unfold

[Stephen Weeks]

> I'd vote for the following:
> 
>       val fold: 'a t * 'b * ('a * 'b -> 'b) -> 'b
>       val foldi: 'a t * 'b * (int * 'a * 'b -> 'b) -> 'b
> 
>       val unfold: 'b * ('b -> ('a * 'b) option) -> 'a t
>       val unfoldi: int * 'b * (int * 'b -> ('a * 'b)) -> 'a t
> 
> (Note: this changes the type of the old unfold.  Why didn't the unfolding
> function get the index? 

It did, that's what unfoldi was for (analogous with foldi).

Anyways, I like your compromise.  There's still unfoldr and unfoldri, but that's
it. 

> How could you have unfoldi and unfoldi'?  Particularly for vector/arrays?

I was only thinking of lists.

> What am I supposed to do if my generator returns NONE before I've filled
> up the vector/array?

I never mix the option ones with ones that specify the size.