How painless dispatches functions

Painless uses receiver, name, and arity to for method dispatch. For example, s.foo(a, b) is resolved by first getting the class of s and then looking up the method foo with two parameters. This is different from Groovy which uses the runtime types of the parameters and Java which uses the compile time types of the parameters.

The consequence of this that Painless doesn’t support overloaded methods like Java, leading to some trouble when it whitelists classes from the Java standard library. For example, in Java and Groovy, Matcher has two methods: group(int) and group(String). Painless can’t whitelist both of them methods because they have the same name and the same number of parameters. So instead it has group(int) and namedGroup(String).

We have a few justifications for this different way of dispatching methods:

  1. It makes operating on def types simpler and, presumably, faster. Using receiver, name, and arity means when Painless sees a call on a def objects it can dispatch the appropriate method without having to do expensive comparisons of the types of the parameters. The same is true for invocations with def typed parameters.
  2. It keeps things consistent. It would be genuinely weird for Painless to behave like Groovy if any def typed parameters were involved and Java otherwise. It’d be slow for it to behave like Groovy all the time.
  3. It keeps Painless maintainable. Adding the Java or Groovy like method dispatch feels like it’d add a ton of complexity which’d make maintenance and other improvements much more difficult.