Unique vectors

[TimWhiting]

Addresses #544

@anfelor can you take a look at this?

I added a few public apis and a few private helpers.
Public:

• set
• update
• update for local variable vectors

The most important function is the copy function, which ensures that the vector is unique prior to applying any mutation operations. For many cases we would expect it to be unique.

[TimWhiting]

We might want to add a few more apis (cutting an array short and dropping the the rest of the array), or extending an array.

[anfelor]

Thanks @TimWhiting, this looks a lot friendlier than my draft! I made a few minor comments regarding naming of variables and documentation.

I worry a bit that the unsafe surface area is quite large still. I think we can probably pay for two is-unique checks per iteration of map (given that the second can be well-predicted). Then we can make this API more clean by incorporating the copy calls into unsafe-assign and unsafe-idx-own.

[anfelor]

We might want to add a few more apis (cutting an array short and dropping the the rest of the array), or extending an array.

Yes, that would be fun! I think we could also support vector.filter, which would require us to write box_null into the tail of the vector and keep a separate length field. In OCaml, this is unsupported since box_null might not be an inhabitant of the type of the elements though, and I wonder if there is a problem that I don't see currently.

[TimWhiting]

We might want to add a few more apis (cutting an array short and dropping the the rest of the array), or extending an array.

Yes, that would be fun! I think we could also support vector.filter, which would require us to write box_null into the tail of the vector and keep a separate length field. In OCaml, this is unsupported since box_null might not be an inhabitant of the type of the elements though, and I wonder if there is a problem that I don't see currently.

I think these sorts of APIs are maybe best left to a resizeable vector type, which could hold on to the length like you mention and have a resize policy for add / extending the vector as well. What do you think?

[anfelor]

lgtm