Re: [Haskell-cafe] Re: FRP for game programming / artifical life simulation

Am Freitag 30 April 2010 17:23:19 schrieb Antoine Latter:

On Fri, Apr 30, 2010 at 3:37 AM, Daniel Fischer

wrote:

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Am Donnerstag 29 April 2010 20:08:00 schrieb Ben:

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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