Case expression

isLeapYear : Int -> Bool
isLeapYear year =
  let
    isDivisibleByFour = (modBy 4 year) == 0
    isDivisibleBy100 = (modBy 100 year) == 0
    isDivisibleBy400 = (modBy 400 year) == 0
  in
    case (isDivisibleByFour, isDivisibleBy100, isDivisibleBy400) of
      (True, _, True) -> True
      (True, False, _) -> True
      _ -> False

In this approach

In this approach we use a case expression, and match on a [Tuple][tuple]. It turns out that, if we are careful to ask questions in the right order, we can always potentially attain certainty about the answer by asking one more question. This is a [truth-table][truth-table] like approach, which can be easier to read for complicated logic.

When to use a case expression?

Using a case expression with a Tuple is idiomatic in Elm, but tuples have a maximum of 3 values, so can't be used for anything larger than this.

Strings and lists can hold more values and can also be used with case expressions, which are useful in many circumstances. Using a list with a case expression and recursion is especially common.

[tuple] https://elmprogramming.com/tuple.html "Tuples in Elm" [truth-table]: https://brilliant.org/wiki/truth-tables/ "Truth tables"