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

Darach Ennis <>
Fri Feb 28 00:34:42 CET 2014

Hi Jesper, all,

Feature flags would be ideal if/when breaking changes are required if only
to resolve issues with having to relax warnings-as-errors which, as you say,
is not good for those of us who prefer to be warning free.

But, feature flags introduce a maintenance concern if flags hang around
for too many releases. They should give offer time enough for code being
actively maintained to catch up, whilst being short-lived enough that the
language can evolve.

One of the first behavioural changes I quickly adopted when moving from
casual to daily erlang use was to enable all warnings and treat warnings as



On Thu, Feb 27, 2014 at 11:06 PM, Jesper Louis Andersen <
> 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
> http://erlang.org/mailman/listinfo/erlang-questions
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