ancestors: [perly_web,perly_sup,<0.58.0>] messages: [] links: [<0.60.0>,#Port<0.965>] dictionary: [{mochiweb_request_body,undefined}, {mochiweb_request_qs,[]}, {mochiweb_request_post,[]}, {mochiweb_request_path,"/online"}, {mochiweb_request_cookie, [{"04c_sid","hG9Oyv"}, {"04c_visitedfid","2"}, {"kQx_cookietime","2592000"}, {"kQx_loginuser","admin"}, {"kQx_activationauth", "98b3mdX86fKT9dI4WyKuL61Tqxk%2BW1r6ACpHp9y8itH2xQ"}, {"smile","1D1"}]}] trap_exit: false status: running heap_size: 1597 stack_size: 24 reductions: 5188 neighbours: The code I write in Mochiweb is start(Options) -> {DocRoot, Options1} = get_option(docroot, Options), Loop = fun (Req) -> ?MODULE:loop(Req, DocRoot) end, % we’ll set our maximum to 1 million connections. (default: 2048) mochiweb_http:start([{max, 1000000}, {name, ?MODULE}, {loop, Loop} | Options1]), mysql:start_link(p1, "10.0.0.123", "root", "root", "test"). stop() -> mochiweb_http:stop(?MODULE). loop(Req, DocRoot) -> "/" ++ Path = Req:get(path), case Req:get(method) of Method when Method =:= 'GET'; Method =:= 'HEAD' -> case Path of "online" -> Result1 = mysql:fetch(p1, <<"SELECT * FROM cdb_forums LIMIT 10">>), Body1 = io:format("Result1: ~p~n", [Result1]), Req:ok({"text/plain", Body1}); The connection looks good but when I added Result1 = mysql:fetch(p1, <<"SELECT * FROM cdb_forums LIMIT 10">>), it crashed. Can someone help me? Thanks in advance~ //================================================== updated: I noticed the follwoing information. If that is correct? =PROGRESS REPORT==== 4-Jul-2009::05:49:32 === supervisor: {local,kernel_safe_sup} started: [{pid,<0.65.0>}, {name,inet_gethost_native_sup}, {mfa,{inet_gethost_native,start_link,[]}}, {restart_type,temporary}, {shutdown,1000}, {child_type,worker}] mysql_conn: greeting version "5.1.33-log" (protocol 10) salt "ne0_m'vA" caps 63487 serverchar <<8,2,0,0, 0,0,0,0, 0,0,0,0, 0,0,0,0>> salt2 "!|o;vabJ*4bt" mysql_auth send packet 1: <<5,162,0,0,64,66,15,0,8,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,114,111,111,116,0,20,52,235,78, 173,36,251,201,242,172,139,113,231,253,181,245,3, 91,198,111,135>> Link: {ok,<0.62.0>} =SUPERVISOR REPORT==== 4-Jul-2009::05:49:32 === Supervisor: {local,perly_sup} Context: start_error Reason: ok Offender: [{pid,undefined}, {name,perly_web}, {mfa, {perly_web,start, [[{ip,"0.0.0.0"}, {port,8000}, {docroot, "/work/mochiweb-read-only/scripts/perly/priv/www"}]]}}, {restart_type,permanent}, {shutdown,5000}, {child_type,worker}]

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