这听起来像你想能够做一些像get_record_field(Field, SomeRecord)
其中Field
是由用户界面代码说在运行时确定。
你是对的,你不能在标准erlang中做到这一点,因为记录和record_info
函数在编译时被扩展和消除。
我已经使用或看过几种解决方案。我的解决方案如下:(例子给出了从inet_dns.hrl
到#dns_rec
和#dns_rr
记录运行时访问)
%% 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).
的这一点,编译一个版本可以在这里找到:rec_test.erl
您还可以扩展这种动态字段查找动态生成匹配specs与ets:select/2
或mnesia:select/2
使用如下所示:
%% 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)]]).
Ulf Wiger也写了一个parse_transform,Exprecs,这或多或少地为你自动完成。我从来没有尝试过,但Ulf的代码通常非常好。
如果你给你想要什么,能够做一个伪代码示例它会帮。 – archaelus 2009-02-26 19:38:18