spazm's solution

to Leap in the Haskell Track

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.

If your language provides a method in the standard library that does this look-up, pretend it doesn't exist and implement it yourself.


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.


To complete this exercise you need to implement the function isLeapYear, that takes a year and determines whether it is a leap year.

You can use the provided signature if you are unsure about the types, but don't let it restrict your creativity:

isLeapYear :: Integer -> Bool

Getting Started

For installation and learning resources, refer to the exercism help page.

Running the tests

To run the test suite, execute the following command:

stack test

If you get an error message like this...

No .cabal file found in directory

You are probably running an old stack version and need to upgrade it.

Otherwise, if you get an error message like this...

No compiler found, expected minor version match with...
Try running "stack setup" to install the correct GHC...

Just do as it says and it will download and install the correct compiler version:

stack setup

Running GHCi

If you want to play with your solution in GHCi, just run the command:

stack ghci

Feedback, Issues, Pull Requests

The exercism/haskell repository on GitHub is the home for all of the Haskell exercises.

If you have feedback about an exercise, or want to help implementing a new one, head over there and create an issue. We'll do our best to help you!


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

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.


{-# OPTIONS_GHC -fno-warn-type-defaults #-}
{-# LANGUAGE RecordWildCards #-}

import Data.Foldable     (for_)
import Test.Hspec        (Spec, describe, it, shouldBe)
import Test.Hspec.Runner (configFastFail, defaultConfig, hspecWith)

import LeapYear (isLeapYear)

main :: IO ()
main = hspecWith defaultConfig {configFastFail = True} specs

specs :: Spec
specs = describe "isLeapYear" $ for_ cases test

    test Case{..} = it explanation assertion
        explanation = unwords [show input, "-", description]
        assertion   = isLeapYear (fromIntegral input) `shouldBe` expected

data Case = Case { description :: String
                 , input       :: Integer
                 , expected    :: Bool

cases :: [Case]
cases = [ Case { description = "year not divisible by 4: common year"
               , input       = 2015
               , expected    = False
        , Case { description = "year divisible by 4, not divisible by 100: leap year"
               , input       = 1996
               , expected    = True
        , Case { description = "year divisible by 100, not divisible by 400: common year"
               , input       = 2100
               , expected    = False
        , Case { description = "year divisible by 400: leap year"
               , input       = 2000
               , expected    = True
module LeapYear (isLeapYear, isLeapYear') where

isLeapYear :: Int -> Bool
isLeapYear year
    | year `divisible_by` 100 && not (year `divisible_by` 400 ) = False
    | year `divisible_by` 4                                     = True
    | otherwise                                                 = False
        where divisible_by x y = x `mod` y == 0

isLeapYear' :: Int -> Bool
isLeapYear' year
    | not (year `divisible_by` 4)                               =  False
    | year `divisible_by` 100 && not (year `divisible_by` 400 ) = False
    | otherwise                                                 = True
        where divisible_by x y = x `mod` y == 0

What can you learn from this solution?

A huge amount can be learnt from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

  • What compromises have been made?
  • Are there new concepts here that I could read more about to develop my understanding?