Bas van Dijk
On Sun, Mar 15, 2009 at 7:35 PM, Will Ness
wrote: which is then just rewritable as:
myCycle xs = ys where ys = foldr (:) ys xs
or (arriving at the same result)
myCycle xs = foldr (:) (myCycle xs) xs
Note that, because 'ys' only has to be calculated once, GHC makes the most efficient code for the former one. In the latter 'myCycle xs' has to be calculated each time 'xs' runs empty.
Here are some efficiency tests:
myCycle1 xs = ys where ys = foldr (:) ys xs myCycle2 xs = foldr (:) (myCycle2 xs) xs myCycle3 xs = ys where ys = xs ++ ys
main = do ns:ms:_ <- getArgs print $ sum $ take (read ns) (myCycle1 [1..read ms])
So you've discovered an inefficiency in GHC, which should probably improve on its deforestation skills. :) "Smart" compiler would NOT allocate any extra storage in the three cases above, only altering the access to it. The meaning of all the three is the same - they are all rewritable one into another - it's a loop around on access past end. The "very bright" compiler would just translate sum $ take 30000000 [1..1000] into sum [1..1000]*30000 producing the same result: Prelude> sum [1..1000]*30000 15015000000 Cheers,