Add another form of unfoldr

[Khudyakov]

Please add following functions for construction of immutable vectors. They are useful for generation of n first element like power series, sequence defined as recurrent relation etc.

iterate :: Int -> (a -> a) -> a -> Vector a

-- I can't think of nice name (unfoldrN is taken already)
unfoldrN2 :: Int -> (b -> (a,b)) -> b -> Vector a

Clearly they are specializations of unfoldr. I think they are quite useful especially first one. Second function could be easily defined in terms of unfoldrN but it should be advantageous from perfomance point of view. There is no need to wrap/unwrap value to/from Maybe.

[Khudyakov]

I have incomplete patch for iterate function. I hope i will finish it a week or so.

[Khudyakov]

Well, week could be a really long time. Here is patch for iterate. It looks like it does work as expected.

[rl]

Applied, thanks. I renamed iterate to iterateN to be consistent with Data.Sequence and friends.

[rl]

Actually, since unfoldrN is inlined, there should be no performance difference between unfoldrN n . Just and unfoldrN2 n so I don't think it's necessary to add the latter. Please shout if you think otherwise!