On Fri, Apr 30, 2010 at 3:37 AM, Daniel Fischer
Am Donnerstag 29 April 2010 20:08:00 schrieb Ben:
A technical question: it seems like the instance of ArrowLoop is too strict (this is something I've wondered about in Liu's paper too.) Shouldn't it be
instance ArrowLoop SFAuto where loop (SFAuto s f) = SFAuto s f' where f' (b, s1) = let (~(c, d), s2) = f ((b, d), s1) in (c, s2)
Let-bindings are already lazy, so the '~' makes no difference here. Apart from the readability, both are the same as
where f' (b,s1) = let x = f ((b, snd $ fst x),s1) in (fst $ fst x, snd x)
Ben's version is slightly lazier - even though the let binding is lazy, pattern matching is strict. so (let ((x,y).z) = (undefined, "hello") in z) will exception out, but (let (~(x,y),z) = (undefined, "hello") in z) will not. I don't know if you need that level of laziness, though. Antoine