22 Sep
2009
22 Sep
'09
4:57 p.m.
Am Dienstag 22 September 2009 22:31:48 schrieb Daniel Fischer:
That doesn't explain the quadratic behaviour of shuffleArr, though. I suspect it's laziness, things aren't actually done until the result is finally demanded, but I would have to take a closer look to really find out.
Yep. Strictifying things gives the expected linear behaviour. In swap a n m = do [n',m'] <- mapM (readArray a) [n,m] mapM (uncurry $ writeArray a) [(m,n'),(n,m')] you don't actually read and write values, but ever longer thunks. Changing swap to swap a m n vm <- readArray a m vn <- readArray a n writeArray a n $! vm writeArray a m $! vn is all you need to do (as long as your values have a simple type).