[erlang-questions] Defining static functions in abstract modules

Matthew Dempsky <>
Wed Apr 9 11:22:46 CEST 2008


Because I want to be able to abstract away how to instantiate a
module, and I want to be able to define internal helper functions to
do it, and I want to be able to call them "foo(X)" instead of
"new(foo, X)".

On Wed, Apr 9, 2008 at 2:20 AM, Hynek Vychodil <> wrote:
> UGH, static functions? Where is benefit to do it? What matter? I don't
> understand.
>
>
>
> On Wed, Apr 9, 2008 at 11:04 AM, Matthew Dempsky <>
> wrote:
>
> >
> >
> >
> > It would be nice to be able to define arbitrary static functions
> > within an abstract module.  Right now you can kludge around this by
> > exploiting that local functions named 'new' are always static and
> > write something like
> >
> >    new(incr, X) -> X + 1.
> >
> > but it would be nicer to just write
> >
> >    -static([incr/1]).
> >    incr(X) -> X + 1.
> >
> > Below is a very rudimentary patch that adds support for this syntax.
> > It's basically only enough to compile and run very simple test cases
> > and to demonstrate that implementing this does not seem too difficult.
> > (One problem I'm already aware of is that a static function foo/1 and
> > an instance function foo/0 silently collide without any warning... so
> > don't do that. ;-))
> >
> > Another option would be to make the attribute statement just
> > "-static([incr])." and then all incr functions are defined static, but
> > that seems unnecessarily limiting.
> >
> > If the long-term goal is to have separate namespaces for static and
> > instance functions (e.g., so (Mod:new(...)):module_info() works and
> > returns something meaningful for the module instance), then maybe a
> > syntax that actually decorates individual function definitions would
> > be better?
> >
> > Are there any plans on adding static functions in a future release?
> >
> >
> > --- stdlib/src/erl_parse.yrl.orig       2007-11-27 07:57:32.000000000
> -0800
> > +++ stdlib/src/erl_parse.yrl    2008-04-09 01:25:31.000000000 -0700
> > @@ -627,6 +627,12 @@
> >            {attribute,La,export,farity_list(ExpList)};
> >        _Other -> error_bad_decl(La, export)
> >     end;
> > +build_attribute({atom,La,static}, Val) ->
> > +    case Val of
> > +        [ExpList] ->
> > +            {attribute,La,static,farity_list(ExpList)};
> > +        _Other -> error_bad_decl(La, static)
> > +    end;
> >  build_attribute({atom,La,import}, Val) ->
> >     case Val of
> >        [Name] ->
> > --- compiler/src/sys_pre_expand.erl.orig        2008-02-05
> 05:37:09.000000000 -0800
> > +++ compiler/src/sys_pre_expand.erl     2008-04-09 01:26:06.000000000
> -0700
> > @@ -38,6 +38,7 @@
> >                  parameters=undefined,          %Module parameters
> >                  package="",                    %Module package
> >                  exports=[],                    %Exports
> > +                 statics=[],                    %Statics
> >                  imports=[],                    %Imports
> >                  mod_imports,                   %Module Imports
> >                  compile=[],                    %Compile flags
> > @@ -109,6 +110,7 @@
> >                 end,
> >             {Fs1,Xs,Ds} = sys_expand_pmod:forms(Fs0, Ps,
> >                                                 St0#expand.exports,
> > +                                                St0#expand.statics,
> >                                                 St0#expand.defined),
> >            St1 = St0#expand{exports=Xs, defined=Ds},
> >            {Fs2,St2} = add_instance(Ps, Fs1, St1),
> > @@ -220,6 +222,8 @@
> >               package = packages:strip_last(M)};
> >  attribute(export, Es, St) ->
> >     St#expand{exports=union(from_list(Es), St#expand.exports)};
> > +attribute(static, Ss, St) ->
> > +    St#expand{statics=union(from_list(Ss), St#expand.statics)};
> >  attribute(import, Is, St) ->
> >     import(Is, St);
> >  attribute(compile, C, St) when is_list(C) ->
> > --- compiler/src/sys_expand_pmod.erl.orig       2008-02-05
> 05:38:12.000000000 -0800
> > +++ compiler/src/sys_expand_pmod.erl    2008-04-09 01:32:36.000000000
> -0700
> > @@ -25,18 +25,19 @@
> >  %% and 'exports'. The automatic 'new/N' function is neither added to the
> >  %% definitions nor to the 'exports'/'defines' lists yet.
> >
> > --export([forms/4]).
> > +-export([forms/5]).
> >
> > --record(pmod, {parameters, exports, defined, predef}).
> > +-record(pmod, {parameters, exports, statics, defined, predef}).
> >
> >  %% TODO: more abstract handling of predefined/static functions.
> >
> > -forms(Fs0, Ps, Es0, Ds0) ->
> > +forms(Fs0, Ps, Es0, Ss0, Ds0) ->
> >     PreDef = [{module_info,0},{module_info,1}],
> > -    forms(Fs0, Ps, Es0, Ds0, PreDef).
> > +    forms(Fs0, Ps, Es0, Ss0, Ds0, PreDef).
> >
> > -forms(Fs0, Ps, Es0, Ds0, PreDef) ->
> > -    St0 = #pmod{parameters=Ps,exports=Es0,defined=Ds0, predef=PreDef},
> > +forms(Fs0, Ps, Es0, Ss0, Ds0, PreDef) ->
> > +    St0 = #pmod{parameters=Ps,exports=Es0,statics=Ss0,defined=Ds0,
> > +                predef=PreDef},
> >     {Fs1, St1} = forms(Fs0, St0),
> >     Es1 = update_function_names(Es0, St1),
> >     Ds1 = update_function_names(Ds0, St1),
> > @@ -50,15 +51,22 @@
> >     [update_function_name(E, St) || E <- Es].
> >
> >  update_function_name(E={F,A}, St) when F =/= new ->
> > -    case ordsets:is_element(E, St#pmod.predef) of
> > +    case ordsets:is_element(E, St#pmod.predef) orelse
> > +        ordsets:is_element(E, St#pmod.statics) of
> >        true -> E;
> >        false -> {F, A + 1}
> >     end;
> >  update_function_name(E, _St) ->
> >     E.
> >
> > -update_forms([{function,L,N,A,Cs}|Fs],St) when N =/= new ->
> > -    [{function,L,N,A+1,Cs}|update_forms(Fs,St)];
> > +update_forms([F0={function,L,N,A,Cs}|Fs],St) when N =/= new ->
> > +    F = case ordsets:is_element({N,A}, St#pmod.statics) of
> > +            true ->
> > +                F0;
> > +            false ->
> > +                {function,L,N,A+1,Cs}
> > +        end,
> > +    [F|update_forms(Fs,St)];
> >  update_forms([F|Fs],St) ->
> >     [F|update_forms(Fs,St)];
> >  update_forms([],_St) ->
> > @@ -74,9 +82,14 @@
> >     {[], St0}.
> >
> >  %% Only function definitions are of interest here. State is not updated.
> > -form({function,Line,Name0,Arity0,Clauses0},St) when Name0 =/= new ->
> > -    {Name,Arity,Clauses} = function(Name0, Arity0, Clauses0, St),
> > -    {{function,Line,Name,Arity,Clauses},St};
> > +form(F = {function,Line,Name0,Arity0,Clauses0},St) when Name0 =/= new ->
> > +    case ordsets:is_element({Name0, Arity0}, St#pmod.statics) of
> > +        true ->
> > +            {F,St};
> > +        false ->
> > +            {Name,Arity,Clauses} = function(Name0, Arity0, Clauses0, St),
> > +            {{function,Line,Name,Arity,Clauses},St}
> > +    end;
> >  %% Pass anything else through
> >  form(F,St) -> {F,St}.
> >
> > @@ -362,7 +375,13 @@
> >  expr({call,Lc,{atom,Lf,F},As0},St) ->
> >     %% Local function call - needs THIS parameter.
> >     As1 = expr_list(As0,St),
> > -    {call,Lc,{atom,Lf,F},As1 ++ [{var,0,'THIS'}]};
> > +    As2 = case ordsets:is_element({F, length(As1)}, St#pmod.statics) of
> > +              true ->
> > +                  As1;
> > +              false ->
> > +                  As1 ++ [{var,0,'THIS'}]
> > +          end,
> > +    {call,Lc,{atom,Lf,F},As2};
> >  expr({call,Line,F0,As0},St) ->
> >     %% Other function call
> >     F1 = expr(F0,St),
> > _______________________________________________
> > erlang-questions mailing list
> > 
> > http://www.erlang.org/mailman/listinfo/erlang-questions
> >
>
>
>
> --
> --Hynek (Pichi) Vychodil



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