dynamic - Erlang动态记录编辑

Question

我正在 mnesia 中存储一些数据，并且我希望能够更改所涉及的大多数值。

天真的

change(RecordId, Slot, NewValue) ->
    [Rec] = do(qlc:q([X || X <- mnesia:table(rec), X#rec.id =:= RecordId])),
    NewRec = Rec#rec{Slot=NewValue},
    F = fun() -> mnesia:write(NewRec) end,
    {atomic, Val} = mnesia:transaction(F),
    Val.

不这样做；编译器提示 Slot不是 atom或 _ .有没有办法像上面一样表达通用插槽编辑功能，或者我会被困在定义一大堆 change_slot ?

稍微好一点的方法是取出 insert和 find件

atomic_insert(Rec) ->
    F = fun() -> mnesia:write(Rec) end,
    {atomic, Val} = mnesia:transaction(F),
    Val.

find(RecordId) -> 
    [Rec] = do(qlc:q([X || X <- mnesia:table(rec), X#rec.id =:= RecordId])),
    Rec.

change(RecordId, name, NewValue) ->
    Rec = find(RecordId),
    NewRec = Rec#rec{name=NewValue},
    atomic_insert(NewRec);
change(RecordId, some_other_property, NewValue) ->
    Rec = find(RecordId),
    NewRec = Rec#rec{some_other_property=NewValue},
    ...

但是那里仍然有一些代码重复。有没有办法抽象出这种模式？是否有允许编辑记录的成熟技术？一般有什么想法吗？

Answer 1

由于记录由元组表示，您可以尝试使用元组操作来设置单个值。

-module(rec).
-export([field_num/1, make_rec/0, set_field/3]).
-record(rec, {slot1, slot2, slot3}).

make_rec() ->
  #rec{slot1=1, slot2=2, slot3=3}.

field_num(Field) ->
  Fields = record_info(fields, rec),
  DifField = fun (FieldName) -> Field /= FieldName end,
  case length(lists:takewhile(DifField, Fields)) of
    Length when Length =:= length(Fields) ->
      {error, not_found};
    Length ->
      Length + 2
  end.

set_field(Field, Value, Record) ->
  setelement(field_num(Field), Record, Value).

set_field 将返回更新的记录:

Eshell V5.9.1  (abort with ^G)
1> c(rec).
{ok,rec}
2> A = rec:make_rec().
{rec,1,2,3}
3> B = rec:set_field(slot3, other_value, A).
{rec,1,2,other_value}