Thanks Francesco - I’ll follow up the link. M

On 27 Jul 2017, at 11:08, Francesco Ariis wrote:

On Thu, Jul 27, 2017 at 09:33:33AM +0100, mike h wrote:

...

What I'd like to do is take one item at a time from the stream. My first shot was

next :: (Stream a) -> (Stream a, a)

but that means I need to keep a ref to the stream that is returned in the tuple which makes for messy code for subsequent calls to next. I sense that the State monad needs to be involved but I'm not sure exactly how as my experiments with State still required I keep a ref to the 'new' stream. Conceptually I see this as a 'global' state and next is just

next :: a

but that's 30 years of imperative programming speaking and is, I think, wrong!

Hello Mike,

I see nothing wrong with a State monad (you signature looks really like the one inside a State monad, after all)

next :: Stream a -> (a, Stream a) next' :: s -> (a, s )

Many many many other ways of dealing elegantly with streams have been proposed; check for example this article [1], which starts exactly from your example, e.g.

data Stream b = SCons (b, Stream b)

And then, if you are sold to the idea, exploring the various libraries on hackage is the other half of the fun :P -F

[1] https://blog.jle.im/entry/intro-to-machines-arrows-part-1-stream-and _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners