erlang

问题 I try to execute simple PostgreSQL query with erlang and epgsql. I do: {ok, C} = pgsql:connect("localhost", "shk", "qwerty", [{database, "mydb"}]). >> {ok,<0.34.0>} Then: {ok, Cols, Rows} = pgsql:squery(C, "select * from users"). But i got error: =ERROR REPORT==== 27-Apr-2012::17:58:23 === ** State machine <0.34.0> terminating ** Last message in was {'EXIT',<0.32.0>, {{badmatch, {error, {error,'ð\236ð¨ð\230ð\221ð\232ð\220',<<"42P01">>, <<208,190,209,130,208,189,208,190,209,136,208,181, 208

问题 Somebody told me that simple_one_for_one is very useful for chat applications, because each chat client is a server process (gen_server). Is this right? And I wonder why do we need it? Why not create only one center server (gen_server) to handle all chat client communication? Because maybe the number of chat clients is very large so only one server couldn't handle fast, make the system slow down? I think maybe creating too many servers like simple_one_for_one may take too much system resource

问题 I have an Ejabberd 17.01 installation where I need to push a notification in case a recipient is offline. This seems the be a common task and solutions using a customized Ejabberd module can be found everywhere. However, I just don't get it running. First, here's me script: -module(mod_offline_push). -behaviour(gen_mod). -export([start/2, stop/1]). -export([push_message/3]). -include("ejabberd.hrl"). -include("logger.hrl"). -include("jlib.hrl"). start(Host, _Opts) -> ?INFO_MSG("mod_offline

问题 I have a process, which opens the file by using file:open. Will this file get automatically closed, when my process will terminate? 回答1: Yes. From the documentation of file:open/2: Returns: {ok, IoDevice} The file has been opened in the requested mode. IoDevice is a reference to the file. [...] IoDevice is really the pid of the process which handles the file. This process is linked to the process which originally opened the file. If any process to which the IoDevice is linked terminates, the

问题 I am studying an erlang based system, and trying to analyze the sequence of events that take place in the system. Is there a way to force erlang run-time or the elang vm to create a new kernel thread, each time "spawn" is called. This would make the system slower, but it would make the study a lot easier. I have tried the +S flag, and enabled smp already, but I suspect the system is still mapping multiple processes to one kernel thread, or erlang scheduler. Are there any inputs/configuration

问题 I have a behaviour X and a callback function with parameter type: %{a: any} Module Y implements behaviour X and the callback function in implementing module Y has parameter type: %{a: any, b: any} Dialyzer doesn't like that and complains: (#{'a':=_, 'b':=_, _=>_}) is not a supertype of #{'a':=_} This implies dialyzer attempts to determine if callback parameter's type in implementing module Y is a supertype of param type in behaviour X. In other words, it asks: Is behaviour X's callback param

问题 I am trying to write a custom module in ejabberd XMPP server. This is the erl module i'v written. -module(mod_wish). -behavior(gen_mod). -include("logger.hrl"). -export([ start/2, stop/1 ]). start(_Host, _Opt) -> ?INFO_MSG("Loading module 'mod_wish' ", []), ok. stop(_Host) -> ok. I can compile it without any errors. But after i add this module to config file and restart the ejabberd server, serve does not start. it says C(<0.38.0>:gen_mod:75) : Problem starting the module mod_wish for host

问题 I'm trying to follow a Url which is in a RSS to find the end URL! so for example I have this URL "http://coder.io/~a27983554a" and the result of following this will be "http://ferd.ca/code-janitor-nobody-s-dream-everyone-s-job-and-how-erlang-can-help.html" The end URL is saved in the header of the first URL, but the problem is I need to send a request to that URL to get response containing the end result for me! I'm using this command {ok, {{Version, 200, ReasonPhrase}, Headers, Body}} =

问题 I'm trying to make Erlang program that finds prime numbers with processes (ineffective, I know, but hey, it's just for fun :) ) - something along the lines of numbersimulation.com. On each "tick" the server spawns new process ("number") that increments its counter. If counter == that number, it's a factor, so we let server know. If server doesn't get any messages, it's a prime number. On small numbers (primes upto N, the server(50,L) line) it's okay, but on the bigger ones it crashes with:

问题 -module(prac). -export([len/1]). len([]) -> 0; len([_|T]) -> 1 + len(T). So I have this code and it works, but I dont know how to simulate it properly. 回答1: Okay, if you're looking for an explanation of why the code works the way it does, it goes something like this. Given the following code: len([]) -> 0; len([_|T]) -> 1 + len(T). If you were to call len/1 like len([a,b,c]) , then you can think of it executing like: call len([a,b,c]) does [a,b,c] match [] ? no does [a,b,c] match [_|T] ? yes,