SML and functional coding style

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?

pad · Accepted Answer · 2013-01-17 09:01:08Z

13

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)

answered Jan 17, 2013 at 9:01

pad

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Comments

Jesper.Reenberg · Accepted Answer · 2013-01-17 17:48:46Z

In general when you have something on the form

if b then
  true
else
  false

you should exchange it with just b, as it is seen trivially to be the same. The solution provided by pad would probably also have been my solution, as it is nice and short.

However when you end up having those nasty/nested if-then-else's, and you don't return something simple (e.g., true/false or a number), then you should consider using a case. Your function is not a prime candidate to use, however I hope the below still shows the idea (that you easily can make structure of those nested if's)

fun is_older((y1, m1, d1), (y2, m2, d2)) =
    case (Int.compare(y1,y2), Int.compare(m1,m2), Int.compare(d1, d2)) of
      (LESS , _    , _   ) => true
    | (EQUAL, LESS , _   ) => true
    | (EQUAL, EQUAL, LESS) => true
    | _ => false

Rupam · Accepted Answer · 2013-01-26 04:11:42Z

1

fun is_older((yr1 : int , mo1 : int , dt1 : int), (yr2 : int , mo2 : int , dt2 : int )) =
  yr1 < yr2 orelse (yr1 = yr2 andalso mo1 < mo2) 
  orelse (yr1 = yr2 andalso mo1 = mo2 andalso dt1 < dt2)

edited Jan 26, 2013 at 4:11

answered Jan 24, 2013 at 22:06

Rupam

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1 Comment

Yaroslav Over a year ago

Adding some comments will help others understand your answer propposal.

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