E4127

Type is not an error type.

MoonBit only allows error types (defined with suberror) to appear after raise in the return type of a function.

Erroneous example

pub fn may_raise_error() -> Unit raise String {
  return
}

Suggestion

You can wrap the type you wish to raise in an error type:

pub suberror StringError {
  StringError(String)
}

pub fn may_raise_error() -> Unit raise StringError {
  raise StringError("Failed")
}