[erlang-questions] Elixir Protocols in Erlang & a Strange Warning

Kaveh Shahbazian <>
Fri Dec 13 09:42:31 CET 2013


Thans José;

And the part "the underlying dispatch mechanism, which is used in the
descried issue, is not deprecated and it won't be removed." really helped
my mind to be in peace!

Kaveh Shahbazian
“Walking on water and developing software from a specification are easy if
both are frozen.”
― Edward Berard
<http://goo.gl/ZZ2TMu>
<http://stackoverflow.com/users/54467/kaveh-shahbazian>


On Fri, Dec 13, 2013 at 11:57 AM, José Valim <
> wrote:

> I have seen this question asked earlier in Stack Overflow and answered it
> there:
> http://stackoverflow.com/questions/20556472/elixir-protocols-in-erlang-a-strange-warning/20561973#20561973
>
> I have also reported a bug to erlang-bugs this time.
>
> One final note: although parameterized modules were removed from R16, the
> underlying dispatch mechanism, which is used in the descried issue, is not
> deprecated and it won't be removed.
>
>
>
>
> *José Valim*
> www.plataformatec.com.br
> Skype: jv.ptec
> Founder and Lead Developer
>
>
> On Fri, Dec 13, 2013 at 8:10 AM, Dmitry Kolesnikov <
> > wrote:
>
>> Hello,
>>
>> Essentially you are trying to use parametrized modules, which get
>> deprecated in R14/15 and dropped from R16.
>> See for details:
>>  * http://www.erlang.org/news/35
>>  * http://www.erlang.se/workshop/2003/paper/p29-carlsson.pdf
>>
>> This type of problems are solvable either via pattern match or currying:
>>
>> -module(stringer).
>> -export([stringer/1,sample/0]).
>>
>> stringer(V) when is_list(V) ->
>>     fun() -> to_string(V, nop) end;
>> stringer(V) when is_atom(V) ->
>>     fun() -> to_string(V, nop) end;
>> stringer(_V) ->
>>     fun() -> not_implemented end.
>>
>> to_string(V, _Nop) ->
>>     Buffer = io_lib:format("~p",[V]),
>>     lists:flatten(Buffer).
>>
>> sample() ->
>>     io:format("~p~n", [(stringer([1,2]))()]),
>>     io:format("~p~n", [(stringer(cute_atom))()]),
>>     io:format("~p~n", [(stringer(13))()]).
>>
>>
>> Best Regards,
>> Dmitry
>>
>> On Dec 13, 2013, at 8:31 AM, Kaveh Shahbazian <>
>> wrote:
>>
>> I wanted to write something like ((IStringer)object).ToString() (in C#)
>> in Erlang. After some studying I've learnt that Elixir has something called
>> Protocols that pretty much resembles the same thing of C# (in an inside-out
>> manner). Then I came up with this idea/code in Erlang - which is nice
>> enough to me like:
>>
>> ?stringer(my_val):to_string().
>>
>> And it either returns the expected value or not_implemented atom!
>>
>> But 2 questions:
>>
>> 1 - Why nobody use this or promote things based on stateful modules in
>> Erlang? (OTP aside and from talking to some Erlangers they did not know
>> that actually OTP is built around this! So really there is a need to change
>> how Erlang is being taught and promoted. It's possible that I am confused.).
>>
>> 2 - Why I get this warning? That call actually never fails.
>>
>> The warning:
>>
>> stringer.erl:18: Warning: invalid module and/or function name; this call will always fail
>> stringer.erl:19: Warning: invalid module and/or function name; this call will always fail
>> stringer.erl:20: Warning: invalid module and/or function name; this call will always fail
>>
>> The code:
>>
>> -module(stringer).
>> -export([to_string/1,sample/0]).
>>
>> -define(stringer(V), {stringer, V}).
>>
>> to_string({stringer, V}) when is_list(V) ->
>>     to_string(V, nop);
>> to_string({stringer, V}) when is_atom(V) ->
>>     to_string(V, nop);
>> to_string({stringer, _V}) ->
>>     not_implemented.
>>
>> to_string(V, _Nop) ->
>>     Buffer = io_lib:format("~p",[V]),
>>     lists:flatten(Buffer).
>>
>> sample() ->
>>     io:format("~p~n", [?stringer([1,2]):to_string()]),
>>     io:format("~p~n", [?stringer(cute_atom):to_string()]),
>>     io:format("~p~n", [?stringer(13):to_string()]).
>>
>> And the output is:
>>
>> "[1,2]"
>> "cute_atom"
>> not_implemented
>>
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>
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