SML if and else if // boolean fuction - if-statement

Question

I am solving this exercise that takes 2 tupples in SML (intintint) (date month and year)
"Write a function is_older that takes two dates and 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.) "
this is what i wrote :
fun is_older (x int*int*int_,y int*int*int_) =
let val r1 = #1 x
val r2 = #2 x
val r3 = #3 x
val t1 = #1 y
val t2 = #2 y
val t3 = #3 y
if r3 < t3 return true
else if r2 < t2 && r3 == t3 return true
else if r1 < t1 && r2 == t2 && r3 == t3 return true
else return false
there is an error in line 10 where we have the "else if r2 < t2 && r3 == t3 return true " . I am new to sml and i dont really know how to solve this issue .
and also can we return a boolean in SMl and how do we pass on values to it (like do we have a main fuction like in java )
i googled the issue and didnt really get an answer pls help me
type here

Answer 1

You have several issues with your syntax. I'm going to guess you meant:
fun is_older (x : int*int*int, y : int*int*int) =
let
val r1 = #1 x
val r2 = #2 x
val r3 = #3 x
val t1 = #1 y
val t2 = #2 y
val t3 = #3 y
in
r3 < t3 orelse
(r2 < t2 andalso r3 = t3) orelse
(r1 < t1 andalso r2 = t2 andalso r3 = t3)
end
But pattern-matching can make that:
fun is_older ((r1, r2, r3), (t1, t2, t3)) =
r3 < t3 orelse
(r2 < t2 andalso r3 = t3) orelse
(r1 < t1 andalso r2 = t2 andalso r3 = t3)
|| is replaced with orelse. && with andalso. The = operator is used for equality checks in SML, rather than ==. There is no need for a return as a function returns the last expression in it. The types are correctly inferred without having to be explicitly annotated.
Let bindings follow the form let ... in ... end.
Pattern-matching removes the need for that construct entirely.