Using a development snapshot of happy

[Ben Gamari]

Vladislav Zavialov <vladislav at serokell.io> writes:

> Hi ghc-devs,
>
> I’m working on the unification of parsers for terms and types, and one
> of the things I’d really like to make use of is a feature I
> implemented in ‘happy’ in October 2019 (9 months ago):
>
>   https://github.com/simonmar/happy/pull/153
>
> It’s been merged upstream, but there has been no release of ‘happy’,
> despite repeated requests:
>
>   1. I asked for a release in December: https://github.com/simonmar/happy/issues/164
>   2. Ben asked for a release a month ago: https://github.com/simonmar/happy/issues/168
>
> I see two solutions here:
>
>   a) Find a co-maintainer for ‘happy’ who could make releases more
>   frequently (I understand the current maintainers probably don’t have
>   the time to do it).
>   b) Use a development snapshot of ‘happy’ in GHC
>
> Maybe we need to do both, but one reason I’d like to see (b) in
> particular happen is that I can imagine introducing more features to
> ‘happy’ for use in GHC, and it’d be nice not to wait for a release
> every time. For instance, there are some changes I’d like to make to
> happy/alex in order to implement #17750
>
> So here are two questions I have:
>
>   1. Are there any objections to this idea?

I'm not entirely keen on the idea: while the cost of the submodule
itself is pretty low (happy is a small package which takes little time
to build), I am skeptical of addressing social issues like happy's lack
of maintenance with technical solutions. Ultimately, shipping happy as a
submodule would merely kick the current problem down the road:
eventually (when we release GHC) we will need a happy release. Unless
the underlying maintainership problem is addressed we will end up right
back where we are today.

Moreover, note that happy requires happy as a build dependency so we won't be
able to drop it as a build dependency of GHC even if we do include it as
a submodule.

For this reason, I would weakly prefer that we first find a maintainer
and try to get a release out before jumping straight to adding happy as
a submodule. I will try to bring up the matter with Simon Marlow to see
if we can't find a solution here.

>   2. If not, could someone more familiar with the build process guide
>   me as to how this should be implemented? Do I add ‘happy’ as a
>   submodule and change something in the ./configure script, or is
>   there more to it? Do I need to modify make/hadrian, and if so, then
>   how?
>
It will be a tad more involved than this. We will need to teach the
build systems to build Happy, use the configure executable, and update
the source distribution packaging rules to include the new submodule.
Moreover, happy (unfortunately) has a make-based build system which will
need to be used to generate its parser.

Updating the build systems likely won't be difficult, but there isn't clear
documentation on what it will involve. This will really be a matter of
finding a similar existing case (e.g. genprimops, perhaps?), following
it as a model, and figuring out how to fill any gaps.

Moreover, build system logic is inevitably a bug-nest; adding the same
logic twice greatly increases the chance that we will have (two sets of)
bugs that will only be caught post-release. For me, this underscores the
need to first try to make the existing decoupled situation work before
moving to a vendored solution.

Cheers,

- Ben
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