Callable classes

To allow an instance of your Dart class to be called like a function, implement the call() method.

In the following example, the WannabeFunction class defines a call() function that takes three strings and concatenates them, separating each with a space, and appending an exclamation. 

class WannabeFunction { 
   call(String a, String b, String c) => '$a $b $c!'; 
} 

main() { 
   var wf = new WannabeFunction(); 
   var out = wf("Hi","there,","gang"); 
   print('$out'); 
}