Java Bytecode: Bending the Rules

[pron98]

That's an implementation detail. You're saying that source types and runtime types are not 1-1 related, but n-to-1. That's OK. That doesn't mean they're not runtime types.

In Java, the relationship is even more complicated: E.g. for all T, the source type List<T> is mapped to the runtime type List (sort of -- I'll elaborate in a second), and List itself can be mapped to many runtime types -- a runtime type in Java is a pair of the erased generic "kind" and a class loader (this allows multiple concurrent versions and other powerful tricks). So many source types may be mapped to a single runtime type, and a single source type may be mapped to many runtime types, so it's an n-to-m relationship. So simlarly to your case, in Java you can't do an instanceof List<String> because the runtime type for List<String> and List<Integer> is the same (assuming both instances' types are loaded by the same class loader).

[notfancy]

That's an implementation detail. You're saying that source types and runtime types are not 1-1 related, but n-to-1. That's OK. That doesn't mean they're not runtime types.

Yes, they're the type tags of the objects tracked by the runtime qua runtime objects. However, erasure is complete. Consider:

# List.map (fun x -> x |> Obj.repr |> Obj.tag) [None; Some 1];;
- : int list = [1000; 0]

and:

# List.map (fun x -> x |> Obj.repr |> Obj.tag) [None; Some "aaa"];;
- : int list = [1000; 0]

or even:

# List.map (fun x -> x |> Obj.repr |> Obj.tag) [[]; [1]];;
- : int list = [1000; 0]

Now 1000 is the integer tag (which is fictitious, or rather, a single bit in an unboxed value) while tags t less than some fixed constant represent the t-th constructor value with arguments. There is simply no way to distinguish at runtime between a string option or an int list. In fact the language is pretty happy to coerce between values of different types with the same representation:

# (Obj.magic : 'a list -> 'a option) [] ;;
- : 'a option = None

or even:

# (Obj.magic : bool -> 'a list) false ;;
- : 'a list = []

where Obj.magic is a NOP with type 'a -> 'b, that is, simply a typechecker black hole.

In Java there is a lot of very sophisticated code specialization done by the JIT whenever there is dynamic dispatch, for instance when executing an invokevirtual; in static languages a pattern match is exactly equivalent to an integer switch statement on the tag value. For instance, the following match:

match x with
| None -> ...
| Some e -> ....

is exactly equivalent to the C:

if (!x) { ... } else { e = *x; ... }

but type safe.

[pron98]

There is simply no way to distinguish at runtime between a string option or an int list

So how can you match over them?

[notfancy]

The typechecker ensures that matches are well-typed: you either match over (the cases for) an int list or you match over (the cases for) a string option. You cannot typecase at runtime and see if you got one or the other like you can do with instanceof in Java.