On 06/11/2009, at 09:50, Henning Thielemann wrote:
Actually in stream-fusion:Data.Stream I found such a class, called Unlifted. How about moving this into a separate package?
class Unlifted a where
-- | This expose function needs to be called in folds/loops that consume -- streams to expose the structure of the stream state to the simplifier -- In particular, to SpecConstr. -- expose :: a -> b -> b expose = seq
-- | This makes GHC's optimiser happier; it sometimes produces really bad -- code for single-method dictionaries -- unlifted_dummy :: a unlifted_dummy = error "unlifted_dummy"
This class serves as a vehicle for performing certain black magic rituals which (usually) help SpecConstr destroy evil artifacts introduced by stream fusion. In particular, expose is *not* seq; it is essentially deepSeq. As to single-method dictionaries, that problem existed two years or so ago. I don't think it still happens today. Roman