Breaking Changes and Long Term Support Haskell

[Mario Blažević]

On 15-10-22 09:29 AM, Geoffrey Mainland wrote:
> ...
>
> 1) What is the master plan, and where is it documented, even if this
> document is not up to the standard of a proposal? What is the final
> target, and when might we expect it to be reached? What is in the
> pipeline after MRP?
>
> Relatedly, guidance on how to write code now so that it will be
> compatible with future changes helps mitigate the stability issue.

	I have been fully in favour of all the proposals implemented so far, 
and I think that having an explicit master plan would be a great idea. 
It would address some of the process-related objections that have been 
raised, and it would provide a fixed long-term target that would be much 
easier to make the whole community aware of and contribute to.

	For that purpose, the master plan should be advertised directly on the 
front page of haskell.org. Once we have it settled and agreed, the 
purpose of the base-library commitee would essentially become to figure 
out the details like the timeline and code migration path. One thing 
they wouldn't need to worry about is whether anybody disagrees with 
their goals.


> 2) How can I write code that makes use of the Prelude so that it will
> work with every new GHC release over the next 3 years? 5 years? For
> example, how can I write a Monad instance now, knowing the changes that
> are coming, so that the instance will work with every new GHC release
> for the next 3 years? 5 years? If the answer is "you can't," then when
> might I be able to do such a thing? As of 8.4? 8.6? I'm embarrassed to
> say I don't know the answer!

	From the discussions so far it appears that the answer for 3 years (or 
at least the next 3 GHC releases) would be to write the code that works 
with the current GHC and base, but this policy has not been codified 
anywhere yet. Knowing the upcoming changes doesn't help with making your 
code any more robust, and I think that's a shame. We could have a 
two-pronged policy:

- code that works and compiles with the latest GHC with no *warnings* 
will continue to work and compile with no *errors* with the following 2 
releases, and
- code that also follows the forward-compatibility recommendations 
current for that version of GHC will continue to work and compile with 
no *errors* with the following 4 releases.

	The forward-compatibility recommendations would become a part of the 
online GHC documentation so nobody complains they didn't know about 
them. Personally, I'd prefer if the recommendations were built into the 
compiler itself as a new class of warnings, but then (a) some people 
would insist on turning them on together with -Werror and then complain 
when their builds break and (b) this would increase the pressure on GHC 
implementors.


> Finally, if none of these changes broke Prelude backwards compatibility,
> far fewer people would be complaining :) Of course, we can't always make
> progress without breaking things, but a more deliberative process might
> offer an opportunity to make progress while still preserving backwards
> compatibility. Take AMP for example. There were at least two [3] [4]
> proposals for preserving backwards compatibility. Investigating them
> would have taken time and delayed AMP, yes, but why the rush?

	Because they have been investigated for years with no effect.


> 3) Can we have a process that allows more deliberation over, and wider
> publicity for, changes that break backwards compatibility? The goal of
> such a process would not be to prevent change, but to allow more time to
> find possible solution to the issue of backwards compatibility.

	I doubt we can, but this question has already been answered by others.