How to specify a default value of a function parameter?

Question

This function should transform each element of the list with the given function transform:

void _doSomething(List<Something> numbers, [transform(Something element)]) {...}

As I don't want to skip this method when the transform should not do anything, I wanted to give a default value to the transform method like this:

void _doSomething(List<Something> numbers, 
                  [transform(Something element) = (v) => v]) {...}

Unfortunately, the editor tells me

Expected constant expected

Is there some workaround or simply not possible (or shouldn't be done like this at all)?

Answer 1

if you want to initialize a Function parameter that is also a field of your class I suggest:

class MyClass{
  Function myFunc;
  MyClass({this.myFunc = _myDefaultFunc}){...}
  static _myDefaultFunc(){...}
}

Or more suitable:

typedef SpecialFunction = ReturnType Function(
                              FirstParameterType firstParameter, 
                              SecondParameterType secondParameter);

class MyClass{
  SpecialFunction myFunc;
  MyClass({this.myFunc = _myDefaultFunc}){...}
  static ReturnType _myDefaultFunc(FirstParameterType firstParameter, 
                                   SecondParameterType secondParameter){...}
}