In Groovy we can invoke an implicit call() method on a Groovy object. We can leave out the call method name and just use (). We can use meta programming to add an implementation for the call() method to a class. In the following example script we add an implementation for the call() method with a single parameter to the String class. The implementation returns the element found at the range specified by the argument when we invoke the method:

String.metaClass.call = { range ->
    delegate[range]
}

def value = 'Groovy is Gr8'
assert value(0) == 'G'
assert value(10) == 'G'
assert value(4) == value[4]
assert value.call(1) == value(1)
assert value(0..5) == 'Groovy'

Inspired by the examples http://groovyconsole.appspot.com/view.groovy?id=21006 and http://groovyconsole.appspot.com/script/21005 we can also write our own class and implement the call() method. This can for example be used in DSLs.

class StringConverter {
    def value
    
    def value(s) {
        value = s
        this
    }

    /** Convert characters in value property if cond is true */    
    def upper(cond) {
        value = value.collect { cond(it) ? it.toUpperCase() : it }.join()
    }

    def call(callable) {
        callable
    }
}

def converter = new StringConverter()
converter.with {
    value 'mrhaki' upper { it < 'm' }
    // Equivalent to:
    // value('mrhaki') upper { it < 'm' }
    // or
    // value('mrhaki').call(upper { it < 'm' })
    // or
    // value('mrhaki').call(upper({ it < 'm' }))
}
assert converter.value == 'mrHAKI'

converter.with {
    value('jdriven') upper { it == 'j' || it == 'd' }

    assert value == 'JDriven'
}

(Code written with Groovy 2.0.4)