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.

Erroneous example

///|
trait T {
  f(Self) -> Int
}

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

Suggestion

You can implement the trait manually:

///|
priv trait T {
  f(Self) -> Int
}

///|
priv struct A(Int)

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

///|
test {
  let value : A = A(1)
  inspect(value.f(), content="1")
}