E3015

The parameter already has default value None.

In MoonBit, the optional parameter has one of the two following forms:

• Optional parameter with default value:

  fn f(a~ : Int = 0) -> Unit {
    // ...
  }
  

• Optional parameter with no default value. In this case, when the parameter is not provided, it is None by default.

  fn f(a? : Int) -> Unit { // a has type Int?
    // ...
  }
  

Therefore, if the optional parameter has a default value of None, it is redundant and should be removed.

Erroneous example

fn f(a? : Int = None) -> Unit { // Error: The parameter a? already has default value `None`.
  println(a)
}

Suggestion

Remove the = None part from the optional parameter.

fn f(a? : Int) -> Unit {
  println(a)
}