Since Paul is now threatening us with a 4.0 beta, I wanted to float an
idea that I've been thinking over for a bit now that might be best to
add there (though maybe it would be okay in 3 still, with the Java 8
baseline).
A large number of the GDK extension methods (particularly stuff like
`with`, `collect`, and similar) have signatures that rely explicitly
on Closure for their strategies. This means that there are
interoperability problems with libraries that provide strategy
implementations as functional types; e.g., imagine this trivial
example:
```
public UnaryOperator<Integer> multiplier(int multiplicand) {
return x -> x * multiplicand;
}
```
I can't say `[1, 2, 3].collect(multiplier(2))`, because `collect`
takes only a Closure.
I would like to (ideally) alter the signatures of the methods in
DefaultGroovyMethods and similar to replace Closure with the "more
static" interfaces where possible; most of these cases would end up
being Function, Consumer, and some of the other usual suspects, with
some of the extension methods being duplicated to account for the
cases where the GDK currently does runtime magic like arity detection.
In the alternative, at least adding overrides for these would support
language-level interoperability.
What is the opinion regarding making this change, and what kinds of
compatibility problems might result from replacing Closure with (e.g.)
Function rather than adding a new override?
