Question

In Groovy, I can make an object invokable like a function by monkey-patching the metaclass’ call method:

myObject.metaClass.call = { "hello world" }
println myObject() // prints "hello world"

patching call only allows me to invoke the object with no arguments. Is there a way of allowing objects to be invoked with arguments using standard function-like syntax?

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edit: one answer is exactly as tim_yates suggests, although it’s worth noting from ataylor’s comment that you can simply override call without explicit metaprogramming:

class MyType {
    def call(...args) {
        "args were: $args"
    }
}

def myObject = new MyType()
println myObject("foo", "bar") // prints 'args were ["foo", "bar"]'

Apparently the trick is the variadic signature using ...args.

Answer 1

You could do:

myObject.metaClass.call = { ...args -> "hello $args" }
assert myObject( 'one', 'two', 'three' ) == 'hello [one, two, three]'

(as you can see, args is an array of Objects)

Or for one parameter:

myObject.metaClass.call = { who -> "hello $who" }

Or if you want that single parameter as an optional param, you could do:

myObject.metaClass.call = { who = null -> "hello ${who ?: 'world'}" }

assert myObject( 'tim' ) == 'hello tim'
assert myObject() == 'hello world'