title: Getting indexes of elements of an array with certain values
body: <p>Consider I have an array of options like <code>[|Some 1;Some0;None;None;Some0|]</code>
  and i am going to get the indexes of elements with <code>None</code> value, in this case the correct answer would be <code>[|2;3|]</code>. </p>
  
  <p>My current idea is to change the array to a list and then go throw it using recursive function but in this case i will need <code>mutable value</code> to compute index, and i do not want to use mutable?</p>
  
  <p>Is there any other solution</p>