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There is a feature request to support functions which can take a variable number of arguments in Dart (the same request was mentioned earlier in a comment to another feature request). But we can implement such functions without modifying the language or the compiler!

typedef dynamic ApplyType(List positionalArguments);

class Variadic implements Function {
  final ApplyType _apply;

  Variadic(this._apply) {}

  @override
  dynamic noSuchMethod(Invocation invocation) {
    if (invocation.memberName == #call) {
      if (invocation.isMethod)
        return _apply(invocation.positionalArguments);
      if (invocation.isGetter)
        return this;
    }
    return super.noSuchMethod(invocation);
  }
}

num sumList(List<num> xs) =>
  xs.fold(0, (num acc, num x) => acc + x);

final Function sumVariadic = new Variadic(sumList);

void main() {
  print(sumList([100, 200, 300]));
  print(sumVariadic(100, 200, 300));
}

Notes:

• The trick is that according to the language specification, sumVariadic(…) is a shorthand for sumVariadic.call(…) (Section 12.15.4, “Function Expression Invocation”). Although we cannot define call as a method of sumVariadic (because then call itself must be a variadic function!), we can define the noSuchMethod method to specify what this method invocation should actually do.

• The declaration of Variadic produces a static warning “Concrete classes that implement Function must implement the method call()”. This is as described in the language specification (Section 15.5, “Function Types”), but I am not sure if this warning is desirable.

• With this approach, the static type of sumVariadic(100, 200, 300) is dynamic, not num.

• Needless to say, if you want to use variable-length argument lists seriously, you probably want the proper support of the language instead of this hack. But it is fun to try.

• The code above supports only positional arguments. Support for named arguments is left as an exercise.