Single parameter functions should implement Java's Function interface

[SimonCockx]

Functions that accept a single input should implement Java's Function interface. That would make the generated code more convenient for consumers.

func Foo:
  inputs: a int (1..1)
  output: b string (1..1)

should generate

public class Foo implements RosettaFunction,/* ADD THIS*/ Function<Integer, String> {
  ...
}