Re[Haskell-cafe] cursive referencing

[Eugene Kirpichov]

Thanks, that clarifies things a lot; I must improve my laziness-fu!

(to Belka: Also, lazy matching may be performed as .... fix
(\(~(aa,bb)) -> ...) - the tilde does the trick)

2009/1/29 Ryan Ingram <ryani.spam at gmail.com>:
> On Thu, Jan 29, 2009 at 12:44 AM, Eugene Kirpichov <ekirpichov at gmail.com> wrote:
>> With the Y combinator, the code becomes as follows:
>>
>> f = \somedata1 somedata2 -> fst $ fix (\(aa,bb) -> (AA somedata1 bb,
>> BB somedata2 aa))
>>
>> I tried to prove to myself that the structure really turns out cyclic,
>> but games with reallyUnsafePtrEquality# didn't lead to success; that's
>> why it's "reallyUnsafe" :-/
>
> Your definition with "fix" isn't lazy enough, so it goes into an
> infinite loop.  You need to match lazily on the pair; here's a better
> body for "fix":
>
> fix (\p -> (AA somedata1 $ snd p, BB somedata2 $ fst p))
>
> To prove that the structure really turns out cyclic, you can use Debug.Trace:
>
> import Debug.Trace (trace)
> import Data.Function (fix)
>
> data AA = AA Int BB deriving Show
> data BB = BB Int AA deriving Show
>
> f = \data1 data2 -> fst $ fix $ \p ->
>     (trace "eval_aa" $ AA data1 $ snd p,
>      trace "eval_bb" $ BB data2 $ fst p)
>
> sumAA 0 _ = 0
> sumAA n (AA v bb) = trace "sumAA" (v + sumBB (n-1) bb)
> sumBB 0 _ = 0
> sumBB n (BB v aa) = trace "sumBB" (v + sumAA (n-1) aa)
>
> main = print $ sumAA 10 $ f 1 2
>
> *Main> main
> eval_aa
> sumAA
> eval_bb
> sumBB
> sumAA
> sumBB
> sumAA
> sumBB
> sumAA
> sumBB
> sumAA
> sumBB
> 15
>
>  -- ryan
>