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?