how to define a variable of type "function"? (using optional typing)

Question

In Dart, functions are first class. The documentation tells us that you can create a variable that is a function like this:

var loudify = (msg) => '!!! ${msg.toUpperCase()} !!!';

to create a function named "loudify" that takes a String and returns another String.
If I don't know beforehand which function I want to assign to the variable, I can do it like this:

// define the variable
var loudify;

// later on set the variable
loudify = (msg) => '!!! ${msg.toUpperCase()} !!!';

But how do I use optional typing so that I know later on that the variable is of type "function" and has input String and output String? I would suggest something like this, but this doesn't work (Dart editor tells me "undefined class 'function'"):

// this gives a syntax error in the Dart editor
function<String, String> loudify;

So what is the proper syntax here?
Regards,
Hendrik

Answer 1

You can use typedef

typedef String OneString(String x);
typedef String OneStringAndInt(String x, int y);

void main() {
  //var f = (String x) => '$x';
  var f = (String x, int y) => '$x$y';
  if(f is OneString) {
    print(f('bla'));
  } else {
    print(f('bla', 10));
  }
}