[erlang-questions] erl_scan, erl_parse, macros and records

[Ulf Wiger]

This sounds in part similar to what the Diameter application (part of OTP R14B03) does.

The Diameter application uses a parse_transform called diameter_exprecs, which is essentially the same as the exprecs module in http://github.com/esl/parse_trans

For one thing, this allows users to work with record definitions generated at compile-time, without having to deal with include files. It also allows the diameter_service agent to check ubiquitous attributes like 'Destination-Host', 'Destination-Realm' in any record, as long as that record is known by the encode-decode ("dictionary") module:

get_avp_value(Dict, Name, Rec)
  when is_tuple(Rec) ->
    try
        Dict:'#get-'(Name, Rec)
    catch
        error:_ ->
            undefined
    end.

(from diameter/src/app/diameter_service.erl)

Another thing that might help is parse_trans_codegen.erl, which provides a (IMHO) more readable way to generate code.
The idea is that you simply wrap the code you want to generate in a fun(), possibly using some simple expansion techniques to import values, etc. If records are known at generation time, they can simply be included and used as-is, with normal record syntax.

https://github.com/esl/parse_trans/blob/master/doc/parse_trans_codegen.md

See also https://github.com/esl/parse_trans/blob/master/examples/ex_codegen.erl for some (admittedly pretty corny) examples.

BR,
Ulf W

On 16 Jun 2011, at 21:48, Dan Kelley wrote:

> 
> Hi,
> 
> I'm working on a router of sorts.  I'd like to have a config file which specifies how to classify messages, what channels are available, and routing rules based on classifications and channels.
> 
> When my application starts, I'd like to transform the configuration into a module (let's call it 'router') so I can do someting like 'router:route(Message)'.  Message will be a record that encapsulates a bunch of things about the message, and the returned value will be the channel to which the message should be delivered.  The Message record is reasonably complicated - the top level thing contains several layers of different records.  I also have a large number of macros that I use to extract different fields/subrecords of a top-level Message.
> 
> When I started looking at how to do this, I quickly found erl_scan, erl_parse, and friends.  I can easily create a module on the fly with these, but everything falls apart when I need to get at the Message record and macro definitions.  Despite the access provided to compiler-ish things by erl_scan, erl_parse and friends, it looks like the only way to get at the preprocessor is to use epp on files.  This is a problem because the code I want to dynamically create needs to use the record definitions and macros from the Message header file.
> 
> Trying to figure a way around this led me to aleppo, which neatly solved the problem of including the header file, but aleppo doesn't process records.  For a brief and exciting moment, it looked like erl_expand_records might save the day, but it expects forms, and I can't get anything with records through erl_parse:parse_form.
> 
> Am I missing an easier way to dynamically create code that references macros and records?
> 
> Thanks,
> 
> Dan
> 
> 
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Ulf Wiger, CTO, Erlang Solutions, Ltd.
http://erlang-solutions.com



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