[erlang-questions] Breaking backwards compatibility in Release 17.0-rc2

Vlad Dumitrescu <>
Fri Feb 28 10:45:34 CET 2014


On Fri, Feb 28, 2014 at 10:09 AM, Loïc Hoguin <> wrote:

> A macro for ?OTP_RELEASE or ?OTP_VERSION makes *absolutely no sense*.
> There are no guarantees that people are going to compile on a fresh Erlang
> release. If I create a release named PONIES, this will not help at all when
> I then use the compiler app I included to compile some stuff at runtime.
>
> Something a little better is ?COMPILER_VERSION, but people might also run
> a custom compiler code with a custom version number, so that's a no go
> either.
>
>
It would make sense for me, that's why I considered it. However, I agree
that my use case is special and is probably not enough to support this
addition by itself.

Since I have a patched debugger for erlide and I don't know which version
will users run their code on, I need to provide separate debuggers for all
supported versions and I am going to compile the code on fresh releases.
This isn't a problem until there are incompatibilities. I suppose that the
debugger is stable enough that I could maintain different patched variants
for the different versions, but it's still work that could be used for
other features/improvements/fixes.

regards,
Vlad


>
> On 02/28/2014 09:36 AM, Vlad Dumitrescu wrote:
>
>> Hi,
>>
>> I've been hit by this too, as I have a patched debugger that I need to
>> compile on older versions too and there are issues with
>> unicode/maps/named funs. Unfortunately, there might be cases where code
>> will still have to be duplicated, because macros can only wrap full forms.
>>
>>  From a brief look att epp.erl, it feels like adding a ?OTP_RELEASE or
>> ?OTP_VERSION predefined macro would be easy and the only possible
>> problem is if there are user-defined macros with the same name.
>>
>> predef_macros(File) ->
>>       Machine = list_to_atom(erlang:system_info(machine)),
>>       {ok, Release0} = file:read_file(code:root_dir()++"/OTP_VERSION"),
>>       Release = string:strip(Release0, right, $\n),
>>       ...
>> {{atom,'OTP_RELEASE'},      {none,[{string,1,Release}]}},
>>       ...
>>
>> By the way, wouldn't it be useful to have an erlang:system_info() that
>> reads the file and strips the 'ok' and the whitespace?
>>
>> best regards,
>> Vlad
>>
>>
>> On Fri, Feb 28, 2014 at 1:08 AM, ANTHONY MOLINARO
>> < <mailto:>> wrote:
>>
>>     I also have felt this pain with the transition from behaviour_info
>>     to callbacks for behaviours.  Ideally, the preprocessor would define
>>     a macro along the lines of ?MODULE, ?MODULE_STRING, ?FILE, ?LINE,
>>     and ?MACHINE which is the full list according to
>>     http://www.erlang.org/doc/reference_manual/macros.html.
>>
>>     If there was one additional macro call ?RELEASE with the major
>>     release, then it would be possible to conditionally compile at least
>>     dialyzer stuff (I don't know about the file encoding, I guess it
>>     would depend on whether the check is done during the preprocessor or
>>     at a later step).  This would probably prevent the proliferation of
>>     different compile macros which seem to crop up as every individual
>>     library adds their own based on a rebar or makefile check.
>>
>>     -Anthony
>>
>>     On Feb 27, 2014, at 3:06 PM, Jesper Louis Andersen
>>     <
>>     <mailto:>> wrote:
>>
>>      Release 17.0 brings two changes which prove to take some work
>>>     getting around.
>>>
>>>     1. utf-8 is now the default encoding.
>>>
>>>     This is a rather insignificant change. The source code which uses
>>>     latin1 can be fixed by one of three ways:
>>>
>>>     * Tell the compiler the file is latin1. This won't work going
>>>     forward but works now.
>>>     * Change the file to utf-8. This won't work going backward a long
>>>     way. But it will work going backwards for a bit.
>>>     * Change the file to ASCII. This works both backward and forward
>>>     as long as we want.
>>>
>>>     This is a benign problem. I have tried compiling some projects and
>>>     it turns out there are numerous repositories which needs fixing
>>>     now. But the fix is rather simple.
>>>
>>>     2. Dialyzer dislikes queue(), dict(), ...
>>>
>>>     Dialyzer now prefers using queue:queue() and the like. This is
>>>     *definitely* the right thing to support as it is much more
>>>     consistent with the rest of the system and doesn't treat certain
>>>     types as magically introduced types.
>>>
>>>     -module(z).
>>>
>>>     -export([f/1]).
>>>
>>>     -spec f(queue:queue()) -> queue:queue().
>>>     f(Q) -> queue:in(3, Q).
>>>
>>>     Which is nice, but this doesn't work on R16B03:
>>>
>>>     z.erl:5: referring to built-in type queue as a remote type; please
>>>     take out the module name
>>>     z.erl:5: referring to built-in type queue as a remote type; please
>>>     take out the module name
>>>
>>>     So here, I have no way of getting my source code to work with both
>>>     R16 and 17.0 easily. There is no transition period so-to-speak.
>>>     Many projects run with warnings-as-errors and they are in trouble:
>>>
>>>     * They can't compile
>>>     * They can remove the warnings-as-errors but this defeats the purpose
>>>     * They will have warnings spewed out over the console all the time
>>>
>>>     In the case of crypto:hash/2, we had somewhat the same situation.
>>>     Prominent projects like Yaws, and lesser projects like Emysql has
>>>     EPP macros in place as well as detection in order to figure out
>>>     what to do. Or you can disable the warnings in this case
>>>     specifically for this. But can I do the same with wrong type
>>>     specs? Also, this workaround is done in almost every project out
>>>     there, which is darn irritating.
>>>
>>>     I don't know what we need to solve this. At one point, I would
>>>     really like to have a set of feature flags
>>>
>>>     http://www.lispworks.com/documentation/HyperSpec/Body/v_featur.htm
>>>     , ZFS, ...
>>>
>>>     where you have a way to compile-time scrutinize what your
>>>     environment supports. Another way to solve it is the variant Go
>>>     uses, namely "build constraints"
>>>
>>>     http://golang.org/pkg/go/build/#pkg-overview
>>>
>>>     which will mention under which circumstances to include a file as
>>>     a part of an application. This would allow for easy handling of
>>>     crypto:hash/2, but I do note it will fail on the dialyzer problem.
>>>     It looks like the only sane way to solve that is to allow both
>>>     queue() and queue:queue() as aliases for a major release and then
>>>     proceed to remove queue().
>>>
>>>     Am I completely wrong here? I can accept languages evolve and that
>>>     Release 17 has maps which will be used and break a lot of software
>>>     for R16 quickly. But I also feel we should have some way of having
>>>     a process so there is a way to handle this gracefully going
>>>     forward. It is natural for libraries and languages to evolve and
>>>     break compatibility. Yet, it should be easy to handle for
>>>     programmers. There is much time wasted, which could be used better
>>>     were there a nice solution.
>>>
>>>     Thoughts?
>>>
>>>     --
>>>     J.
>>>     _______________________________________________
>>>     erlang-questions mailing list
>>>      <mailto:>
>>>     http://erlang.org/mailman/listinfo/erlang-questions
>>>
>>
>>
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> --
> Loïc Hoguin
> http://ninenines.eu
>
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