Question

I’m start to learn Standard ML with Programming languages course.

In the first homework, I try to write a function is_older that takes two dates and evaluates to true or false. It evaluates to true if the first argument is a date that comes before the second argument (If the two dates are the same, the result is false.).

So I write the following code:

fun is_older(first: int * int * int, second: int * int * int) =
  if(#1 first = #1 second andalso #2 first = #2 second andalso #3 first = #3 second) then false
  else if (#1 first < #1 second) then true
  else if (#1 first = #1 second andalso #2 first < #2 second) then true
  else if (#1 first = #1 second andalso #2 first = #2 second andalso #3 first < #3 second) then true
  else false

The code is works fine, but it looks ugly.

How can I rewrite this code in functional style?

Answer 1

Two suggestions:

• Use pattern matching to decompose tuples.
• Use boolean operators (andalso, orelse, etc.) when if/else constructs return boolean.

A more readable version:

(* Compare two dates in the form of (year, month, day) *)
fun is_older((y1, m1, d1), (y2, m2, d2)) =
  y1 < y2 orelse (y1 = y2 andalso m1 < m2) 
  orelse (y1 = y2 andalso m1 = m2 andalso d1 < d2)