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

Kaveh Shahbazian <>
Fri Dec 13 09:25:19 CET 2013


I do not thing so. Parametrized modules are different than stateful
modules. Parametrized modules are deprecated and they were always
provisional.

Joe Armstrong in Programming Erlang (2nd Edition, Page 418) talked about
them and in this same book he was promoting "coming soon" features of R17
and It's highly unlikely for them to be deprecated.

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 11:40 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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