On Mon, Oct 31, 2011 at 3:19 PM, John Lask
On 1/11/2011 1:35 AM, Captain Freako wrote:
you need to study ArrowLoop and understand that.
Thanks, John. I'm working my way through Hughes' suggested exercise in `Programming with Arrows', to wit: "The reader who finds this argument difficult should work out the sequence of approximations in the call `runSF (loop (arr swap)) [1,2,3]'" I think I understand how wrapping `cs' in `stream' provides the needed delay of evaluation, in order to avoid a self propagating `undefined'. However, it's not clear to me exactly what finally triggers the evaluation of c0. So, for instance, I think the `sequence of approximations' looks like this: 1) let (bs, cs) = unzip (map swap (zip as (_:_:_:...))) in bs : bs = [c0, (_:_:...)] cs = [1,2,3] At this point, `c0' is equal to 1, but what triggers the evaluation of c0? Is it simply the binding of it to `b0'? If so, then why aren't `c1' and `c2' also evaluated, which would yield undefined values? Thanks, -db