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noahsummers's solution

to Leap in the OCaml Track

Published at Feb 11 2021 · 0 comments
Test suite

Given a year, report if it is a leap year.

The tricky thing here is that a leap year in the Gregorian calendar occurs:

on every year that is evenly divisible by 4
  except every year that is evenly divisible by 100
    unless the year is also evenly divisible by 400

For example, 1997 is not a leap year, but 1996 is. 1900 is not a leap year, but 2000 is.


Though our exercise adopts some very simple rules, there is more to learn!

For a delightful, four minute explanation of the whole leap year phenomenon, go watch this youtube video.

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JavaRanch Cattle Drive, exercise 3 http://www.javaranch.com/leap.jsp


(* leap - 1.5.1 *)
open OUnit2
open Leap

let ae exp got _test_ctxt = assert_equal exp got ~printer:string_of_bool

let tests = [
  "year not divisible by 4 in common year" >::
  ae false (leap_year 2015);
  "year divisible by 2, not divisible by 4 in common year" >::
  ae false (leap_year 1970);
  "year divisible by 4, not divisible by 100 in leap year" >::
  ae true (leap_year 1996);
  "year divisible by 100, not divisible by 400 in common year" >::
  ae false (leap_year 2100);
  "year divisible by 400 in leap year" >::
  ae true (leap_year 2000);
  "year divisible by 200, not divisible by 400 in common year" >::
  ae false (leap_year 1800);

let () =
  run_test_tt_main ("leap tests" >::: tests)
let divisible_by d n = n mod d = 0
let even_divisor_for n d = n |> divisible_by d

let leap_year y =
  List.filter (even_divisor_for y) [4;100;400] |> function
  | [4] | [4;100;400] -> true
  | _ -> false

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noahsummers's Reflection

This probably isn't fast or efficient; I just wanted to try writing a clearly readable function without using if/else/when/etc, and without requiring the reader to map literal true/false values to a series of statements on a different line (my original function matched against a tuple of three booleans).