[Haskell-cafe] Deprecation cycle for classes

Tom Ellis tom-lists-haskell-cafe-2017 at jaguarpaw.co.uk
Sun Jun 16 07:02:20 UTC 2019

On Sat, Jun 15, 2019 at 11:51:33PM -0400, Viktor Dukhovni wrote:
> > On Jun 15, 2019, at 12:20 PM, Tom Ellis <tom-lists-haskell-cafe-2017 at jaguarpaw.co.uk> wrote:
> > 
> > I am unsure how to change the name of a class in a library whilst providing
> > a safe deprecation cycle for the users of the library.  Suppose I have
> > version 1 of my library with a class
> Is this perhaps essentially:
>    https://gitlab.haskell.org/ghc/ghc/issues/7543

It's closely related and indeed option 2 that Edward Yang proposes below
would allow me to do what I want:

    "Allow instance declarations on constraint synonyms, but only if after
    desugaring the synonym, you end up with a single class head."


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