E4122

E4122#

Invalid raise operation. Can only be used inside a function with error types in its signature.

There are 3 raise operations in MoonBit, and they must be used inside a function with error types in its signature or a try block.

  • Use the raise keyword to raise an error directly.

  • Re-raise an error by annotating the function application with the ! operator.

  • Explicitly handle the error using the try ... catch! ... block. The catch! means re-raise all the errors that are not handled in the catch block.

Erroneous example#

Raise the error directly:

type! ArithmeticError {
  DivisionByZero
}

pub fn checked_div(a : Double, b : Double) -> Double {
  if b == 0.0 {
    raise DivisionByZero // Error: raise can only be used inside ...
  }
  return a / b
}

Re-raise the error:

pub fn rethrow() -> Unit {
  fail!("throwing") // Error: `!` operator will rethrow the error raised in the function application, and can only be used inside ...
}

Catch all errors using catch!:

pub fn catch_all() -> Double {
  try {
    fail!("Failed") // Error: catch! will rethrow unhandled error, and can only be used inside ...
  } catch! {
  }
}

Suggestion#

You can either modify the surrounding function to have error types in its signature:

pub fn checked_div(a : Double, b : Double) -> Double!ArithmeticError {
  ...
}

pub fn rethrow() -> Unit! {
  ...
}

pub fn catch_all() -> Double! {
  ...
}

Or use the try ... catch ... block to handle the error:

pub fn checked_div(a : Double, b : Double) -> Double {
  try {
    if b == 0.0 {
      raise DivisionByZero // Error: raise can only be used inside ...
    }
    return a / b
  } catch {
    _ => {
      println("DivisionByZero")
      @double.not_a_number
    }
  }
}
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