[erlang-questions] Two questions on dynamic code loading example in Joe's book

[Joe Armstrong]

-module(a)

loop(...) ->
      ....
      loop(..)     <- call loop in the *same* module

Is different to

loop(...) ->
      ...
      a:loop(...)    <- call loop in "the latest version of a:

/Joe



On Sat, Jun 27, 2009 at 2:37 AM, Kaiduan Xie<kaiduanx@REDACTED> wrote:
> Hi, all,
>
> While trying example from Joe's book on dynamic code loading, I am
> puzzled by the following two things,
>
> 1. Why we need to compile code under the same erlang shell to make
> dynamic loading module b work? If I change b, then compile b under a
> unix shell, new code is not loaded dynamically.
>
> kaiduanx@REDACTED:~/joebook/code$ erl
> Erlang (BEAM) emulator version 5.5.2 [source] [async-threads:0]
> [kernel-poll:false]
>
> Eshell V5.5.2  (abort with ^G)
> 1> c(b).
> {ok,b}
> 2> c(a).
> {ok,a}
> 3> a:start(one).
> <0.43.0>
> Vsn1 (one) b:x() = 1
> Vsn1 (one) b:x() = 1
> Vsn1 (one) b:x() = 1
> Vsn1 (one) b:x() = 1
> Vsn1 (one) b:x() = 1
> Vsn1 (one) b:x() = 1
> Vsn1 (one) b:x() = 1
> Vsn1 (one) b:x() = 1
> Vsn1 (one) b:x() = 1
> Vsn1 (one) b:x() = 1
>
> kaiduanx@REDACTED:~/joebook/code$ more b.erl
> %% ---
> %%  Excerpted from "Programming Erlang",
> %%  published by The Pragmatic Bookshelf.
> %%  Copyrights apply to this code. It may not be used to create training materia
> l,
> %%  courses, books, articles, and the like. Contact us if you are in doubt.
> %%  We make no guarantees that this code is fit for any purpose.
> %%  Visit http://www.pragmaticprogrammer.com/titles/jaerlang for more book infor
> mation.
> %%---
> -module(b).
> -export([x/0]).
>
> x() -> 1.
>
>
> Then I change b.erl from x() -> 1 to x() -> 2, erlc b.erl
>
> The code is not dynamically loaded.
>
> 2. If I change module a, even under the same erlang shell, the new
> code is still not loaded.
>
> kaiduanx@REDACTED:~/joebook/code$ more a.erl
> %% ---
> %%  Excerpted from "Programming Erlang",
> %%  published by The Pragmatic Bookshelf.
> %%  Copyrights apply to this code. It may not be used to create training materia
> l,
> %%  courses, books, articles, and the like. Contact us if you are in doubt.
> %%  We make no guarantees that this code is fit for any purpose.
> %%  Visit http://www.pragmaticprogrammer.com/titles/jaerlang for more book infor
> mation.
> %%---
> -module(a).
> -compile(export_all).
>
>  start(Tag) ->
>    spawn(fun() -> loop(Tag) end).
>
> loop(Tag) ->
>    sleep(),
>    Val = b:x(),
>    io:format("Vsn1 (~p) b:x() = ~p~n",[Tag, Val]),
>    loop(Tag).
>
> sleep() ->
>    receive
>        after 3000 -> true
>    end.
>
> After changing to io:format("Vsn2  (~p) b:x() = ~p~n",[Tag, Val]),
> even I compile it under the same erlang shell, the new code is still
> not loaded.
>
> 4> c(a).
> {ok,a}
> Vsn1 (one) b:x() = 1
> Vsn1 (one) b:x() = 1
> Vsn1 (one) b:x() = 1
> Vsn1 (one) b:x() = 1
>
> Can anyone explain this? Why changing b works but changing a does not work?
>
> Thanks,
>
> kaiduan
>
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