Not necessarily. You just store a pointer to the type info inline with the data (like C++'s vtables). You can have unboxed "value-types" (const structs, essentially) in dynamic languages. In fact, you could even differentiate between a "boxed" ref type and a value type at runtime, because refs don't need all 64 bits of the pointer. So a ref is a 64 bit pointer with the first bit set to, say, 0, and a value (struct) type always begins with a 64 bit pointer to it's type information, only it's tagged with MSB of 1. Since you can't extend concrete types, you can easily store value types inline in an array, and just have the type-info pointer (which must be the same for all elements, b/c there is no inheritance) at the beginning of the array. And if your structs are aligned OK, you could easily pass them to C by skipping the type-info pointer both in the single value case and in the array case.
Ok, having read this comment again, here's a more measured response. You're assuming in this comment the value-type vs. object-type dichotomy that's used in, e.g., C#. That's one way to go, but I'm not sold that it's the best way. Deciding whether you want something to be storable inline in arrays or not when you define a type is kind of a strange thing. Maybe sometimes you do and sometimes you don't. So the bigger question is really if that's the best way to go about the matter.
It seems that in dynamically typed languages, you either need to have two kinds of objects (value types vs. object types), or two kinds of storage slots (e.g. arrays that hold values inline vs. arrays that hold references to heap-allocated values). The boxing aspect is really only part of that since you can't get the shared-reference behavior unless the storage is heap-allocated, regardless of whether there's a box header or not.
This is an interesting scheme and seems like it might work, but I'm not sure. Would you be willing to pop onto julia-dev@googlegroups.com and post this suggestion there so we can have a full-blown discussion of it? Hard to do here — and some of the other developers would need to chime in on this too.
