On Tue, Jun 28, 2011 at 6:20 PM, Eric Rasmussen <ericrasmussen@gmail.com> 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. For an example of a similar technique (minus the freezing part), I did a similar thing in the hashtables library: https://github.com/gregorycollins/hashtables/blob/master/src/Data/HashTable/... It's unlikely to be worth the extra effort except for extremely performance-critical code. G -- Gregory Collins <greg@gregorycollins.net>