spawn function in erlang using function in another module - erlang

I have been learning erlang for the past week and am going through Joe Armstrong's Pragmatic erlang book . I was writing some code to spawn processes and have come across a situation
I have a function in module myatom.erl which looks like this
start(anatom,Fun) ->
case whereis(anatom) of
undefined ->
Pid = spawn(Fun),
try register(anatom,Pid) of
true -> true
catch
error:Reason ->
Reason
end;
Other -> {error,already_defined}
end.
There is a function in another module named tloop.erl
loop() ->
receive
{ From , No } -> From ! { self(), No*4};
Other -> void
end.
if I am to use start() to spawn loop in the erlang shell , how can I do it ?
I get the following error when I do
anatom:start(atomname,tloop:loop).
Thanks in advance !
anatom:start(myatom,fun tloop:loop).
* 2: syntax error before: ')

You must write the following
anatom:start(myatom, fun tloop:loop/0).
You have to specify the arity (number of arguments) of the function, as in erlang functions with the same name but different arity are not considered to be the same function.

Related

How to find where a process started in elixir

I am using observer in elixir and the following is the snapshot of an Application [under applications tab]:
I need to exit these processes once their work is done. Somehow, I am not able to figure out where some of the processes are originating. Is there a way in elixir/erlang to figure out the module/function where a particular process was created?
Suggestions will be highly appreciated. Thanks.
First you must always have the process's PID or its reference name.
Process.info/2
will give you information about that Process. You may get more documentation and information on how this function works in the Erlang's function it is calling:
process_info-2
There are also arity 1 variants: Process Docs
[erlang:process_info(Pid, initial_call) || Pid <- erlang:processes()].
But note that gen_server, etc., all have the same initial call, so you need to dig a little deeper.
The following is adapted from https://gist.github.com/rlipscombe/a8e87583d47799170f8b:
lists:map(
fun(Pid) ->
InitialCall = case erlang:process_info(Pid, initial_call) of
{initial_call,{proc_lib,init_p,A}} ->
case erlang:process_info(Pid, dictionary) of
{dictionary, D} ->
proplists:get_value('$initial_call', D, undefined);
_ ->
{proc_lib,init_p,A}
end;
{initial_call,{erlang,apply,A}} ->
case erlang:process_info(Pid, current_function) of
{current_function,MFA} -> MFA;
_ -> {erlang,apply,A}
end;
{initial_call,IC} ->
IC;
Other ->
Other
end,
{Pid, InitialCall}
end, erlang:processes()).
Using process_info/1 you can get a list of process information from which initial_call and current_function could help you to find the initial function call with which the process was spawned and the current function call of the process respectively.
Also process_info(Pid, initial_call) and process_info(Pid, current_function) functions are using as shortcut.

What am I doing wrong with erl_parse:parse_form?

I wrote a Hello-World module and compiled it successfully. And then I tried to learn the things under the hood by using erl_scan and erl_parse.
-module(useless).
-export([hello/0]).
hello() -> io:format("hello world\n").
and I type in the erl shell
{ok, S} = file:read_file("useless.erl").
and
{ok, T, _} = erl_scan:string(binary_to_list(S)).
It works fine apparently. But when I try erl_parse:parse_form(T).
It gives {error,{2,erl_parse,["syntax error before: ","'-'"]}}
What am I doing wrong?
Edit:
the module compile is helpful.
Also this is cool.
The function erl_parse:parse_form/1 works on one form only. So you must split the result of erl_scan:string/1 into individual forms first.
you can use erl_scan:tokens to achieve this (this code wors, but I am not sure I use the function in the right way):
-module(useless).
-export([eval/1]).
eval(File) ->
{ok, B} = file:read_file(File),
Forms = scan(erl_scan:tokens([],binary_to_list(B),1),[]),
F = fun(X) -> {ok,Y} = erl_parse:parse_form(X), Y end,
[F(X) || X <- Forms].
scan({done,{ok,T,N},S},Res) ->
scan(erl_scan:tokens([],S,N),[T|Res]);
scan(_,Res) ->
lists:reverse(Res).
Robert suggestion is to use the re-entrant feature of erl_scan:tokens/3.
The docs on this function are not explicit and it took me some time to understand that the initial string needed to be closed by eof. (if not the tokens function runs an endless loop).
Here is the code I finally came up with, I hope that will help others.
eval(File)->
{ok, Data} = file:read_file(File),
String=erlang:binary_to_list(Data),
scan(String).
scan(String) when is_list(String)->
scan(String++eof,[]). %% appended eof
scan({done, Result, LeftOverChars},Acc)->
scan_done(Result,LeftOverChars,Acc);
scan({more, Continuation},Acc)->
scan(erl_scan:tokens(Continuation,[],1),Acc);
scan(String,Acc) when is_list(String)->
scan(erl_scan:tokens([],String,1),Acc).
scan_done({error,ErrorMsg,_Location},_LeftOverChars,_Acc)->
ErrorMsg;
scan_done({eof,Location},LeftOverChars,Acc)->
Acc;
scan_done({ok,Tokens,Location},LeftOverChars,Acc)->
case erl_parse:parse_form(Tokens) of
{ok,R}->scan(LeftOverChars,Acc++[R]);
{error,R}->scan(LeftOverChars,R)
end.

spawn_link is starting an infinite loop, but does not return?

I have an init function which I call start_server:
start_server() ->
spawn_link(game_loop(0,0)).
The purpose is to start a new process that begin to loop and waiting for someone to send a message there:
game_loop(X,Y) ->
receive
{move, left} ->
tell_client(X+1,Y),
game_loop(X+1,Y);
{move, right} ->
tell_client(X-1,Y),
game_loop(X-1,Y)
end.
My thaught was that start_server would return the Pid so that I could write something like this in the Erlang terminal:
> Server = server:start_server().
And then use the variable Server to handle the servers via functions like:
move_left(Pid) ->
Pid ! {move, left}.
But that does not work since start_server() never returns, why is that?
The function spawn_link/1 takes a function as an argument. But in your code you dont pass a function into it:
start_server() ->
spawn_link(game_loop(0,0)).
That sample means that the function game_loop/2 will be called first and after it returns spawn_link/1 will be called with an argument which is a result of calling of game_loop/2. But your function game_loop/2 implements infinite loop so it will never returns and so spawn_link/1 will never be called. If we even assume that game_loop/2 returns it must return a function to call spawn_link/1 properly, otherwise an exception will rise.
To do what you want you should pass game_loop/2 as a function into spawn_link/1:
start_server() ->
spawn_link(fun () -> game_loop(0,0) end).

How to show Erlang call stack?

I need to debug some module in foreign system. The module has public function foo() - how can I know place (module and function name)
from which foo() given module was called? I mean stack of calls.
I cannot stop system, all work I can do by reload this module (but with some debug info).
-module(given).
-export(foo/0).
foo() ->
%% here is my debug - and
%% i need here(!) known about unknown_module:unknown_foo!
ok.
---
-module(unknown_module).
..
unknown_foo() ->
given:foo(). %% see above
Here's a simple trick:
Trace = try throw(42) catch 42 -> erlang:get_stacktrace() end,
erlang:display(Trace)
This might work:
where_am_i() ->
try throw(a)
catch throw:a:Stacktrace ->
Stacktrace
end.
Except that it doesn't work for tail calls. For example, given these two functions:
foo() ->
where_am_i().
bar() ->
X = where_am_i(),
{ok, X}.
I get these results:
4> foo:foo().
[{foo,where_am_i,0},
{erl_eval,do_apply,5},
{shell,exprs,6},
{shell,eval_exprs,6},
{shell,eval_loop,3}]
5> foo:bar().
{ok,[{foo,where_am_i,0},
{foo,bar,0},
{erl_eval,do_apply,5},
{shell,exprs,6},
{shell,eval_exprs,6},
{shell,eval_loop,3}]}
That is, I can only see bar, since foo's call frame has been left already when where_am_i is called.
io:format("~s~n", [element(2, process_info(self(), backtrace))]).
self() can be replaced by any other pid (rpc:pinfo should even work with remote procs). This helps if you cannot even modify the source or beam.
Here is my code for doing this:
format_stack_entry(S) ->
{Module,Fun,Arity,[{file,File},{line,Line}]}=S,
io_lib:format("{~p,~p,~p,[{file,~p},{line,~p]}",[Module,Fun,Arity,File,Line]).
stacktop([Top|_]) ->
Top.
ancestor(N) ->
{_,Stacktrace}=erlang:process_info(self(),current_stacktrace),
ancestor(N+1,Stacktrace).
ancestor(1,S) ->
format_stack_entry(stacktop(S));
ancestor(N,[_|T]) ->
ancestor(N-1,T).
info(Format) -> io:format(lists:concat([ancestor(2),Format,"\r"])).
info(Format,Args) -> io:format(lists:concat([ancestor(2),Format,"\r"]),Args).
Lists is a custom module in the system. Use your foo module instead.

Declaration of ETS in Erlang

The following code gives me an error: "syntax error before: Some_ets"
-module(tut).
-export([incr/1]).
Some_ets = ets:new(?MODULE, [bag]).
incr(X) ->
X+1.
But I am able to declare the ETS within a function, like:
-module(tut).
-export([incr/1]).
incr(X) ->
Some_ets = ets:new(?MODULE, [bag]),
X+1.
Can't I declare a ETS outside a function?
No - unlike other languages there isn't a concept of static initialization - there's no appropriate time for an Erlang system to execute that piece of code.
Erlang does have the concept of a parameterized module however, and that may be what you're after. Have a look here http://www.lshift.net/blog/2008/05/18/late-binding-with-erlang which is a good write up of that - it would allow you to instantiate an "instance" of your tut module bound to a given ets table and save passing around that handle explicitly in your module function calls.
Or if you are into OTP you could have the handle to the ets table passed around in the state variable:
init(_) ->
Some_ets = ets:new(?MODULE, [bag]),
{ok, Some_ets}.
and then use it in your handle_call methods:
get_ets_handle() ->
gen_server:call(?MODULE, {getETSHandle}, infinity).
handle_call({getETSHandle}, _From, Some_ets) ->
{reply, Some_ets, Some_ets}.
You can't do variable assignments like that in a module. See here.

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