[erlang-questions] Breaking backwards compatibility in Release 17.0-rc2
Loïc Hoguin
essen@REDACTED
Fri Feb 28 10:09:06 CET 2014
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.
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
> <anthonym@REDACTED <mailto: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
> <mailto: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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--
Loïc Hoguin
http://ninenines.eu
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