[erlang-questions] Question about record_info

[Ladislav Lenart]

Daniel Luna wrote:
> On Mon, 8 Jan 2007, Ladislav Lenart wrote:
>> How can I obtain info about a record given its name?
>> For example let's suppose I have an obligate person
>> record with fields: name, phone and address. I would
>> like to be able to do the following:
>> record_info(fields, person) %% -> {name, phone, address}
>>
>> The following is what the reference manual says about the
>> topic (section 8.7 Internal Representation of Records):
>>
>> ----------------------------------------------------------------
>> To each module using records, a pseudo function is added
>> during compilation to obtain information about records:
>>
>> record_info(fields, Record) -> [Field]
>> record_info(size, Record) -> Size
>>
>> Size is the size of the tuple representation, that is one
>> more than the number of fields.
>> ----------------------------------------------------------------
>>
>> But whatever I try, I get either:
>>  * undef for record_info function,
>>  * illegal record info,
>>  * record Name undefined.
> 
> record_info is not a proper function. It only exists during compilation, 
> which means that it cannot take variable arguments.
> 
> record_info(fields, address) %% works
> 
> Rec = address,
> record_info(fields, Rec) %% doesn't work!
> 
> Of course it only works if the address record is defined in that module.
> 
> If you do want a function that uses record_info, try to use a macro 
> instead.
> 
> This will work:
> 
> -define(print_fields(Rec), io:format("~p~n", [record_info(fields, Rec)])).
> 
> And then use: ?print_fields(address)
> 
> This will not work:
> 
> print_fields(Rec) ->
>   io:format("~p~n", [record_info(fields, Rec)]).
> 
> 
> So you need two things:
> 
> 1. the record needs to be defined in the module where you use 
> record_info (through an -include or through -record).
> 
> 2. The argument to record_info needs to be an 'atom' and not a Variable.

Thank you, I finally managed to make it work. My problem was that I
didn't know that record_info is not a normal function. I thought that
it is automatically compiled to each module that uses records and not
that it only exists during compilation of the module.

Thanks again,

Ladislav Lenart