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

You can define the default function as private method :

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

Or check argument like this :

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

Or like Ladicek suggests :

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