Creating function with variable number of arguments or parameters in Dart

Question

I am looking for a way to create a function with a variable number of arguments or parameters in Dart. I know I could create an array parameter instead, but I would prefer to not do that because I'm working on a library where syntactic brevity is important.

For example, in plain JavaScript, we could do something like this (borrowed from here):

function superHeroes() {
  for (var i = 0; i < arguments.length; i++) {
    console.log("There's no stopping " + arguments[i]);
  }
}

superHeroes('UberMan', 'Exceptional Woman', 'The Hunk');

However, in dart, that code will not run. Is there a way to do the same thing in dart? If not, is this something that is on the roadmap?

Answer 1

You can't do that for now.

I don't really know if varargs will come back - they were there some times ago but have been removed.

However it is possible to emulate varargs with Emulating functions. See the below code snippet.

typedef OnCall = dynamic Function(List arguments);

class VarargsFunction {
  VarargsFunction(this._onCall);
  
  final OnCall _onCall;

  noSuchMethod(Invocation invocation) {
    if (!invocation.isMethod || invocation.namedArguments.isNotEmpty)
      super.noSuchMethod(invocation);
    final arguments = invocation.positionalArguments;
    return _onCall(arguments);
  }
}

main() {
  final superHeroes = VarargsFunction((arguments) {
    for (final superHero in arguments) {
      print("There's no stopping ${superHero}");
    }
  }) as dynamic;
  superHeroes('UberMan', 'Exceptional Woman', 'The Hunk');
}

Answer 2

I played around a little with Alexandre Ardhuin's answer and found that we can tweak a couple of things to make this work in the current version of Dart:

class VarArgsClass {
  noSuchMethod(InvocationMirror invocation) {
    if (invocation.memberName == 'superheroes') {
      this.superheroes(invocation.positionalArguments);
    }
  }

  void superheroes(List<String> heroNames) {
    for (final superHero in heroNames) {
      print("There's no stopping ${superHero}!");
    }
  }
}

main() {
  new VarArgsClass().superheroes('UberMan', 'Exceptional Woman', 'The Hunk');
}

This has lots of problems, including:

• A warning is generated wherever you call superheroes() because the signature doesn't match your parameters.
• More manual checking would need to be done to make sure the list of arguments passed to superheroes is really a List<String>.
• Needing to check the member name in noSuchMethod() makes it more likely you'll forget to change the 'superheroes' string if you change the method name.
• Reflection makes the code path harder to trace.

BUT if you are fine with all of those issues, then this gets the job done.

Answer 3

This version:

1. Works with both positional and keyword arguments.
2. Supports typing of the return value.
3. Works with modern Dart.

typedef VarArgsCallback = void Function(List<dynamic> args, Map<String, dynamic> kwargs);

class VarArgsFunction {
  final VarArgsCallback callback;
  static var _offset = 'Symbol("'.length;

  VarArgsFunction(this.callback);

  void call() => callback([], {});

  @override
  dynamic noSuchMethod(Invocation inv) {
    return callback(
      inv.positionalArguments,
      inv.namedArguments.map(
        (_k, v) {
          var k = _k.toString();
          return MapEntry(k.substring(_offset, k.length - 2), v);
        },
      ),
    );
  }
}

void main() {
    dynamic myFunc = VarArgsFunction((args, kwargs) {
      print('Got args: $args, kwargs: $kwargs');
    });
    myFunc(1, 2, x: true, y: false); // Got args: [1, 2], kwargs: {x: true, y: false}
}

Thanks, Alexandre for your answer!

Answer 4

Dart does indirectly support var-args as long as you as you aren't too much into syntactic brevity.

void testFunction([List<dynamic> args=[]])
{
  for(dynamic arg:args)
  {
    // Handle each arg...
  }
}

testFunction([0, 1, 2, 3, 4, 5, 6]);
testFunction();
testFunction([0, 1, 2]);

Note: You can do the same thing with named parameters, but you'll have to handle things internally, just in case if the user (of that function; which could be you) decides to not pass any value to that named parameter.

---

I would like to thank @Ladicek for indirectly letting me know that a word like brevity exists in English.

Answer 5

For the example you've written, I think you're best off using a list. Sorry about that!

I'm looking at dartbug.com, but I don't see a feature request for this. You're definitely welcome to create one!

Answer 6

If you are really into syntactic brevity, just declare a function/method with say 10 optional positional parameters and be done. It's unlikely someone will call that with more than 10 arguments.

If it sounds like a hack, that's because it is a hack. But I've seen the Dart team doing the same :-)