Typechecker plugins: request for review and another workflow question

[Simon Peyton Jones]

Iavor, Adam, Eric

I’m happy with the general direction of the plugins stuff, so I’m mostly going to leave it to you guys to plough ahead; but I am happy to respond to questions.

 * I still think it would be better to provide an escape hatch to the
TcS, not merely the TcM, alongside the nice TcPluginM wrapper. Notably,
Simon's new TcFlatten.unflatten needs TcS...

It think the only reason for this is that ‘unflatten’ needs to set evidence bindings, which in turn requires access to tcs_ev_binds.  I think that everything else is in TcM.  So I suppose you could carry around the EvBindsVar if you really didn’t want TcS.  (And I can see why you wouldn’t; TcS has a lot of stuff you don’t need.)

Simon


From: Iavor Diatchki [mailto:iavor.diatchki at gmail.com]
Sent: 10 November 2014 19:15
To: Adam Gundry
Cc: ghc-devs at haskell.org; Simon Peyton Jones
Subject: Re: Typechecker plugins: request for review and another workflow question

Hi,

On Mon, Nov 10, 2014 at 1:31 AM, Adam Gundry <adam at well-typed.com<mailto:adam at well-typed.com>> wrote:

On the subject of that "nearly", I'm interested to learn whether you
have a suggestion to deal with unflattening, because the interface still
works with flat constraints only. Simon's changes should make it more
practical to unflatten inside the plugin, but it would be far easier (at
least for my purposes) if it was simply given unflattened constraints. I
realise that this would require the plugin loop to be pushed further
out, however, which has other difficulties.

Not sure what to do about this.  With the current setup, I think either way, the plugin would have to do some extract work.   Perhaps we should run the plugins on the unflattened constraints, and leave to the plugins to manually temporarily "flatten" terms from external theories?  For example, if the type-nat plugin saw `2 * F a ~ 4`, it could temporarily work with `2 * x ~ 4`, and then when it figures out that `x ~ 2`, it could emit `F a ~ 2` (non-canonical), which would go around again, and hopefully get fully simplified.



A few other issues, of lesser importance:

 * I still think it would be better to provide an escape hatch to the
TcS, not merely the TcM, alongside the nice TcPluginM wrapper. Notably,
Simon's new TcFlatten.unflatten needs TcS...
I don't mind that but, if I recall correctly, to do this without more recursive modules, we had to split `TCSMonad` in two parts, one with the types, and one with other stuff.  Eric and I did this once, but we didn't commit it, because it seemed like an orthogonal change.


 * Is there a way for my plugin to "solve" a given constraint (e.g. to
discard the uninformative "a * 1 ~ a")?
Yes, you'd say something like: `TcPluginOK [(evidence, "a * 1 ~ a")] []`

The first field of `TcPluginOK` are things that are solved, the second one is new work.

 * It is unfortunately easy to create infinite loops by writing plugins
that emit wanteds but make no useful progress. Perhaps there should be a
limit on the number of times round the loop (like SubGoalDepth but for
all constraints)?

Indeed, if a plugin keeps generating new work, we could go in a loop, so maybe a limit of some sort is useful.  However, note that if the plugin generates things that are already in the inert set, GHC should notice this and filter them, so we won't keep going.


 * Perhaps runTcPlugin should skip invoking the plugin if there are no
wanteds?

Oh, there is an important detail here that needs documentation!   GHC will call the plugin twice: once to improve the givens, and once to solve the wanteds.   The way to distinguish the two is exactly by the presence of the wanteds.

Why might you want to improve the givens?  Suppose you had something like `x * 2 ~ 4` as a given:  then you'd really want the plugin to generate another given: `x ~ 2`, as this is likely to help the rest of the constraint solving process.


 * The use of ctev_evar in runTcPlugin is partial, and fails with a
nasty error if the plugin returns a non-wanted in the solved constraints
list. Would it be worth catching this error and issuing a sensible
message that chastises the plugin author appropriately?

Aha, good idea. Bad plugin! :-)


 * Finally, I presume the comment on runTcPlugin that "The plugin is
provided only with CTyEq and CFunEq constraints" is simply outdated and
should be removed?
Yep, thanks!

Apologies for the deluge of questions - please take them as evidence of
my eagerness to use this feature!

Thanks for your feedback!  Also, if you feel like doing some hacking please do so---I am quite busy at the moment so I don't  have a ton of time to work on it, so any help you be most appreciated.  I know Eric is also quite keen on helping out so we can just coordinate over e-mail.

-Iavor




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