seq as type class method

[Roman Leshchinskiy]

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