On Mon, 2009-11-09 at 12:19 +0000, Henning Thielemann wrote:
On Mon, 9 Nov 2009, Neil Mitchell wrote:
| -- | 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.
I assume the case in stream-fusion:Data.Stream was vice versa: A one member class seemed to make performance worse .Thus they added a dummy method.
I believe Roman reported the problem at the time and I think that it got fixed. Duncan