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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:36.0pt">
1. What prevents us from adding `NamedFieldPuns` as a proper constructor for the `Extension` type and in principle remove `RecordPuns`? Backward compatibility I assume?<o:p></o:p></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">You mean, essentially, rename `LangExt.RecordPuns` to `NamedFieldPuns`.<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’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!<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>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><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> 06 July 2021 10:14<br>
<b>To:</b> Simon Peyton Jones via ghc-devs <ghc-devs@haskell.org><br>
<b>Subject:</b> Can NamedFieldPuns be added to `GHC.LanguageExtensions.Types.Extension`?<o:p></o:p></span></p>
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Dear all,<o:p></o:p></p>
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<o:p> </o:p></p>
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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.<o:p></o:p></p>
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<o:p> </o:p></p>
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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.<o:p></o:p></p>
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<o:p> </o:p></p>
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Today I was working on turning this error into a proper Haskell type:<o:p></o:p></p>
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<o:p> </o:p></p>
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badPun :: Located RdrName -> TcRnMessage<o:p></o:p></p>
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badPun fld = TcRnUnknownMessage $ mkPlainError noHints $<o:p></o:p></p>
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vcat [text "Illegal use of punning for field" <+> quotes (ppr fld),<o:p></o:p></p>
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text "Use NamedFieldPuns to permit this"]<o:p></o:p></p>
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<o:p> </o:p></p>
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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`:<o:p></o:p></p>
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<o:p> </o:p></p>
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...<o:p></o:p></p>
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depFlagSpec' "RecordPuns" LangExt.RecordPuns<o:p></o:p></p>
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(deprecatedForExtension "NamedFieldPuns"),<o:p></o:p></p>
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...<o:p></o:p></p>
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flagSpec "NamedFieldPuns" LangExt.RecordPuns,<o:p></o:p></p>
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...<o:p></o:p></p>
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<o:p> </o:p></p>
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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 <o:p></o:p></p>
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"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).<o:p></o:p></p>
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<o:p> </o:p></p>
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I am not sure this behaviour is unique for just `NamedFieldPuns`, but my question is:<o:p></o:p></p>
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<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">
1. What prevents us from adding `NamedFieldPuns` as a proper constructor for the `Extension` type and in principle remove `RecordPuns`? Backward compatibility I assume?<o:p></o:p></p>
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<o:p> </o:p></p>
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<o:p></o:p></p>
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Many thanks,<o:p></o:p></p>
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<o:p> </o:p></p>
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Alfredo<o:p></o:p></p>
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<o:p> </o:p></p>
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<o:p> </o:p></p>
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<o:p> </o:p></p>
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<o:p> </o:p></p>
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<o:p> </o:p></p>
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<o:p> </o:p></p>
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<o:p> </o:p></p>
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