handling exceptions in ML

问题

everyone, I'm trying to understand how exceptions work in ML, but I have strange error, and I can't figure out what is wrong:

exception Factorial 

fun checked_factorial n =
  if n < 0 then
    raise Factorial 
  else n;

fun factorial_driver () =
    checked_factorial(~4)
  handle
    Factorial => print "Out of range.";

what may be wrong? thanks in advance for any help.

回答1:

You need to make sure that factorial_driver has a consistent type. The non-exceptional case returns int, so ML infers the function to be of type unit -> int, but the exceptional case (that is, the print expression) returns unit, not int.

Generally, you basically need to return a value of the same type in all cases.

来源：https://stackoverflow.com/questions/4497243/handling-exceptions-in-ml