title: how to handle divide by zero error in ML
body: <p>I am new to ML.</p>
  
  <p>I need to define a function taking an conditional expression as argument, the problem is if the expression is invalid like <code>"10 div 0 = 0"</code>. How can I handle this?</p>
  
  <p>For example, the function is defined as following: <code>foo exp1 = if (exp1) then ... else...</code>, and <code>exp1</code> is <code>"10 div 0 = 0"</code>, how to handle this division error.</p>