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copied!<p>It sounds like you want to be able to do something like <code>get_record_field(Field, SomeRecord)</code> where <code>Field</code> is determined at runtime by user interface code say.</p> <p>You're right in that you can't do this in standard erlang as records and the <code>record_info</code> function are expanded and eliminated at compile time.</p> <p>There are a couple of solutions that I've used or looked at. My solution is as follows: (the example gives runtime access to the <code>#dns_rec</code> and <code>#dns_rr</code> records from <code>inet_dns.hrl</code>)</p> <pre><code>%% Retrieves the value stored in the record Rec in field Field. info(Field, Rec) -> Fields = fields(Rec), info(Field, Fields, tl(tuple_to_list(Rec))). info(_Field, _Fields, []) -> erlang:error(bad_record); info(_Field, [], _Rec) -> erlang:error(bad_field); info(Field, [Field | _], [Val | _]) -> Val; info(Field, [_Other | Fields], [_Val | Values]) -> info(Field, Fields, Values). %% The fields function provides the list of field positions %% for all the kinds of record you want to be able to query %% at runtime. You'll need to modify this to use your own records. fields(#dns_rec{}) -> fields(dns_rec); fields(dns_rec) -> record_info(fields, dns_rec); fields(#dns_rr{}) -> fields(dns_rr); fields(dns_rr) -> record_info(fields, dns_rr). %% Turns a record into a proplist suitable for use with the proplists module. to_proplist(R) -> Keys = fields(R), Values = tl(tuple_to_list(R)), lists:zip(Keys,Values). </code></pre> <p>A version of this that compiles is available here: <a href="http://gist.github.com/71050" rel="nofollow noreferrer">rec_test.erl</a></p> <hr> <p>You can also extend this dynamic field lookup to dynamic generation of matchspecs for use with <code>ets:select/2</code> or <code>mnesia:select/2</code> as shown below:</p> <pre><code>%% Generates a matchspec that does something like this %% QLC psuedocode: [ V || #RecordKind{MatchField=V} <- mnesia:table(RecordKind) ] match(MatchField, RecordKind) -> MatchTuple = match_tuple(MatchField, RecordKind), {MatchTuple, [], ['$1']}. %% Generates a matchspec that does something like this %% QLC psuedocode: [ T || T <- mnesia:table(RecordKind), %% T#RecordKind.Field =:= MatchValue] match(MatchField, MatchValue, RecordKind) -> MatchTuple = match_tuple(MatchField, RecordKind), {MatchTuple, [{'=:=', '$1', MatchValue}], ['$$']}. %% Generates a matchspec that does something like this %% QLC psuedocode: [ T#RecordKind.ReturnField %% || T <- mnesia:table(RecordKind), %% T#RecordKind.MatchField =:= MatchValue] match(MatchField, MatchValue, RecordKind, ReturnField) when MatchField =/= ReturnField -> MatchTuple = list_to_tuple([RecordKind | [if F =:= MatchField -> '$1'; F =:= ReturnField -> '$2'; true -> '_' end || F <- fields(RecordKind)]]), {MatchTuple, [{'=:=', '$1', MatchValue}], ['$2']}. match_tuple(MatchField, RecordKind) -> list_to_tuple([RecordKind | [if F =:= MatchField -> '$1'; true -> '_' end || F <- fields(RecordKind)]]). </code></pre> <hr> <p>Ulf Wiger has also written a parse_transform, <a href="http://forum.trapexit.org/viewtopic.php?p=21790" rel="nofollow noreferrer">Exprecs</a>, that more or less does this for you automagically. I've never tried it, but Ulf's code is usually very good. </p> <hr>

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