The relevant ticket is - https://ghc.haskell.org/trac/ghc/ticket/12203
On Wed, Aug 24, 2016 at 11:11 PM, David Feuer <david.fe...@gmail.com> wrote: > I'm trying to write a bidirectional pattern synonym which is morally > simple, but as far as I can tell there's no way to write it as a > simple bidirectional pattern synonym. > > What I want to say is > > pattern TSnoc (TSnocList xs) x = TSnocList (CL.TCons (Dual x) xs) > > (where TSnocList and Dual are both newtype constructors) > > But I don't see a way to do it without being explicitly bidirectional: > > pattern TSnoc xs x <- (tsViewR -> ViewR xs x) where > TSnoc (TSnocList xs) x = TSnocList (CL.TCons (Dual x) xs) > > tsViewR :: TSnocList c x y -> ViewR c (TSnocList c) x y > tsViewR (TSnocList CL.TNil) = EmptyR > tsViewR (TSnocList (CL.TCons (Dual x) xs)) = ViewR (TSnocList xs) x > > Would it be possible to make this simple thing simple? > _______________________________________________ > 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
