[Haskell-cafe] Sequence Classes

[Donald Bruce Stewart]

jcast:
> I was just meditating on array fusion using streams, and list fusion using 
> streams, and it struck me: the definition of the list functions over arrays 
> is the same as that of the list functions over lists.  From the ByteString 
> paper, we get:

Yes indeed, hence Data.Stream provides the common implemetations for
both arrays, lists and a few other fusible sequence types.
  
> map f = transformerBi (mapS f)
> foldr f z = consumerDn (foldrS f z)
> foldl f z = consumerUp (foldlS f z)
> replicate n = producerBi (replicateS n)
> 
> etc.
> 
> So why not say
> 
> class Sequence t alpha where
>   consumerUp, consumerDn, consumerBi
>     :: (Stream alpha -> beta) -> t alpha -> beta
>   producerUp, producerDn, producerBi :: Stream alpha -> t alpha
>   transformerUp, transformerDn, transformerBi
>     :: Sequence t beta => (Stream alpha -> Stream beta) -> t alpha -> t beta
> 
> map :: Sequence t alpha => (alpha -> beta) -> t alpha -> t beta
> map f = transformerBi (mapS f)
> 

You could do that, yep.

-- Don