as at hacks.yi.org
Sat Jan 22 00:59:32 CET 2011
On Fri, Jan 21, 2011 at 3:13 AM, Simon Peyton-Jones
<simonpj at microsoft.com> wrote:
> | So, given that 7.2 will be released much earlier than the normal
> | release cycle, is there any room for anything else to get into HEAD
> | for the 7.2 release before everything is switched? In particular I
> | fixed up Max Bolingbroke's old compiler plugin work to be usable with
> | the latest HEAD, and all the fundamental work is there and done, just
> | some additional small things are needed (notably having ghc dump
> | plugin information a la -ddump flags, and testsuite patches are about
> | it I think.) The patch itself is pretty small and doesn't touch *too*
> | much code, mostly adding dynamic loading and the plugin API, but it's
> | arguably adding a 'big' feature for users of GHC to start utilizing,
> | and perhaps a release in 7.2 would cause problems merging changes
> | until you cut a new STABLE branch with git, like you said.
> I'm sorry I've been slow on this. "Review and apply the plugins patch" is in my inbox, but it's been queued up behind too many other things, notably making the new typechecker work.
That's fine! One thing at a time. The typechecker overhaul is great
IMO, and I've noticed it's fixed many strange GHC bugs relating to
type system feature interactions (including a few I hit.) Much
> I'm pretty keen on the whole plugin idea, because it makes the compiler more extensible and lowers the barrier to entry. My only reason for delay is that I wanted to review the design (as seen by a plug-in author). Once we provide it, we have to support it, and it's harder to change.
This is very reasonable and I was going to mention it once someone
responded. Up to this point, Max has really been the main person to
write any plugins for GHC using the current interface. I would really
welcome anybody interested in hacking on GHC look at the current API
in the patch - my main concern up to this point is that the actual
means of installing passes into the Core pipeline, the installation
function of type `[CoreToDo] -> CoreM [CoreToDo]`, is a little
Scala's approach seems to be to have plugins specify 'runAfter' and
'runBefore' constraints, which specify what compiler phases should run
before and after the plugin's pass in question (and certain phases may
run multiple times, so as a consequence, so does your plugin.) If I
remember correctly, this was somewhat similar to the original design
Max proposed which was to specify in which phase the pass is run, and
it essentially amounted to a phase constraint. Perhaps Max can
elaborate on why this design was rejected in favor of the current one,
so we can see how and where it falls down, and what we really want.
Thomas pointed out the Scala compiler plugin design document, so I'll
be sure to read over it this weekend when I get the chance to cook up
Also worth noting, like I said, is I would inevitably like to extend
compiler plugins to work on Cmm, once the new codegen (using hoopl)
hits. Maybe even allow you to write a new backend ultimately if you
wanted to, although I'd call that the last and ultimate challenge at
this point (because from my cursory glances, it would require quite a
bit of re-engineering and re-design of the driver and compilation
pipeline, much more so than just the Core plugins did or Cmm plugins
might. That's a bit more time than I have... at the moment ;)
To accumulate ideas and comments I've spent the past few days writing
up a wiki page (mostly written in the wee hours of the morning no
less), documenting the current API, how it might be improved, and
future work like C-- plugins or plugins implementing new backends.
Most of it is half baked. And half-written. Probably grammatical
errors and run-on sentences. Incomplete sections. Likely with big,
stupid ideas. Caveat emptor, etc.
The wiki page in all its (horrid, half-written and mangled) glory can
be located here:
If people can comment and add ideas, it will be much easier to
determine if this should go into 7.2.1. If the current API is too
fragile or needs to be expanded, it would likely be best to back out
and do some work, and try again for 7.4.1. That will also give a much
bigger window for feedback and testing, since 7.2 will be on a much
shorter cycle it seems.
I'll probably end up throwing up a RFC on both glasgow-haskell-users
and cvs-ghc sometime in the next week to get feedback after more
brainstorming, because it's the most important thing as of right now
and for any future work in this area, in my opinion, since it will be
a publicly exposed API for users to write code against.
> The fact that you are actively engaged, have done the work with Max, and are (I assume) happy to respond to user queries, fix bugs etc, is a major incentive. Thank you!
I'm more than happy to respond to any user queries and respond to bugs
people come up with, yes (Max seems to be working on lots of other
things at the moment, and I like this functionality and would be
willing to support it.)
> So yes, because of that I undertake to do this for 7.2 [unless Simon M tells me no :-)]. I'll add a few thoughts to the ticket right now. http://hackage.haskell.org/trac/ghc/ticket/3843. Interested parties, add yourselves to the cc list of the ticket.
Thanks for your time Simon.
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