how to handle divide by zero error in ML

Question

I am new to ML.

I need to define a function taking an conditional expression as argument, the problem is if the expression is invalid like "10 div 0 = 0". How can I handle this?

For example, the function is defined as following: foo exp1 = if (exp1) then ... else..., and exp1 is "10 div 0 = 0", how to handle this division error.

Answer 1

It looks like you want to ask about exception handling mechanism in SML.

The div function in SML basis library raise Div exception when you invoke 10 div 0. It depends on whether you need the value or not to handle the exception. You can either return true/false or option type in this case:

(* only catch exception, ignore value *)
fun div_check (x, y) = (
  ignore (x div y);
  false
) handle Div => true

(* catch exception and return option value *)
fun div_check2 (x, y) = (
  SOME (x div y)
) handle Div => NONE

UPDATE:

It is really weird that the compiler doesn't raise Div exception in this case. I suggest that you define a custom div function and raise/handle exceptions yourself:

exception DivByZero;

(* custom div function: raise DivByZero if y is zero *)
infix my_div;
fun x my_div y =
  if y=0 then raise DivByZero else x div y

fun div_check (x, y) = (
  ignore (x my_div y);
  false
) handle DivByZero => true

fun div_check2 (x, y) = (
  SOME (x my_div y)
) handle DivByZero => NONE