Am Freitag 30 April 2010 17:23:19 schrieb Antoine Latter: > On Fri, Apr 30, 2010 at 3:37 AM, Daniel Fischer > > <daniel.is.fisc...@web.de> wrote: > > 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.
Probably not. Although, you're right, if only s2 is ever looked at and not c, Ben's version can give a result where the library instance throws an exception. Was fooled by the use of c in the result. > > Antoine _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
