[Haskell-cafe] Re: FRP for game programming / artifical life
daniel.is.fischer at web.de
Fri Apr 30 11:45:38 EDT 2010
Am Freitag 30 April 2010 17:23:19 schrieb Antoine Latter:
> On Fri, Apr 30, 2010 at 3:37 AM, Daniel Fischer
> <daniel.is.fischer at 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
Was fooled by the use of c in the result.
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