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-bounces@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