E4007

When constructing an enum, the arguments passed to the constructor cannot be unit (). MoonBit does not allow constructors to take unit as an argument because it would be redundant - a constructor without arguments already represents a singleton value.

错误示例

enum Status {
  Done
  Pending
}

fn main {
  let status = Done() // Error: constructor can't take unit as argument
}

建议

Remove the unit argument from the constructor since it adds no value:

// ...
fn main {
  let status = Done
}

If you really need the constructor to take a unit arguments, explicitly use Unit as its argument

enum Status {
  Done(Unit)
  Pending
}

fn main {
  let status = Done(())
}