Really by "boxing" I mean "dictionary passing" (I've also seen the term "intensional type analysis" for it). That encompasses approaches like Java as well as those of Swift.

Hybrid approaches in which the compiler (or JIT!) decides whether to monomorphize or use vtables are also possible.

"Dictionary passing" is a good term for this implementation strategy, I haven't heard it named before. Do you know of any other languages that take a similar approach?

Haskell and Ocaml, e.g. http://okmij.org/ftp/Computation/typeclass.html .

I thought Ocaml still boxed values, so there's no equivalent of value witness tables?

Sure, but it still does basically the same thing of passing around witness tables. Values happen to have a uniform representation, but this seems seems like variation within a cluster of languages that do similar thing rather than something fundamentally new.

The Swift approach shares many of the same characteristics and trade-offs as "true" (aka Java-style) boxing. Of course, it does avoid some downsides of that, but also brings along some additional ones.

I think the main difference is that the Swift approach ends up being more memory-efficient, eg consider Swift's Array<Int> is stored as an array of integer values compared to a Java's ArrayList<Integer> where each element is boxed.

Also the Swift optimizer can clone and specialize generic functions within a module, eliminating the overhead of indirection through reified metadata.

Yes, there's some pitfalls of Java that Swift avoids, like compulsory allocation, but it fundamentally has the same style of indirection (often pointers to stack memory rather than heap, but still pointers), type erasure and pervasive virtual calls, and brings with it even more virtual calls due to needing to go through the value witness table to touch any value (which might be optimised out... or might not, especally with widespread resilience).

The compiler specializing/monomorphizing as an optimisation shows that Swift has a hybrid approach that tries to balance the trade-offs of both (like many other languages, like Haskell, and, with explicit programmer direction, Rust), but the two schemes still fall into the broad categories of monomorphizing vs dictionary passing, not something fundamentally different.

Oh, hi Huon.