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

Dan Kelley <>
Thu Jun 16 21:48:50 CEST 2011


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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