[moelf]

>happens at runtime

is not technically true, because that implies a massive slow-down. instead it's more accurate to say behavior-wise it's always equivalent to a dynamic dispatch, but because Julia's Just-Ahead-of-Time compilation, often you eliminate the dynamic dispatch during run time.

[sfpotter]

It is technically true, Julia and other programming language's implementation of multiple dispatch notwithstanding.

First sentence from the Wikipedia article on multiple dispatch:

"Multiple dispatch or multimethods is a feature of some programming languages in which a function or method can be dynamically dispatched based on the run-time (dynamic) type or, in the more general case, some other attribute of more than one of its arguments."

And later:

"Multiple dispatch should be distinguished from function overloading, in which static typing information, such as a term's declared or inferred type (or base type in a language with subtyping) is used to determine which of several possibilities will be used at a given call site, and that determination is made at compile or link time (or some other time before program execution starts) and is thereafter invariant for a given deployment or run of the program."

[DNF2]

The dispatch is based on the "runtime type", but that does not necessarily "happen at runtime", because the runtime/dynamic type can often be determined statically.