There's always pattern synonyms as an option for cases like this, free of backwards compat issues.
-Edward On Tue, Jul 6, 2021 at 3:00 AM Alfredo Di Napoli <alfredo.dinap...@gmail.com> wrote: > > Hello Simon, > > Yes, renaming and perhaps keeping `RecordPuns` as a pattern synonym to not > break backward-compat, if that's feasible to define as we are in > `ghc-boot-th` here. Not sure if `PatternSynonyms` and `COMPLETE` would be > available there. > > I am not sure how many libs that depend on the ghc API would break (I > haven't grepped on Hackage yet), but that might tip the benefits/troubles > ratio towards keeping the status quo. > > This is not a "problem" I have to solve today, and it might not be > considered a problem by others (just an inconsistency I guess): as a > colleague of mine pointed out, GHC is not necessarily "lying" here. It's > still the same underlying extension, it just happens that there are two > names that refer to it. > > Perhaps I could think about adding to `GhcHint` some kind of mapping which > would give to IDEs or third-party libs the correct extension name given an > input `LangExt.Extension`, the problem then becomes making sure that we > keep this mapping in sync with the information contained in > `GHC.Driver.Session`. > > I will let it simmer. > > Thanks! > > A. > > On Tue, 6 Jul 2021 at 11:19, Simon Peyton Jones <simo...@microsoft.com> > wrote: > >> 1. What prevents us from adding `NamedFieldPuns` as a proper constructor >> for the `Extension` type and in principle remove `RecordPuns`? Backward >> compatibility I assume? >> >> You mean, essentially, rename `LangExt.RecordPuns` to `NamedFieldPuns`. >> >> >> >> I’d be fine with that. There might be back-compat issues, but only with >> other plugins, and probably with vanishingly few of them. Grep in Hackage! >> >> >> >> Simon >> >> >> >> *From:* ghc-devs <ghc-devs-boun...@haskell.org> *On Behalf Of *Alfredo >> Di Napoli >> *Sent:* 06 July 2021 10:14 >> *To:* Simon Peyton Jones via ghc-devs <ghc-devs@haskell.org> >> *Subject:* Can NamedFieldPuns be added to >> `GHC.LanguageExtensions.Types.Extension`? >> >> >> >> Dear all, >> >> >> >> As some of you might know, for the past few months I have been working on >> changing GHC's diagnostic messages from plain SDocs to richer Haskell types. >> >> >> >> As part of this work, I have added a mechanism to embed hints into >> diagnostics, defined in `GHC.Types.Hint` in `HEAD`. One of the main >> workhorse of this `GhcHint` type is the `SuggestExtension >> LangExt.Extension` constructor, which embeds the extension to enable to use >> a particular feature. The `LangExt.Extension` type comes from >> `GHC.LanguageExtensions.Types`, and up until now there has always been a >> 1:1 mapping between the language pragma for the extension and the type >> itself. >> >> >> >> Today I was working on turning this error into a proper Haskell type: >> >> >> >> badPun :: Located RdrName -> TcRnMessage >> >> badPun fld = TcRnUnknownMessage $ mkPlainError noHints $ >> >> vcat [text "Illegal use of punning for field" <+> quotes (ppr fld), >> >> text "Use NamedFieldPuns to permit this"] >> >> >> >> I was ready to yield a `SuggestExtension LangExt.NamedFieldPuns` when I >> discovered that there is no `NamedFieldPuns` constructor. Rather, there is >> a `RecordPuns` , which refer to a deprecated flag, and we simply map >> `NamedFieldPuns` back to it in `GHC.Driver.Session`: >> >> >> >> ... >> >> depFlagSpec' "RecordPuns" LangExt.RecordPuns >> >> (deprecatedForExtension "NamedFieldPuns"), >> >> ... >> >> flagSpec "NamedFieldPuns" LangExt.RecordPuns, >> >> ... >> >> >> >> This is problematic for the `GhcHint` type, because now if I was to yield >> `SuggestExtension LangExt.RecordPuns` to the user, I could still >> pretty-print the suggestion to turn `RecordPuns` into `NamedFieldPuns`, but >> this means that IDEs or third-party library would have access to the >> >> "raw" Haskell datatype, and at that point they will be stuck with a >> suggestion to enable a deprecated extension! (or best case scenario they >> will have to transform the suggestion into something more sensible, which >> partially defeats the point of this refactoring work I have been doing). >> >> >> >> I am not sure this behaviour is unique for just `NamedFieldPuns`, but my >> question is: >> >> >> >> 1. What prevents us from adding `NamedFieldPuns` as a proper constructor >> for the `Extension` type and in principle remove `RecordPuns`? Backward >> compatibility I assume? >> >> >> >> >> >> Many thanks, >> >> >> >> Alfredo >> >> >> >> >> >> >> >> >> >> >> >> >> >> >> >> >> > _______________________________________________ > ghc-devs mailing list > ghc-devs@haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs >
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