Typechecker plugins: request for review and another workflow question

[Eric Seidel]

Indeed, I spoke too soon. The type-nat plugin in fact causes an infinite loop when run on the unflattened constraints for one of our examples..

I think it would be reasonable for TcM to track how many times it has called the plugins and disable them after N iterations. But I don't know what would be a sensible default for N :/


> On Nov 14, 2014, at 10:50, Iavor Diatchki <iavor.diatchki at gmail.com> wrote:
> 
> Hi,
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> depending on what the plugin does, it is not too hard to get into a loop.  For example, the plugin could keep generating things like:  x > 5, x + 1 > 5, x + 2 > 5, x + 3 > 5.   Of course, this is not a good plan, but one has to be careful to avoid doing it accidentally.   I think that a good general guideline for plugin behavior is like this:
> 
>    - Avoid generating new variables
>    - Only generate constrains of the form: `x = y` or `x = K`: these always make things more defined and could help another solver to make progress
>    - These two ensure that eventually you will run out of new things to generate.
>    - It is occasionally useful to also generate more complex equations:  (e.g. x = y + 1), but then one needs to be careful to ensure progress.
> 
> Despite this, I also don't think that plugins need to be told how many times they've been called: it seems hard for a plugin author to know what to do with this number.  Besides, a buggy plugin could also get stuck in loops in other ways (e.g., get stuck in some internal loop, without going back to GHC).
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> -Iavor
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> On Fri, Nov 14, 2014 at 10:09 AM, Eric Seidel <eric at seidel.io> wrote:
> 
> > On Nov 14, 2014, at 09:14, Adam Gundry <adam at well-typed.com> wrote:
> >
> > Thanks, Simon! I've been convinced that TcS is more than we need, and I
> > think the right thing to do is to (optionally) invoke the plugin after
> > the constraints have been unflattened anyway. I've just pushed a commit
> > to wip/tc-plugins-amg that does this. Iavor, Eric, your views on how
> > convenient this alternative is would be most welcome. I'm also wondering
> > if the plugin should be told how many times it has been called, to make
> > it easier to prevent infinite loops.
> 
> Thanks, I was just sitting down to figure out how to call the plugins
> with the unflattened constraints. Is there really a point though in
> calling the plugins with flattened and unflattened versions of the same
> constraints? Perhaps we should just settle on one or the other? I think
> Iavor and I are fine with only passing unflattened constraints to the
> plugins. It means a bit more work for the type-nat plugin, but shouldn't
> be a huge issue (I'll confirm this before next week).
> 
> I don't think the plugins should be told how often they're being called,
> I'd prefer for GHC to track that and break the loop, if anything. I'm
> generally unconvinced that tracking the invocations is necessary at all
> though. The only way I can see an infinite loop occurring is if the plugins
> keep emitting *new* facts. Disregarding the possibility of a malicious
> plugin, that should mean that we're making progress towards a solution,
> right?
> 
> Eric
>