[erlang-questions] Accessing 'spec' information at runtime

Kenneth Lundin <>
Tue Mar 8 16:48:21 CET 2011

The -spec and -type is included in the abstract_code chunk in the beam
files. If
they are compiled to contain debug_info (erlc +debug_info my_module.erl).

The appear as attributes and the can be abstracted like this (only the
-spec(s) in this example):

%% Beam is a string contaning the path to a beam file i.e
%% you can for example call it like this from the shell:
%% m:getspecs(code:which(lists)).

        {ok,{_,[{abstract_code,{_,AC}}]}} =
    Specs = [Spec|| Spec = {attribute,_,'spec',{FA,_}} <- AC],
    io:fwrite("~s~n", [erl_prettypr:format(erl_syntax:form_list(Specs))]).

Hmm, just thinking. Maybe it would be a good idea to have the -spec
info available as a separate
chunk who is always present even without debug_info.
The -spec info could for example be used for a smarter tab-completion
of functions in the shell.
Including argument names and types.

/Kenneth, Erlang/OTP Ericsson

On Tue, Mar 8, 2011 at 3:41 PM, Eric Merritt <> wrote:
> Hello All,
>  I have been doing some digging and it I haven't found anything that
> indicates that spec information survives compilation. It may in the
> beam files, but there isn't any documentation to support that
> assumption in beam_lib. So before I go digging in the source I thought
> I would ping the list and see if anyone has any ideas on accessing
> 'speced' type information at run time.
> Thanks,
> Eric
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