Closure with typed arguments in Groovy

Question

I'd like to be more explicit about my closures regarding their argument types. So I would write something like

List<Y> myCollect(List<X> list, Closure<X,Y> clos) { ... }

I know that Groovy won't use that type information, but Groovy++ may use it at compile time. Can this be be achieved (other than putting it into a comment)?

UPDATE: The title may sound misleading, but I thought the above example would make it clearer. I'm interested in specifying types of a closure which is the argument of some function. Suppose, I want to redefince the built-in collect. So I'm interested in writing myCollect, not in writing clos. What I want to achieve is get compile time errors

myCollect(['a', 'ab'], { it / 2 }) // compile error
myCollect(['a', 'ab'], { it.size() })  // OK 

Answer 1

You can define the types of a closure's parameters, but the syntax shown above is incorrect. Here is a closure without parameter types:

def concatenate = {arg1, arg2 ->
  return arg1 + arg2
}

And here is the same closure with parameter types

def concatenate = {String arg1, String arg2 ->
  return arg1 + arg2
}

I know that Groovy won't use that type information, but Groovy++ may use it at compile time.

Groovy does do some compile-time type checking, but not as much as Groovy++ (or Java). Even if the type information is not used at compile-time it will be checked at runtime, and is also valuable as a form of documentation.

Answer 2

I presume you aren't using Groovy++ anymore, but even if you are this may work. It certainly works for statically typed Groovy 2.x

interface Z {
  void callback(X x, Y y)
}

List<Y> myCollect(List<X> list, Z clos) { 
  ... 

  clos.callback(x, y)
}

the caller then calls it with the normal:

List<Y> object.myConnect(list) { X x, Y y -> 
}

If you leave a parameter out and have @CompileStatic, the compiler picks up missing params or bad types.

This works because a 1 method interface is equivalent to a closure in Groovy.

Answer 3

For anyone still looking for the answer: have a look at @ClosureParams, which provides IDE hints as to the expected closure parameter types