E4077

Don’t know how to derive trait for type.

MoonBit allows you to derive implementation of some pre-defined traits for your types. It means that you cannot have MoonBit automatically derive a trait that you defined yourself.

错误示例

trait T {
  f(Self) -> Int
}

type A Int derive(T) // Error: Don't know how to derive trait T for type A

建议

You can implement the trait manually:

trait T {
  f(Self) -> Int
}

// Remove derive(T)
type A Int

impl T for A with f(self : A) -> Int {
  0
}