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I recently started learning Erlang from https://learnyousomeerlang.com
in this chapter errors and processes, I understood what the program does and how it executes but I'm unable to figure out what's the purpose of this receive the statement in judge function when and how will it be called?
From What I understand if tuple pattern matches with Pid and atom it returns an atom. How will i send message to receive inside judge?
start_critic() ->
spawn(?MODULE, critic, []).
judge(Pid, Band, Album) ->
Pid ! {self(), {Band, Album}},
receive
{Pid, Criticism} -> Criticism
after 2000 ->
timeout
end.
critic() ->
receive
{From, {"Rage Against the Turing Machine", "Unit Testify"}} ->
From ! {self(), "They are great!"};
{From, {"System of a Downtime", "Memoize"}} ->
From ! {self(), "They're not Johnny Crash but they're good."};
{From, {"Johnny Crash", "The Token Ring of Fire"}} ->
From ! {self(), "Simply incredible."};
{From, {_Band, _Album}} ->
From ! {self(), "They are terrible!"}
end,
critic().
Output
c(linkmon).
{ok,linkmon}
Critic = linkmon:start_critic().
<0.109.0>
linkmon:judge(Critic, "Genesis", "The Lambda Lies Down on Broadway").
"They are terrible!"
linkmon:judge(Critic, "Genesis", "A trick of the Tail Recursion").
"They are terrible!"
linkmon:judge(Critic, "Johnny Crash", "The Token Ring of Fire").
"Simply incredible."
The line Pid ! ... sends a message to the critic. The critic will then send a response via one of the From ! ... lines. The receive in the judge function waits for said response and then simply returns the string contained in the response.
So, I am trying to read and write into a file.
While writing into the file, I need to check if a particular index exist in file then I don't write and throw error.
The data in file will look like this:
{1,{data,dcA,1}}.
{2, {data, dcA, 2}}.
{3,{data,dcA,3}}.
I added the dot at the end of each line because file:consult() needs the file like this.
Which is in this format.
{Index, {Data, Node, Index}}
When I have to add a new file, I check with this Index.
Here's what I have tried so far - https://pastebin.com/apnWLk45
And I run it like this:
193> {ok, P9} = poc:start(test1, self()).
{ok,<0.2863.0>}
194> poc:add(P9, Node, {6, data}).
In poc:add/3, P9 is the process id from the file:open.
I defined before in shell as dcA
And the third is the data - which is in this format - {Index, data}
Since I am using file:consult/1, it takes the filename as parameter. At that point, I only have process id. So I take the name from
file:pid2name(_Server).
This runs perfectly when I run it for the first time.
When I run this again - poc:add(P9, Node, {6, data2}), I get an error in this line file:pid2name(_Server).
exception error: no match of right hand side value undefined
How can I solve this issue?
I am new to Erlang. Just been a week that I started learning.
I am trying to read and write into a file. While writing into the
file, I need to check if a particular index exist in file then I don't
write and throw error.
A DETS table can easily do what you want:
-module(my).
-compile(export_all).
open_table() ->
dets:open_file(my_data, [{type, set}, {file, "./my_data.dets"}]).
close_table() ->
dets:close(my_data).
clear_table() ->
dets:delete_all_objects(my_data).
insert({Key, _Rest}=Data) ->
case dets:member(my_data, Key) of
true -> throw(index_already_exists);
false -> dets:insert(my_data, Data)
end.
all_items() ->
dets:match(my_data, '$1').
In the shell:
~/erlang_programs$ erl
Erlang/OTP 20 [erts-9.2] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:10] [hipe] [kernel-poll:false]
Eshell V9.2 (abort with ^G)
1> c(my).
my.erl:2: Warning: export_all flag enabled - all functions will be exported
{ok,my}
2> my:open_table().
{ok,my_data}
3> my:clear_table().
ok
4> my:all_items().
[]
5> my:insert({1, {data, a, b}}).
ok
6> my:insert({2, {data, c, d}}).
ok
7> my:insert({3, {data, e, f}}).
ok
8> my:all_items().
[[{1,{data,a,b}}],[{2,{data,c,d}}],[{3,{data,e,f}}]]
9> my:insert({1, {data, e, f}}).
** exception throw: index_already_exists
in function my:insert/1 (my.erl, line 15)
When I run this again - poc:add(P9, Node, {6, data2}), I get an error
in this line file:pid2name(_Server):
exception error: no match of right hand side value undefined
When a process opens a file, it becomes linked to a process that handles the file I/O, which means that if the process that opens the file terminates abnormally, the I/O process will also terminate. Here is an example:
-module(my).
-compile(export_all).
start() ->
{ok, Pid} = file:open('data.txt', [read, write]),
spawn(my, add, [Pid, x, y]),
exit("bye").
add(Pid, _X, _Y) ->
timer:sleep(1000), %Let start() process terminate.
{ok, Fname} = file:pid2name(Pid),
io:format("~s~n", [Fname]).
In the shell:
1> c(my).
my.erl:2: Warning: export_all flag enabled - all functions will be exported
{ok,my}
2> my:start().
** exception exit: "bye"
in function my:start/0 (my.erl, line 7)
3>
=ERROR REPORT==== 25-Jun-2018::13:28:48 ===
Error in process <0.72.0> with exit value:
{{badmatch,undefined},[{my,add,3,[{file,"my.erl"},{line,12}]}]}
According to the pid2name() docs:
pid2name(Pid) -> {ok, Filename} | undefined
the function can return undefined, which is what the error message is saying happened.
I am trying to understand process communication in erlang. Here I have a master process and five friends process. If a friend sends a message to any of the other 5 they have to reply back. But the master should be aware of all this. I am pasting the code below.
-module(prog).
-import(lists,[append/2,concat/1]).
-import(maps,[from_lists/1,find/2,get/2,update/3]).
-import(string,[equal/2]).
-import(file,[consult/1]).
-export([create_process/1,friends/4, master/1, main/0,prnt/1]).
%% CREATE PROCESS
create_process([])->ok;
create_process([H|T])->
{A,B} = H,
Pid = spawn(prog,friends,[B,self(),0,A]),
register(A,Pid),
create_process(T).
%% FRIENDS PROCESS
friends(Msg, M_pid, State, Self_name)->
S = lists:concat([Self_name," state =",State,"\n"]),
io:fwrite(S),
if
State == 0 ->
timer:sleep(500),
io:fwrite("~p~n",[Self_name]),
lists:foreach(fun(X) -> whereis(X)!{Self_name,"intro",self()} end, Msg),
friends(Msg, M_pid, State + 1, Self_name);
State > 0 ->
receive
{Process_name, Process_msg, Process_id} ->
I = equal(Process_msg,"intro"),
R = equal(Process_msg,"reply"),
XxX = lists:concat([Self_name," recieved ",Process_msg," from ",Process_name,"\n"]),
io:fwrite(XxX),
if
I == true ->
io:fwrite("~p~n",[whereis(Process_name)]),
M_pid!{lists:concat([Self_name," received intro message from ", Process_name , "[",Process_id,"]"]), self()},
io:fwrite(I),
whereis(Process_name)!{Self_name, "reply",self()},
friends(Msg, M_pid, State + 1, Self_name);
R == true ->
M_pid!{lists:concat([Self_name," received reply message from ", Process_name , "[",Process_id,"]"]), self()},
io:fwrite(R),
friends(Msg, M_pid, State + 1, Self_name)
end
after
1000->
io:fwrite(lists:concat([Self_name," has received no calls for 1 second, ending..."]))
end
end.
master(State)->
receive
{Process_message, Process_id} ->
io:fwrite(Process_message),
master(State+1)
after
2000->
ok
end.
main() ->
B = [{john, [jill,joe,bob]},
{jill, [bob,joe,bob]},
{sue, [jill,jill,jill,bob,jill]},
{bob, [john]},
{joe, [sue]}],
create_process(B),
io:fwrite("~p~n",[whereis(sue)]),
master(0).
I think the line in friends() function,
M_pid!{lists:concat([Self_name," received intro message from ", Process_name , "[",Process_id,"]"]), self()}
is the cause of error but I cannot understand why. M_pid is known and I am concatenating all the info and sending it to master but I am confused why it isnt working.
The error I am getting is as follows:
Error in process <0.55.0> with exit value: {function_clause,[{lists,thing_to_list,
[<0.54.0>],
[{file,"lists.erl"},{line,603}]},
{lists,flatmap,2,[{file,"lists.erl"},{line,1250}]},
{lists,flatmap,2,[{file,"lists.erl"},{line,1250}]},
{prog,friends,4,[{file,"prog.erl"},{line,45}]}]}
I dont know what is causing the error. Sorry for asking noob questions and thanks for your help.
An example of what Dogbert discovered:
-module(my).
-compile(export_all).
go() ->
Pid = spawn(my, nothing, []),
lists:concat(["hello", Pid]).
nothing() -> nothing.
In the shell:
2> c(my).
my.erl:2: Warning: export_all flag enabled - all functions will be exported
{ok,my}
3> my:go().
** exception error: no function clause matching
lists:thing_to_list(<0.75.0>) (lists.erl, line 603)
in function lists:flatmap/2 (lists.erl, line 1250)
in call from lists:flatmap/2 (lists.erl, line 1250)
4>
But:
-module(my).
-compile(export_all).
go() ->
Pid = spawn(my, nothing, []),
lists:concat(["hello", pid_to_list(Pid)]).
nothing() -> nothing.
In the shell:
4> c(my).
my.erl:2: Warning: export_all flag enabled - all functions will be exported
{ok,my}
5> my:go().
"hello<0.83.0>"
From the erl docs:
concat(Things) -> string()
Things = [Thing]
Thing = atom() | integer() | float() | string()
The list that you feed concat() must contain either atoms, integers, floats, or strings. A pid is neither an atom, integer, float, nor string, so a pid cannot be used with concat(). However, pid_to_list() returns a string:
pid_to_list(Pid) -> string()
Pid = pid()
As you can see, a pid has its own type: pid().
I ran your code.
Where you went wrong was to pass Process_id(which is of type pid()) to lists:concat/1.
Let us try to understand this error:
{function_clause,[{lists,thing_to_list,
[<0.84.0>],
[{file,"lists.erl"},{line,603}]},
{lists,flatmap,2,[{file,"lists.erl"},{line,1250}]},
{lists,flatmap,2,[{file,"lists.erl"},{line,1250}]},
{prog,friends,4,[{file,"prog.erl"},{line,39}]}]}
It states the function lists:thing_to_list/1 has no definition(see the word function_clause in the error log) which accepts an argument of type pid() as denoted here by [<0.84.0>].
Strings are represented as lists in erlang, which is why we use lists:concat/1.
As #7stud pointed out these are the valid types which can be passed to lists:concat/1 as per the documentation:
atom() | integer() | float() | string()
There are 2 occurrences of the following line. Fix them and you are good to go:
Incorrect Code:
M_pid!{lists:concat([Self_name," received intro message from ", Process_name , "[",Process_id,"]"]), self()},
Corrected Code
M_pid!{lists:concat([Self_name," received intro message from ", Process_name , "[",pid_to_list(Process_id),"]"]), self()},
Notice the use of the function erlang:pid_to_list/1. As per the documentation the function accepts type pid() and returns it as string().
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_push loading", []),
ejabberd_hooks:add(offline_message_hook, Host, ?MODULE, push_message, 10),
ok.
stop(Host) ->
?INFO_MSG("mod_offline_push stopping", []),
ejabberd_hooks:add(offline_message_hook, Host, ?MODULE, push_message, 10),
ok.
push_message(From, To, Packet) ->
?INFO_MSG("mod_offline_push -> push_message", [To]),
Type = fxml:get_tag_attr_s(<<"type">>, Packet), % Supposedly since 16.04
%Type = xml:get_tag_attr_s(<<"type">>, Packet), % Supposedly since 13.XX
%Type = xml:get_tag_attr_s("type", Packet),
%Type = xml:get_tag_attr_s(list_to_binary("type"), Packet),
?INFO_MSG("mod_offline_push -> push_message", []),
ok.
The problem is the line Type = ... line in method push_message; without that line the last info message is logged (so the hook definitely works). When browsing online, I can find all kinds of function calls to extract elements from Packet. As far as I understand it changed over time with new releases. But it's not good, all variants lead in some kind of error. The current way returns:
2017-01-25 20:38:08.701 [error] <0.21678.0>#ejabberd_hooks:run1:332 {function_clause,[{fxml,get_tag_attr_s,[<<"type">>,{message,<<>>,normal,<<>>,{jid,<<"homer">>,<<"xxx.xxx.xxx.xxx">>,<<"conference">>,<<"homer">>,<<"xxx.xxx.xxx.xxx">>,<<"conference">>},{jid,<<"carl">>,<<"xxx.xxx.xxx.xxx">>,<<>>,<<"carl">>,<<"xxx.xxx.xxx.xxx">>,<<>>},[],[{text,<<>>,<<"sfsdfsdf">>}],undefined,[],#{}}],[{file,"src/fxml.erl"},{line,169}]},{mod_offline_push,push_message,3,[{file,"mod_offline_push.erl"},{line,33}]},{ejabberd_hooks,safe_apply,3,[{file,"src/ejabberd_hooks.erl"},{line,382}]},{ejabberd_hooks,run1,3,[{file,"src/ejabberd_hooks.erl"},{line,329}]},{ejabberd_sm,route,3,[{file,"src/ejabberd_sm.erl"},{line,126}]},{ejabberd_local,route,3,[{file,"src/ejabberd_local.erl"},{line,110}]},{ejabberd_router,route,3,[{file,"src/ejabberd_router.erl"},{line,87}]},{ejabberd_c2s,check_privacy_route,5,[{file,"src/ejabberd_c2s.erl"},{line,1886}]}]}
running hook: {offline_message_hook,[{jid,<<"homer">>,<<"xxx.xxx.xxx.xxx">>,<<"conference">>,<<"homer">>,<<"xxx.xxx.xxx.xxx">>,<<"conference">>},{jid,<<"carl">>,<<"xxx.xxx.xxx.xxx">>,<<>>,<<"carl">>,<<"xxx.xxx.xxx.xxx">>,<<>>},{message,<<>>,normal,<<>>,{jid,<<"homer">>,<<"xxx.xxx.xxx.xxx">>,<<"conference">>,<<"homer">>,<<"xxx.xxx.xxx.xxx">>,<<"conference">>},{jid,<<"carl">>,<<"xxx.xxx.xxx.xxx">>,<<>>,<<"carl">>,<<"xxx.xxx.xxx.xxx">>,<<>>},[],[{text,<<>>,<<"sfsdfsdf">>}],undefined,[],#{}}]}
I'm new Ejabberd and Erlang, so I cannot really interpret the error, but the Line 33 as mentioned in {mod_offline_push,push_message,3,[{file,"mod_offline_push.erl"}, {line,33}]} is definitely the line calling get_tag_attr_s.
UPDATE 2017/01/27: Since this cost me a lot of headache -- and I'm still not perfectly happy -- I post here my current working module in the hopes it might help others. My setup is Ejabberd 17.01 running on Ubuntu 16.04. Most stuff I tried and failed with seem to for older versions of Ejabberd:
-module(mod_fcm_fork).
-behaviour(gen_mod).
%% public methods for this module
-export([start/2, stop/1]).
-export([push_notification/3]).
%% included for writing to ejabberd log file
-include("ejabberd.hrl").
-include("logger.hrl").
-include("xmpp_codec.hrl").
%% Copied this record definition from jlib.hrl
%% Including "xmpp_codec.hrl" and "jlib.hrl" resulted in errors ("XYZ already defined")
-record(jid, {user = <<"">> :: binary(),
server = <<"">> :: binary(),
resource = <<"">> :: binary(),
luser = <<"">> :: binary(),
lserver = <<"">> :: binary(),
lresource = <<"">> :: binary()}).
start(Host, _Opts) ->
?INFO_MSG("mod_fcm_fork loading", []),
% Providing the most basic API to the clients and servers that are part of the Inets application
inets:start(),
% Add hook to handle message to user who are offline
ejabberd_hooks:add(offline_message_hook, Host, ?MODULE, push_notification, 10),
ok.
stop(Host) ->
?INFO_MSG("mod_fcm_fork stopping", []),
ejabberd_hooks:add(offline_message_hook, Host, ?MODULE, push_notification, 10),
ok.
push_notification(From, To, Packet) ->
% Generate JID of sender and receiver
FromJid = lists:concat([binary_to_list(From#jid.user), "#", binary_to_list(From#jid.server), "/", binary_to_list(From#jid.resource)]),
ToJid = lists:concat([binary_to_list(To#jid.user), "#", binary_to_list(To#jid.server), "/", binary_to_list(To#jid.resource)]),
% Get message body
MessageBody = Packet#message.body,
% Check of MessageBody is not empty
case MessageBody/=[] of
true ->
% Get first element (no idea when this list can have more elements)
[First | _ ] = MessageBody,
% Get message data and convert to string
MessageBodyText = binary_to_list(First#text.data),
send_post_request(FromJid, ToJid, MessageBodyText);
false ->
?INFO_MSG("mod_fcm_fork -> push_notification: MessageBody is empty",[])
end,
ok.
send_post_request(FromJid, ToJid, MessageBodyText) ->
%?INFO_MSG("mod_fcm_fork -> send_post_request -> MessageBodyText = ~p", [Demo]),
Method = post,
PostURL = gen_mod:get_module_opt(global, ?MODULE, post_url,fun(X) -> X end, all),
% Add data as query string. Not nice, query body would be preferable
% Problem: message body itself can be in a JSON string, and I couldn't figure out the correct encoding.
URL = lists:concat([binary_to_list(PostURL), "?", "fromjid=", FromJid,"&tojid=", ToJid,"&body=", edoc_lib:escape_uri(MessageBodyText)]),
Header = [],
ContentType = "application/json",
Body = [],
?INFO_MSG("mod_fcm_fork -> send_post_request -> URL = ~p", [URL]),
% ADD SSL CONFIG BELOW!
%HTTPOptions = [{ssl,[{versions, ['tlsv1.2']}]}],
HTTPOptions = [],
Options = [],
httpc:request(Method, {URL, Header, ContentType, Body}, HTTPOptions, Options),
ok.
Actually it fails with second arg Packet you pass to fxml:get_tag_attr_s in push_message function
{message,<<>>,normal,<<>>,
{jid,<<"homer">>,<<"xxx.xxx.xxx.xxx">>,<<"conference">>,
<<"homer">>,<<"xxx.xxx.xxx.xxx">>,<<"conference">>},
{jid,<<"carl">>,<<"xxx.xxx.xxx.xxx">>,<<>>,<<"carl">>,
<<"xxx.xxx.xxx.xxx">>,<<>>},
[],
[{text,<<>>,<<"sfsdfsdf">>}],
undefined,[],#{}}
because it is not xmlel
Looks like it is record "message" defined in tools/xmpp_codec.hrl
with <<>> id and type 'normal'
xmpp_codec.hrl
-record(message, {id :: binary(),
type = normal :: 'chat' | 'error' | 'groupchat' | 'headline' | 'normal',
lang :: binary(),
from :: any(),
to :: any(),
subject = [] :: [#text{}],
body = [] :: [#text{}],
thread :: binary(),
error :: #error{},
sub_els = [] :: [any()]}).
Include this file and use just
Type = Packet#message.type
or, if you expect binary value
Type = erlang:atom_to_binary(Packet#message.type, utf8)
The newest way to do that seems to be with xmpp:get_type/1:
Type = xmpp:get_type(Packet),
It returns an atom, in this case normal.
When invoking a custom command, I noticed that only the logs are displayed. For example, if my Custom Comand script contains a retrun statement return "great custom command", I can't find it in the result. Both in API Java client or shell execution cases.
What can I do to be able to retrieve that result at the end of an execution?
Thanks.
Command definition in service description file:
customCommands ([
"getText" : "getText.groovy"
])
getText.groovy file content:
def text = "great custom command"
println "trying to get a text"
return text
Assuming that you service file contains the following :
customCommands ([
"printA" : {
println "111111"
return "222222"
},
"printB" : "fileB.groovy"
])
And fileB.groovy contains the following code :
println "AAAAAA"
return "BBBBBB"
Then if you run the following command : invoke yourService printA
You will get this :
Invocation results:
1: OK from instance #1..., Result: 222222
invocation completed successfully.
and if you run the following command : invoke yourService printB
You will get this :
Invocation results:
1: OK from instance #1..., Result: AAAAAA
invocation completed successfully.
So if your custom command's implementation is a Groovy closure, then its result is its return value.
And if your custom command's implementation is an external Groovy file, then its result is its last statement output.
HTH,
Tamir.