It compiles to lift f d = f (d `cast` blah) which seems fine to me. Are you unhappy with that? Simon From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users-bounces@haskell.org] On Behalf Of Louis Wasserman Sent: 09 July 2010 03:30 To: glasgow-haskell-users@haskell.org Subject: Casting + eta reduction Consider newtype Foo = Foo Int lift :: (Int -> a) -> Foo -> a lift f (Foo x) = f x Now, I'd expect this to compile with -O2 down to something like lift f = f `cast` (Foo -> a) but it doesn't. It seems that GeneralizedNewtypeDeriving assumes that these two things *are* equivalent, and it just directly casts the class dictionary. The implication would be that that GeneralizedNewtypeDeriving gives more efficient instances than you could *possibly* get if you wrote them by hand, which is very sad. Louis Wasserman wasserman.louis@gmail.commailto:wasserman.louis@gmail.com http://profiles.google.com/wasserman.louis