E4125

? operator cannot be used with ...

.. operator allows chaining of method call that returns Unit. This is especially useful when the method performs side-effects, like modifying the object itself, and thus returns Unit.

Using ? materializes the error that will be raised by the method to Result[T, E]. If one combine the ? with .., the method call will return Result[T, E] instead of Unit, which is not allowed by the .. operator.

错误示例

type A Double derive(Show)

pub fn A::check(self : A) -> Unit! {
  if self._.is_nan() {
    fail!("NaN")
  }
}

pub fn A::div(self : A, other : A) -> A {
  A(self._ / other._)
}

fn main {
  let a : A = 1.0
  let b : A = 2.0
  println(a.div(b)..check?())
}

建议

Use the single dot syntax to call the method, and store the result in a variable.

fn main {
  let a : A = 1.0
  let b : A = 2.0
  println(a.div(b).check?())
}