Comment by seertaak

Just to clarify the above:

1. I use the term "performance" slightly vaguely. It's comprised of two distinct things: the time it takes to compile the code, and the execution time. The issue is the compilation time: there are certain cases where it's exponential in the number of types which could unify with the callsite's type params.

2. IIRC, Julia compiler has heuristics to ensure things don't explode for common cases. If I'm not mistaken, not only do compile times explode, but certain (very common) things don't even typecheck. There's an excellent video about it by the designer of the language, Jeff Bezanson -- https://www.youtube.com/watch?v=TPuJsgyu87U . Note: Julia experts, please correct me if this has been fixed.

3. The difficulty in intuiting which combinations of types will unify at a given callsite isn't theoretical; there are reports of libraries which unexpectedly fail to work together. I want to qualify this statement: Julia is light years ahead of any language lacking multimethods when it comes to library composability. But my guess is that those problems would be reduced with non-symmetric multimethods.

4. The non-symmetric multimethod system I'm "proposing" isn't my idea. They are referred to variously as encapsulated or parasitic multimethods. See http://lucacardelli.name/Papers/Binary.pdf

I have huge respect for Jeff Bezanson, for the record!