[MLton] refs in ssa

[Matthew Fluet]

> So I assumed that ref_ref would just take a variable.

That would seem to be a reasonable assumption.

> This is not always the case. For
> instance, when I try to snag the first variable of the args field for a
> ref_ref prim op sometimes it is not there, IE we have a length zero arg
> vector.

Can you post some code that demonstrates this?  I'd be very surprised if
this were the case.

> This occurs when we reference a variable which is null.

Not sure what you mean by this.

> In general is the first argument to ref_ref going
> to be the ssa variable that was referenced?

Well, the first (and I would think only) argument to Ref_ref would be the
SSA variable whose value should be allocated in a ref cell.