The problem is that GHC doesn't do eta expansion for product types. See #7259, which won't be fixed by 7.10, as I would imagine a fair amount of theory needs to be done for this to be valid.
Luckily, you can do eta expansion manually yourself: either by requiring that the caller know the eta-expanded form (1) or by storing that knowledge in a GADT (2): I was afraid that would be the answer. Unfortunately, I think I really need the non-eta-expanded form, because either of the eta forms you suggest break other things. The problem is that Twice and Dbl occur in some definitions that go by polymorphic recursion, an the eta-expanded
On 11/08/2014 07:30 PM, Richard Eisenberg wrote: forms don't seem to work. I'll see if I can get by without this... Cheers, Rob