If expression

isLeapYear : Int -> Bool
isLeapYear year =
  let
    divisibleBy number = 
      modBy number year == 0 
  in
    if divisibleBy 100 then
      divisibleBy 400

    else
      divisibleBy 4

An if expression (if … then … else …) is a compound expression that uses a test to determine which of two alternatives to evaluate to. Many other languages feature a similar construct, often termed 'ternary operator'.

In this approach

This approach uses exactly two tests to determine whether a year is a leap year.

The first test is for divisibility by 100. Once we know if the year is a multiple of 100, we know which further test to perform.

• If the year is evenly divisible by 100, then divisibleBy 100 will evaluate to True and the entire if expression will evaluate to whatever divisibleBy 400 evaluates to.
• If the year is not evenly divisible by 100, then divisibleBy 100 is False and so the if expression evaluates to divisibleBy 4.

This approach is not as concise as the logical-expression, but it is easier to describe, which makes it easier to comunicate the problem domain.

When to use if?

if expressions might be a good fit when you

• need an expression that
• chooses between exactly two options
• depending on a single Bool.

When you have something other than a Bool, use case instead.