[eeps] allow external function as literals

[Tony Rogvall]

On 2 okt 2014, at 11:45, Björn-Egil Dahlberg <egil@REDACTED> wrote:

> On 2014-10-01 23:55, Tony Rogvall wrote:
>> To my (mild) surprise I discovered that the tuples above where NOT generated as literals!
>> So of course I checked if there was any particular reason for this.
> Was there? =)

Not really :-)

>> 
>> I developed a patch produced to patch function return values:
>> 
>> '__ffe_sqr'() -> <<"sqr">>.
>> 
>> here I replaced <<"sqr">>.
>> with
>>  { 0, <<"sqr">>, fun ffe:docol/4, fun ffe:dupe/0, fun ffe:star/0, fun ffe:semis/0 }.
>> 
>> in the literal section of the beam file using beam_lib and a simple match.
>> 
>> The loader worked and the code now return constant tuples with functions in them.
> 
> Could you use these literals? Even if the  external module wasn't loaded?
> 
Yupp. This is handled by the external format, which is the format used by the literal table. And also the reason why my
workaround works.

> I only checked the code briefly but I think that if you load an external as a literal you have to make a stub for the module if it isn't loaded, so the
> error handler can load the module later on. But you are a savvy person, I think you can figure it out. =)
> 
When literal table is loaded the EXPORT_EXT (external.c: around 3490) it will create the stub needed.
This is of course the same mechanism used by binary_to_term et al.

My worry is that there are some intricate detail in code loading that will stop my scheme from working flawlessly,
it may have been forgotten for some good reason?

> I'm sure there are other things to consider as well.
> 

One reason I can think of is that is "trick" to add this form of literal in the assembler format (not impossible) and
is possibly not backwards compatible.

Lets say we want to produce a literal with a export entry fun in it,  we can not use fun key word since funs are
not terms. We need to whip together a term somehow.

For a list / string
	{move, {literal, {hello, 0, {erlang,foo,0}}}, {x,0}}

wont work

Some kind of expression format may do the trick (choose some, this is abstract format without line numbers)

	{move, {literal_expr, {tuple, [{atom,hello},{integer,0},{'fun',{function,{atom,erlang},{atom,display},{integer,1}}} ] }}}

The literal_expr must then be parsed by the assembler and then converted to "normal" term before packed into
the literal section of the beam file.

Not to difficult. And a bit more general than the {literal, X} thing.

/Tony

>> 
>> Question: 
>> 1. Is my code going to crash later ?  :-)
>> 2. if so can I do anything about it ?
>> 3. if not can we get external funs as supported literals ?
>> 
>> /Tony
>> 
>> 
>> _______________________________________________
>> eeps mailing list
>> 
>> eeps@REDACTED
>> http://erlang.org/mailman/listinfo/eeps
> 
> _______________________________________________
> eeps mailing list
> eeps@REDACTED
> http://erlang.org/mailman/listinfo/eeps

"Installing applications can lead to corruption over time. Applications gradually write over each other's libraries, partial upgrades occur, user and system errors happen, and minute changes may be unnoticeable and difficult to fix"