[erlang-questions] Maps branch and disclaimers

[Tomas Abrahamsson]

> Here you go, Maps!
>
> I've pushed a Maps branch to Erlang/OTPs repository at GitHub.

Hi, this is cool!

I did some performance experiments with gpb, a google
protobuf encoder/decoder, in which I had already implemented
experimental support for maps. It seems map creation is fast!

                    representation of msg
                    records       maps
    msg to binary:  27.6 MB/s     22.9 MB/s  (83% of record speed)
    binary to msg:  27.3 MB/s     29.9 MB/s (110% of record speed)

The figures are number of megabytes processed per second, so
higher number means faster/better. The "binary to msg" involves
creating a record or map, with several fields, once for each
parsed message or sub message. The "msg to binary" uses a match
for several message fields to assign each field to a variable,
once for each message or sub message to encode. Included in
these figures is of course also some binary processing.


I also stumbled upon an what appears to be a compiler error.
Given this program:

    -module(mapbug).
    -export([verify_msg/1]).

    verify_msg(Msg) ->
        v_msg_m1(Msg).

    v_msg_m1(#{f2 := F2}) ->
        if is_list(F2) -> [v_type_uint32(Elem) || Elem <- F2];
           true -> ok
        end,
        ok;
    v_msg_m1(_X) ->
        ok.

    v_type_uint32(_N) ->
        ok.

compiling it results in:

    1> c("/path/to/mapbug", []).
    mapbug: function v_msg_m1/1+17:
      Internal consistency check failed - please report this bug.
      Instruction: return
      Error:       {stack_frame,undecided}:

    error

BRs
Tomas