I agree that this kind of backward-compatibility pattern synonym is good and shouldn't be prefixed with PS_.
But do you have a concrete example of this leakage of an internal GHC type via TH? While I can imagine this happening, I don't know of any examples in practice. Note that even enumeration types (like Role) have separate TH counterparts. Richard > On Dec 29, 2021, at 6:12 PM, Viktor Dukhovni <ietf-d...@dukhovni.org> wrote: > > Some "GHC-internal" types leak to users via TH, and their constructors > occasionally pick up new fields, causing breakage downstream. The extra > field often has a sensible default (Nothing, [], ...) and it should be > best practice to rename the constructor when adding the new field, while > replacing the original constructor with a pattern synonym with the "old" > signature. > > data Foo = ... > | NewImprovedMkFoo X Y Z -- was MkFoo Y Z > > pattern MkFoo :: Foo > pattern MkFoo Y Z = NewImprovedMkFoo Nothing Y Z > > When pattern synonyms are used to maintain a backwards-compatible API, > there should of course be no special signalling to differentiate them > from "real" constructors. > > The boundary between "GHC-internal" and external may not always be > obvious, some care is required to reduce leaking breakage via TH. > > -- > Viktor. > _______________________________________________ > ghc-devs mailing list > ghc-devs@haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
