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<p class="MsoNormal"><span style="mso-fareast-language:EN-US">I think the main reason is that for Template Haskell the renamer/type-checker need to run the desugarer. See the call to initDsTc in GHC.Tc.Gen.Splice.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">I suppose an alternative is that the TcGblEnv could have a second IORef to use for error messages that come from desugaring during TH splices.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Nothing deeper than that I think.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Simon<o:p></o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> ghc-devs <ghc-devs-bounces@haskell.org>
<b>On Behalf Of </b>Alfredo Di Napoli<br>
<b>Sent:</b> 30 March 2021 08:42<br>
<b>To:</b> Simon Peyton Jones via ghc-devs <ghc-devs@haskell.org><br>
<b>Subject:</b> Why TcLclEnv and DsGblEnv need to store the same IORef for errors?<o:p></o:p></span></p>
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Hello folks,<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
as some of you might know me and Richard are reworking how GHC constructs, emits and deals with errors and warnings (See <a href="https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.haskell.org%2Fghc%2Fghc%2F-%2Fwikis%2FErrors-as-(structured)-values&data=04%7C01%7Csimonpj%40microsoft.com%7C49c033aa2865495eb07c08d8f34f70cd%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637526870280012102%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=ui4JWOp1gl5Yh%2FOYDqcBLXxTm%2FGnQRi0cDshUVEjwmk%3D&reserved=0">https://gitlab.haskell.org/ghc/ghc/-/wikis/Errors-as-(structured)-values</a>
and #18516).<o:p></o:p></p>
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To summarise very briefly the spirit, we will have (eventually) proper domain-specific types instead of SDocs. The idea is to have very precise and "focused" types for the different phases of the compilation pipeline, and a "catch-all" monomorphic `GhcMessage`
type used for the final pretty-printing and exception-throwing:<o:p></o:p></p>
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data GhcMessage where<o:p></o:p></p>
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GhcPsMessage :: PsMessage -> GhcMessage<o:p></o:p></p>
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GhcTcRnMessage :: TcRnMessage -> GhcMessage<o:p></o:p></p>
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GhcDsMessage :: DsMessage -> GhcMessage<o:p></o:p></p>
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GhcDriverMessage :: DriverMessage -> GhcMessage<o:p></o:p></p>
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GhcUnknownMessage :: forall a. (Diagnostic a, Typeable a) => a -> GhcMessage<o:p></o:p></p>
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While starting to refactor GHC to use these types, I have stepped into something bizarre: the `DsGblEnv` and `TcLclEnv` envs both share the same `IORef` to store the diagnostics (i.e. warnings and errors) accumulated during compilation. More specifically, a
function like `GHC.HsToCore.Monad.mkDsEnvsFromTcGbl` simply receives as input the `IORef` coming straight from the `TcLclEnv`, and stores it into the `DsGblEnv`.<o:p></o:p></p>
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This is unfortunate, because it would force me to change the type of this `IORef` to be<o:p></o:p></p>
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`IORef (Messages GhcMessage)` to accommodate both diagnostic types, but this would bubble up into top-level functions like `initTc`, which would now return a `Messages GhcMessage`. This is once again unfortunate, because is "premature": ideally it might still
be nice to return `Messages TcRnMessage`, so that GHC API users could get a very precise diagnostic type rather than the bag `GhcMessage` is. It also violates an implicit contract: we are saying that `initTc` might return (potentially) *any* GHC diagnostic
message (including, for example, driver errors/warnings), which I think is misleading.<o:p></o:p></p>
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Having said all of that, it's also possible that returning `Messages GhcMessage` is totally fine here and we don't need to be able to do this fine-grained distinction for the GHC API functions. Regardless, I would like to ask the audience:<o:p></o:p></p>
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* Why `TcLclEnv` and `DsGblEnv` need to share the same IORef?<o:p></o:p></p>
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* Is this for efficiency reasons?<o:p></o:p></p>
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* Is this because we need the two monads to independently accumulate errors into the<o:p></o:p></p>
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same IORef?<o:p></o:p></p>
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Thanks!<o:p></o:p></p>
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Alfredo<o:p></o:p></p>
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