Question

I just watched a video on protocols in Clojure and it explained how ‘multimethods’ work. It seems to me that they look very similar to how extension methods in C# work. Are they basically the same thing (with the exception that you don’t need to create a static class in Clojure) or is there a fundamental difference? Is there an advantage or disadvantage in using either?

Answer 1

The mutlimethod feature in Clojure is for multiple dispatch scenarios. It effectively enables runtime polymorphism where the method that is invoked depends on the type of the arguments to the method (traditional single-dispatch polymorphism depends on the runtime type of the object receiving the method call). Basically, you can think of single-dispatch polymorphism as a method M

M(arg1, arg2, arg3, ..., argn)

and the actual method that is invoked depends on the runtime type of arg1 (so we’re rewriting the usual syntax of

arg1.M(arg2, arg3, ..., argn)

as

M(arg1, arg2, arg3, ..., argn)

to make the analogy clear. In multiple dispatch, the method that is invoked by

M(arg1, arg2, arg3, ..., argn)

depends on the runtime types of arg1, arg2, …, argn as well.

You can achieve similar functionality in C# with dynamic.

Frankly, it’s not related to extension methods at all.