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

[Vlad Dumitrescu]

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 <
anthonym@REDACTED> 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 <
> jesper.louis.andersen@REDACTED> 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.
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