Lightweight pure functional monad-free type-safe coroutines.

> import Prelude hiding (read, return)

> read cont (i, is) = cont i is
> write o cont is = (x, (o, os)) where (x, os) = cont is
> return x is = (x, ())

Unlikely as it sounds, that code was motivated by
http://www.dpmms.cam.ac.uk/~martin/Research/Publications/2007/hp07.pdf

> chat alice bob = (alice_value, bob_value) where
>       (alice_value, alice_output) =  alice bob_output
>       (bob_value,   bob_output  ) =  bob   alice_output

Example: Diffie-Hellman key exchange.
See http://en.wikipedia.org/wiki/Diffie–Hellman_key_exchange

> p = 23
> g = 5

> alice a =
>   write "Hello" $
>   write (g^a `mod` p) $
>   read $ \b ->
>   let s = b^a `mod` p in
>   write "Goodbye" $
>   return s

> bob b =
>   read $ \message1 ->
>   write (g^b `mod` p) $
>   read $ \a ->
>   let s = a^b `mod` p in
>   read $ \message2 ->
>   return s

> main = print $ chat (alice 6) (bob 15)