Re: [Haskell-cafe] attoparsec and vectors

Gregory Collins wrote:

On Tue, Jun 28, 2011 at 6:20 PM, Eric Rasmussen wrote:

...

It runs quickly, but I naively thought I could outperform it by reworking "many" to build a vector directly, instead of having to build a list first and then convert it to a vector:

manyVec :: Alternative f => f a -> f (V.Vector a) manyVec v = many_v   where many_v = some_v <|> pure V.empty         some_v = V.cons <$> v <*> many_v

That's an O(n^2) loop, and a thunk leak to boot. If you don't know the size of the vector ahead of time, the only way I can think of to beat Vector.fromList is to use a mutable vector with a "highwater" mark, and double the size if you fill it. At the end, you'd use "unsafeFreeze" to turn the mutable vector into a pure one, and "unsafeTake" to truncate the vector into the correct size.

That's basically what fromList does. You could do this at a higher abstraction level by generating a Stream rather than a list and then using unstream to create a vector. I don't know if it's possible to do that with attoparsec. But you'd only save allocating and deallocating a lazily consumed list anyway. I'm not sure if it will be even noticable compared to how much parsing costs.

For an example of a similar technique (minus the freezing part), I did a similar thing in the hashtables library:

You might be interested in 'grow' :-) http://hackage.haskell.org/packages/archive/vector/0.7.1/doc/html/Data-Vecto... Roman