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  • Newbie, deciding Python or Erlang

    - by Joe
    Hi Guys, I'm a Administrator (unix, Linux and some windows apps such as Exchange) by experience and have never worked on any programming language besides C# and scripting on Bash and lately on powershell. I'm starting out as a service provider and using multiple network/server monitoring tools based on open source (nagios, opennms etc) in order to monitor them. At this moment, being inspired by a design that I came up with, to do more than what is available with the open source at this time, I would like to start programming and test some of these ideas. The requirement is that a server software that captures a stream of data and store them in a database(CouchDB or MongoDB preferably) and the client side (agent installed on a server) would be sending this stream of data on a schedule of every 10 minutes or so. For these two core ideas, I have been reading about Python and Erlang besides ruby. I do plan to use either Amazon or Rackspace where the server platform would run. This gives me the scalability needed when we have more customers with many servers. For that reason alone, I thought Erlang was a better fit(I could be totally wrong, new to this game) and I understand that Erlang has limited support in some ways compared to Ruby or Python. But also I'm totally new to the programming realm of things and any advise would be appreciated grately. Jo

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  • Exception error in Erlang

    - by Jim
    So I've been using Erlang for the last eight hours, and I've spent two of those banging my head against the keyboard trying to figure out the exception error my console keeps returning. I'm writing a dice program to learn erlang. I want it to be able to call from the console through the erlang interpreter. The program accepts a number of dice, and is supposed to generate a list of values. Each value is supposed to be between one and six. I won't bore you with the dozens of individual micro-changes I made to try and fix the problem (random engineering) but I'll post my code and the error. The Source: -module(dice2). -export([d6/1]). d6(1) - random:uniform(6); d6(Numdice) - Result = [], d6(Numdice, [Result]). d6(0, [Finalresult]) - {ok, [Finalresult]}; d6(Numdice, [Result]) - d6(Numdice - 1, [random:uniform(6) | Result]). When I run the program from my console like so... dice2:d6(1). ...I get a random number between one and six like expected. However when I run the same function with any number higher than one as an argument I get the following exception... **exception error: no function clause matching dice2:d6(1, [4|3]) ... I know I I don't have a function with matching arguments but I don't know how to write a function with variable arguments, and a variable number of arguments. I tried modifying the function in question like so.... d6(Numdice, [Result]) - Newresult = [random:uniform(6) | Result], d6(Numdice - 1, Newresult). ... but I got essentially the same error. Anyone know what is going on here?

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  • Technically why is processes in Erlang more efficient than OS threads?

    - by Jonas
    Spawning processes seam to be much more efficient in Erlang than starting new threads using the OS (i.e. by using Java or C). Erlang spawns thousands and millions of processes in a short period of time. I know that they are different since Erlang do per process GC and processes in Erlang are implemented the same way on all platforms which isn't the case with OS threads. But I don't fully understand technically why Erlang processes are so much more efficient and has much smaller memory footprint per process. Both the OS and Erlang VM has to do scheduling, context switches and keep track of the values in the registers and so on... Simply why isn't OS threads implemented in the same way as processes in Erlang? Does it have to support something more? and why does it need a bigger memory footprint? Technically why is processes in Erlang more efficient than OS threads? And why can't threads in the OS be implemented and managed in the same efficient way?

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  • How to read integer in Erlang?

    - by Jace Jung
    I'm trying to read user input of integer. (like cin nInput; in C++) I found io:fread bif from http://www.erlang.org/doc/man/io.html, so I write code like this. {ok, X} = io:fread("input : ", "~d"), io:format("~p~n", [X]). but when I input 10, the erlang terminal keep giving me "\n" not 10. I assume fread automatically read 10 and conert this into string. How can I read integer value directly? Is there any way to do this? Thank you for reading this.

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  • Erlang code critique

    - by dagda1
    Hi, I am trying to get my head round some basic erlang functionality and I could do with some comments on the following. I have the following erlang code that takes a list of tuples and returns a list minus an element if a key is found: delete(Key, Database) -> remove(Database, Key, []). remove([], Key, Acc) -> Acc; remove([H|T], Key, Acc) -> if element(1, H) /= Key -> [H| remove(T, Key, Acc)]; true -> remove(T, Key, Acc) end. Is this a good way of doing this? The if statement seems incorrect. Also is my use of the accumulator Acc making this tail recursive? Cheers Paul

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  • Using trace and dbg in Erlang

    - by Gordon Guthrie
    I am trying to start using erlang:trace/3 and the dbg module to trace the behaviour of a live production system without taking the server down. The documentation is opaque (to put it mildly) and there don't appear to be any useful tutorials online. What I spent all day trying to do was capture what was happening in a particular function by trying to apply a trace to module:function using dbg:c and dbg:p but with no success at all... Does anyone have a succinct explanation of how to use trace in a live Erlang system?

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  • Erlang: Interfacing with Xalan: port driver or nif?

    - by mwt
    I'd like to get a real XSLT processor working with erlang. Which would be the best interface, nif or port driver? According to the nif documentation, nif calls block the runtime, so they should not take long. Is processing a long xml document too long? Also, I'd like to allow erlang callbacks during the transformation. Does that seem possible? Possible with nif but not port drivers or vice versa? I've never written any C, so I figured this would be good introduction. Xalan is C++. I assume nif can work with that, right?

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  • Erlang bit indexing

    - by GTDev
    I am currently trying to learn erlang and what I am trying to do is to perform an operation on specific indices of an array stored in a bit array or int. If there is a 0 in a position, the index into the array at that position is not used. So envision the following: Example the array is: [1, 3, 5, 42, 23] My bit array is: 21 = 10101 in binary so I'm using indicies 1,3,5 so I'm calling a function on [1, 5, 23] my function is of the form my_function(Array, BitArray) -> SubArray = get_subarray_from_bitarray(Array, BitArray), process_subarray(SubArray). And I need help with the get_subarray_from_bitarray(). I know erlang has special syntax around bit strings (something like <<) so is there an efficient way of indexing into the bit array to get the indicies?

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  • Understanding the workflow of the messages in a generic server implementation in Erlang

    - by Chiron
    The following code is from "Programming Erlang, 2nd Edition". It is an example of how to implement a generic server in Erlang. -module(server1). -export([start/2, rpc/2]). start(Name, Mod) -> register(Name, spawn(fun() -> loop(Name, Mod, Mod:init()) end)). rpc(Name, Request) -> Name ! {self(), Request}, receive {Name, Response} -> Response end. loop(Name, Mod, State) -> receive {From, Request} -> {Response, State1} = Mod:handle(Request, State), From ! {Name, Response}, loop(Name, Mod, State1) end. -module(name_server). -export([init/0, add/2, find/1, handle/2]). -import(server1, [rpc/2]). %% client routines add(Name, Place) -> rpc(name_server, {add, Name, Place}). find(Name) -> rpc(name_server, {find, Name}). %% callback routines init() -> dict:new(). handle({add, Name, Place}, Dict) -> {ok, dict:store(Name, Place, Dict)}; handle({find, Name}, Dict) -> {dict:find(Name, Dict), Dict}. server1:start(name_server, name_server). name_server:add(joe, "at home"). name_server:find(joe). I tried so hard to understand the workflow of the messages. Would you please help me to understand the workflow of this server implementation during the executing of the functions: server1:start, name_server:add and name_server:find?

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  • rabbitmq-erlang-client, using rebar friendly pkg, works on dev env fails on rebar release

    - by lfurrea
    I am successfully using the rebar-friendly package of rabbitmq-erlang-client for a simple Hello World rebarized and OTP "compliant" app and things work fine on the dev environment. I am able to fire up an erl console and do my application:start(helloworld). and connect to the broker, open up a channel and communicate to queues. However, then I proceed to do rebar generate and it builds up the release just fine, but when I try to fire up from the self contained release package then things suddenly explode. I know rebar releases are known to be an obscure art, but I would like to know what are my options as far as deployment for an app using the rabbitmq-erlang-client. Below you will find the output of the console on the crash: =INFO REPORT==== 18-Dec-2012::16:41:35 === application: session_record exited: {{{badmatch, {error, {'EXIT', {undef, [{amqp_connection_sup,start_link, [{amqp_params_network,<<"guest">>,<<"guest">>,<<"/">>, "127.0.0.1",5672,0,0,0,infinity,none, [#Fun<amqp_auth_mechanisms.plain.3>, #Fun<amqp_auth_mechanisms.amqplain.3>], [],[]}], []}, {supervisor2,do_start_child_i,3, [{file,"src/supervisor2.erl"},{line,391}]}, {supervisor2,handle_call,3, [{file,"src/supervisor2.erl"},{line,413}]}, {gen_server,handle_msg,5, [{file,"gen_server.erl"},{line,588}]}, {proc_lib,init_p_do_apply,3, [{file,"proc_lib.erl"},{line,227}]}]}}}}, [{amqp_connection,start,1, [{file,"src/amqp_connection.erl"},{line,164}]}, {hello_qp,start_link,0,[{file,"src/hello_qp.erl"},{line,10}]}, {session_record_sup,init,1, [{file,"src/session_record_sup.erl"},{line,55}]}, {supervisor_bridge,init,1, [{file,"supervisor_bridge.erl"},{line,79}]}, {gen_server,init_it,6,[{file,"gen_server.erl"},{line,304}]}, {proc_lib,init_p_do_apply,3, [{file,"proc_lib.erl"},{line,227}]}]}, {session_record_app,start,[normal,[]]}} type: permanent

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  • Erlang function does not exist in module?

    - by Sepehr Samini
    In lager.elr (the main module of https://github.com/basho/lager) there is no function with name "debug" but I have an application that call debug function from lager module like: lager:debug(Str, Args) I am beginner in Erlang but I know when we call a function from a module lile "mymodule:myfunction" there should be a function with name "myfunction" in file mymodule.erl but in this case when I search in lager.erl for function "debug" I can't find it.

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  • Is there a full REPL for Erlang?

    - by Gabriel Cuvillier
    Is there a way to have a full interpreter in Erlang, not just a "shell". Since it is a dynamic language, not being able to define named functions in the interpreter is a little bit disappointing... I suspect that it is because compilation units (modules) must be explicitly compiled for their execution by the VM, but maybe a REPL acting as a dynamic/recompilable module exist?

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  • erlang node not responding

    - by vinnitu
    I received such message in erlang condose at first@localhost node =ERROR REPORT==== 1-Jan-2011::23:19:28 === ** Node 'second@localhost' not responding ** ** Removing (timedout) connection ** My question is - what is timeout in this case? How much time before causes this event? Howto prevent this "horror"? I can restore\recover to normal work only by restart node... But what is the right way? Thank you, and Happy New Year!

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  • Erlang Facebook Example

    - by werg
    Does anyone know of an example facebook app (or facebook connect app) done in Erlang? I'm looking for something that includes the whole process. Specifically I don't seem to find anything on user authentication. I've checked out erlang_facebook, erlang2facebook and erlyface but none of them seem to offer a simple and comprehensive example accessible to me as a beginner . I'd be happy for just a bit of code to plough through though, preferably using mochiweb as backend.

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  • Erlang: HTTP Accept Header with Inets

    - by Ted Karmel
    I am trying to do the equivalent of the following curl command : curl -H "Accept: text/plain" http://127.0.0.1:8033/stats I tried with an Inets simple http request. But, it isn't processed. How can I specify in Inets (or some other Erlang http client for that matter) the accept header requirement?

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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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