I filed a ticket for precisely this: https://ghc.haskell.org/trac/ghc/ticket/12203
Edward Excerpts from David Feuer's message of 2016-08-24 18:11:16 -0400: > 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