[Haskell-cafe] Clearly, Haskell is ill-founded

Conor McBride ctm at Cs.Nott.AC.UK
Sun Jul 15 21:29:32 EDT 2007


Whoops! Only just spotted this. Many apologies.

On 10 Jul 2007, at 20:35, Creighton Hogg wrote:

> Me:

> > No, an
> > operating system is supposed to remain responsive. And that's what
> > total coprograms do.
> I'm sorry, but can you expand a little further on this?  I guess I  
> don't understand how a corecursion => responsive to input but not  
> terminating.  Where does the idea of waiting for input fit into  
> corecursion?

You'll be needing a bit of higher-order corecursion for that.
Here's a coprogram for haskell-cafe:

 > data{-codata-} Punter = Speak String (String -> Punter)

A Punter is guaranteed to ask a question, and whatever answer you give
them, they've always got another question, forever! Meanwhile, a
String -> Punter is a good-natured soul, always up for answering  
Hence haskell-cafe is a productive coprogram(*) producing a stream of  
and answers!

 > data{-codata-} Stream x = x :> (Stream x)

 > cafe :: Punter -> (String -> Punter) -> Stream (String, String)
 > cafe (Speak question learn) guru =
 >   let  Speak answer guru' = guru question
 >   in   (question, answer) :> (cafe (learn answer) guru')

All the best


(*) and yes, I know what that means in Greek...

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