Hi, 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). -Iavor On Fri, Nov 14, 2014 at 10:09 AM, Eric Seidel <e...@seidel.io> wrote: > > > On Nov 14, 2014, at 09:14, Adam Gundry <a...@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 >
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