Why TcLclEnv and DsGblEnv need to store the same IORef for errors?

Wed Mar 31 06:05:23 UTC 2021

Morning all,

*Richard*: sorry! Unfortunately MR !4798 is the cornerstone of this
refactoring work but it's also gargantuan. Let's discuss a plan to attack
it, but fundamentally there is a critical mass of changes that needs to
happen atomically or it wouldn't make much sense, and alas this doesn't
play in our favour when it comes to MR size and ease of review. However, to
quickly reply to your remak: currently (for the sake of the
"minimum-viable-product") I am trying to stabilise the external interfaces,
by which I mean giving functions their final type signature while I do
what's easiest to make things typecheck. In this phase what I think is the
easiest is to wrap the majority of diagnostics into the `xxUnknownxx`
constructor, and change them gradually later. A fair warning, though: you
say "I would think that a DsMessage would later be wrapped in an envelope."
This might be true for Ds messages (didn't actually invest any brain cycles
to check that) but in general we have to turn a message into an envelope as
soon as we have a chance to do so, because we need to grab the `SrcSpan`
and the `DynFlags` *at the point of creation* of the diagnostics. Carrying
around a message and make it bubble up at some random point won't be a good
plan (even for Ds messages). Having said that, I clearly have very little
knowledge about this area of GHC, so feel free to disagree :)

*John*: Although it's a bit hard to predict how well this is going to
evolve, my current embedding, to refresh everyone's memory, is the
following:

data DsMessage =

    DsUnknownMessage !DiagnosticMessage

  -- ^ Stop-gap constructor to ease the migration.

  | DsLiftedTcRnMessage !TcRnMessage

  -- ^ A diagnostic coming straight from the Typecheck-renamer.

  -- More messages added in the future, of course

At first I thought this was the wrong way around, due to Simon's comment,
but this actually creates pleasant external interfaces. To give you a bunch
of examples from MR !4798:

deSugar :: HscEnv -> ModLocation -> TcGblEnv -> IO (Messages DsMessage,
Maybe ModGuts)
deSugarExpr :: HscEnv -> LHsExpr GhcTc -> IO (Messages DsMessage, Maybe
CoreExpr)

Note something interesting: the second function actually calls
`runTcInteractive` inside the body, but thanks to the `DsLiftedTcRnMessage`
we can still expose to the consumer an opaque `DsMessage` , which is what I
would expect to see from a function called "deSugarExpr". Conversely, I
would be puzzled to find those functions returning a `TcRnDsMessage`.

Having said all of that, I am not advocating this design is "the best". I
am sure we will iterate on it. I am just reporting that even this baseline
seems to be decent from an API perspective :)

On Wed, 31 Mar 2021 at 05:45, John Ericson <john.ericson at obsidian.systems>
wrote:

> Alfredo also replied to this pointing his embedding plan. I also prefer
> that, because I really wish TH didn't smear together the phases so much.
> Moreover, I hope with
>
>  - GHC proposals https://github.com/ghc-proposals/ghc-proposals/pull/412
> / https://github.com/ghc-proposals/ghc-proposals/pull/243
>
>  - The parallelism work currently be planned in
> https://gitlab.haskell.org/ghc/ghc/-/wikis/Plan-for-increased-parallelism-and-more-detailed-intermediate-output
>
> we might actually have an opportunity/extra motivation to do that. Splices
> and quotes will still induce intricate inter-phase dependencies, but I hope
> that could be mediated by the driver rather than just baked into each phase.
>
> (One final step would be the "stuck macros" technique of
> https://www.youtube.com/watch?v=nUvKoG_V_U0 /
> https://github.com/gelisam/klister, where TH splices would be able to
> making "blocking queries" of the the compiler in ways that induce more of
> these fine-grained dependencies.)
>
> Anyways, while we could also do a "RnTsDsError" and split later, I hope
> Alfredo's alternative of embedding won't be too much harder and prepare us
> for these exciting areas of exploration.
>
> John
> On 3/30/21 10:14 AM, Richard Eisenberg wrote:
>
>
>
> On Mar 30, 2021, at 4:57 AM, Alfredo Di Napoli <alfredo.dinapoli at gmail.com>
> wrote:
>
> I'll explore the idea of adding a second IORef.
>
>
> Renaming/type-checking is already mutually recursive. (The renamer must
> call the type-checker in order to rename -- that is, evaluate -- untyped
> splices. I actually can't recall why the type-checker needs to call the
> renamer.) So we will have a TcRnError. Now we see that the desugarer ends
> up mixed in, too. We could proceed how Alfredo suggests, by adding a second
> IORef. Or we could just make TcRnDsError (maybe renaming that).
>
> What's the disadvantage? Clients will have to potentially know about all
> the different error forms with either approach (that is, using my combined
> type or using multiple IORefs). The big advantage to separating is maybe
> module dependencies? But my guess is that the dependencies won't be an
> issue here, due to the fact that these components are already leaning on
> each other. Maybe the advantage is just in having smaller types? Maybe.
>
> I don't have a great sense as to what to do here, but I would want a clear
> reason that e.g. the TcRn monad would have two IORefs, while other monads
> will work with GhcMessage (instead of a whole bunch of IORefs).
>
> Richard
>
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