[erlang-questions] Converting list of tagged tuples to a record

[Rapsey]

I use these two functions (rec2prop, prop2rec):

rec2prop(Rec, RecordFields) ->
    loop_rec(RecordFields, 1, Rec, []).

loop_rec([H|T], N, Rec, L) ->
    loop_rec(T, N+1, Rec, [{H, element(N+1, Rec)}|L]);
loop_rec([], _, _, L) ->
    L.

prop2rec(Prop, RecName, DefRec, RecordFields) ->
    loop_fields(erlang:make_tuple(tuple_size(DefRec), RecName),
RecordFields, DefRec, Prop, 2).

loop_fields(Tuple, [Field|T], DefRec, Props, N) ->
    case lists:keysearch(Field, 1, Props) of
        {value, {_, Val}} ->
            loop_fields(setelement(N, Tuple, Val), T, DefRec, Props, N+1);
        false ->
            loop_fields(setelement(N, Tuple, element(N, DefRec)), T, DefRec,
Props, N+1)
    end;
loop_fields(Tuple, [], _, _, _) ->
    Tuple.


And then for every record a define (#cdat{} is the name of the record):
-define(R2P(Record), rec2prop(Record, record_info(fields, cdat))).
-define(P2R(Prop), prop2rec(Prop, cdat, #cdat{}, record_info(fields,
cdat))).

It's not exactly optimal, but at least I don't have to write a function for
every record (just two short defines).



Sergej

On Thu, Sep 2, 2010 at 8:48 AM, Bengt Kleberg <bengt.kleberg@REDACTED>wrote:

> Greetings,
>
> After 10 years of programming Erlang I still use the method described
> below (1) when moving data from a user interface.
> The same goes for using records in records. It is rather awkward to
> update the innermost records, so it helps to use functions.
>
>
> bengt
>
> On Thu, 2010-09-02 at 08:23 +0200, Silas Silva wrote:
> > Hello all,
> >
> > In my newbie application, I receive a bunch of data from HTTP's POST
> > (I'm using Mochiweb), in the following format (supposing a person's
> > data):
> >
> >     [{first_name, value}, {last_name, value}, ...]
> >
> > And I have the definition of the record for the person:
> >
> >     -record(person, {first_name, last_name, ...}).
> >
> > alistair in the #erlang channel in freenode, gave me a nice solution to
> > put this list in the record:
> >
> >     lists:foldl(fun add_to_record/2, #person{}, List).
> >
> >     add_to_record({first_name, value}, rec) ->
> >         rec#person{first_name = value};
> >
> >     add_to_record({last_name, value}, rec) ->
> >         rec#person{last_name = value};
> >
> >     ...
> >
> > That works.  My questions are:
> >
> > 1. Are there other [good practice] ways to transform information coming
> >    from a UI (which, must of the times, come as a list) in well
> >    specified records?
> >
> > 2. Is it common to use records of records?  Is it possible?  For example:
> >
> >     -record(name, {last_name, first_name}).
> >     -record(person, name, ...)
> >
> > Thanks!
> >
>
>
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