Re: seq as type class method

|    -- | This makes GHC's optimiser happier; it sometimes produces really bad |    -- code for single-method dictionaries |    -- |    unlifted_dummy :: a |    unlifted_dummy = error "unlifted_dummy"

If you have such cases, please please boil it down and file it as a bug.  We should get *better* code not worse for single-method dictionaries.

I benchmarked this when doing the Uniplate work - 1-member classes gave a noticeable performance benefit over 2 or more members. I think it was 6% or so in my setup, and as a result Uniplate has a single member type class. Thanks, Neil