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

Dan Kelley <>
Thu Jun 16 22:19:16 CEST 2011

On Thu, Jun 16, 2011 at 4:12 PM, Mihai Balea <> wrote:

> On Jun 16, 2011, at 3:48 PM, 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,
> Why not generate a temporary file and apply compile:file/2 on it?

Two reasons:

First, I can provide better error messages, in the sense that I'll have
smaller compile units that I'll trace back to specific parts of the config.

Second,  I'm not sure that I can make that non-ugly.  I had hoped to write
my dynamic module to disk using erl_pp so it'd be human-readable.

But yeah, that's my fallback position.

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