Add a variant unfoldrExactN, returning the resulting generator

[Bodigrim]

I have a job for

unfoldrExactN_ :: Int -> (b -> (a, b)) -> b -> (Vector a, b)

but it is surprisingly difficult to emulate: the best I managed to do was put b into State and use replicateM.

How do you feel about extending vector with such function?

[lehins]

Going through State + replicateM is slow, unless maybe if we specialize it as well like it is done for ST and IO when doing unstreaming.

One fallback always is to use mutable interface instead, but that means no fusion.

So, this function is incredibly useful, as you probably saw in haskell/random#133 so I am definitely all for adding unfoldr that returns the last value. Any volunteers?

[Shimuuar]

This is very tricky function to write inside stream fusion framework. b is hidden inside existential state of stream and we don't have any way to access it:

data Stream m a = forall s. Stream (s -> m (Step s a)) s

data Step s a where
  Yield :: a -> s -> Step s a
  Skip  :: s -> Step s a
  Done  :: Step s a

On top of it Done discards state.

[Shimuuar]

There's consensus that such function is useful. Although it could be only implemented without fusion support. I'm mrking it as help wanted