Syntax Error while accessing a field in a record

问题:

I'm having a problem that should be stupidly easy to fix. Following this, I'm trying to access a field in a record. Here's a simplified example that exhibits my problem:

-module(test).
-export([test/0]).

-record(rec, {f1=[], f2=[], f3=[]}).

test() ->
    Rec = #rec{f1=[1,2,3], f3=[4,5,6]},
    Fields = record_info(fields, rec),
    loop(Fields, Rec).

loop([Field|Fields], Rec) ->
    [Rec#rec.Field|loop(Fields, Rec)]; %% <-- This is line 12.
loop([], _Rec) ->
    [].

When I try to compile test, I get a syntax error:

./test.erl:12: syntax error before: Field

What am I doing wrong?

回答1:

If you only want to enumerate record values you can use element/2 and enumerate elements from 2 (the first element is a record name) to tuple_size(Record).

If you want to access record fields by name at run time you can create auxiliary proplist at compile time like this:

Fields = lists:zip(record_info(fields, rec),
                   lists:seq(2, record_info(size, rec)))

Note that record_info() always evaluated at compile time.

And then query field value with function similar to this:

get_record_value(Name, Record, Fields) ->
    case proplists:get_value(Name, Fields) of
        undefined ->
            undefined;
        N when is_integer(N) ->
            element(N, Record)
    end.

回答2:

Records are converted into arrays in compile-time, meaning that all field accesses are converted to erlang:element calls as well. Thus variables cannot be used as field names must be known at compile-time - as Damg already answered.

The "workarounds" I am aware of are either using proplists, dicts, etc. instead of records, or use Ulf Wiger's exprecs to generate record access functions.

回答3:

Another way is converting the record into a proplist and using the proplists library module to iterate or access specific fields. This example:

-module(test).
-export([start/0]).
-record(test, {value1, value2, value3}).

start() ->
    R = #test{value1=1, value2=2, value3=3},
    Fields = record_info(fields, test),
    Values = tl(tuple_to_list(R)),
    lists:zip(Fields, Values).

Will produce the following proplist:

> c("test").
> Proplist = test:start().
[{value1,1},{value2,2},{value3,3}]

Then for example to get value of field value2:

> proplists:get_value(value2, Proplist).
2