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

Anders Nygren <>
Thu Jun 16 22:42:55 CEST 2011


On Thu, Jun 16, 2011 at 3:19 PM, Dan Kelley <> wrote:
>
>
> 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.
> Dan
>
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Hi
Take a look at erl_syntax.

/Anders



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