What am I doing wrong with erl_parse:parse_form? - erlang

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.

Related

Should I not call gen_server:stop() directly?

In the LYSE book the author handles the termination of the server as follows:
%% Synchronous call
close_shop(Pid) -> gen_server:call(Pid, terminate).
handle_call(terminate, _From, Cats) ->
{stop, normal, ok, Cats}.
terminate(normal, Cats) ->
[io:format("~p was set free.~n",[C#cat.name]) || C <- Cats],
ok.
So it returns a stop value from the handle_call callback.
Here is how I wrote it:
close_shop(Pid) -> gen_server:stop(Pid).
terminate(_Reason, {Cats, Money}) ->
io:format("Made $~w~n", [Money]),
[io:format("~p was set free.~n",[C#cat.name]) || C <- Cats].
Is this not a good practice then to call gen_server:stop() directly?
It is not a bad practice to call gen_server:stop/1,3 directly. It does an almost same thing as the example from LYSE but without calling handle_call/3 from your module. Try and check it out. You can even read the source code to be sure.

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.

spawn function in erlang using function in another module

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.

Erlang -- How to convert a fun() object to a String

Is there a straightforward way to convert an Erlang fun to a string? A call to io_lib:format only prints the function reference, e.g. something like "#Fun<erl_eval.20.67289768>". For example, I'd like to be able to do this:
1> Fun = fun() -> atom_to_list('hello world') end.
2> FunStr = fun_to_str(Fun).
"fun() -> atom_to_list('hello world') end."
I'm looking for how to implement fun_to_str. In javascript, some interpreters have a .toSource() function that can be called on any object, including functions, that print their string representation. Any info is appreciated, thanks.
First, get the environment variables for the fun (which includes the abstract code):
1> {env, [{_, _, _, Abs}]} = erlang:fun_info(Fun, env).
{env,[{[],
{eval,#Fun<shell.21.83096281>},
{value,#Fun<shell.5.83096281>},
[{clause,1,[],[],
[{call,1,{atom,1,atom_to_list},[{atom,1,hello_world}]}]}]}]}
Pretty print the abstract code using erl_pp:
3> Str = erl_pp:expr({'fun', 1, {clauses, Abs}}).
[[[["fun",
[[[[["()"]," ->"],
["\n ",
[["atom_to_list",[[40,["'hello world'",41]]]]]]]]]]],
[10,["end"]]]]
4> io:format([Str|"\n"]).
fun() ->
atom_to_list('hello world')
end
ok
(You have to add {'fun', 1, {clauses, ...}} around it to make it a complete Erlang expression)
You might be able to use erlang:fun_info/2 for that, atleast i get some information from the shell when doing
1> erlang:fun_info(fun() -> test,ok end, env).
{env,[[],
{value,#Fun<shell.7.37281544>},
{eval,#Fun<shell.24.85590193>},
[{clause,1,[],[],[{atom,1,test},{atom,1,ok}]}]]}
2>
You want the last list with the clause atom and then pretty print it using for instance erl_pp

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.

Resources