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# not-null's solution

## to Leap in the Elm Track

Published at Aug 12 2020 · 0 comments
Instructions
Test suite
Solution

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.

## Notes

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.

## Elm Installation

Refer to the Installing Elm page for information about installing elm.

## Writing the Code

The code you have to write is located inside the `src/` directory of the exercise. Elm automatically installs packages dependencies the first time you run the tests so we can start by running the tests from the exercise directory with:

``````\$ elm-test
``````

To automatically run tests again when you save changes:

``````\$ elm-test --watch
``````

As you work your way through the tests suite in the file `tests/Tests.elm`, be sure to remove the `skip <|` calls from each test until you get them all passing!

## Source

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

## Submitting Incomplete Solutions

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

### Tests.elm

``````module Tests exposing (tests)

import Expect
import Leap
import Test exposing (..)

tests : Test
tests =
describe "Leap"
[ test "leap year" <|
\() -> Expect.equal True (Leap.isLeapYear 1996)
, skip <|
test "non-leap year" <|
\() -> Expect.equal False (Leap.isLeapYear 1997)
, skip <|
test "non-leap even year" <|
\() -> Expect.equal False (Leap.isLeapYear 1998)
, skip <|
test "century" <|
\() -> Expect.equal False (Leap.isLeapYear 1900)
, skip <|
test "second century" <|
\() -> Expect.equal False (Leap.isLeapYear 1800)
, skip <|
test "fourth century" <|
\() -> Expect.equal True (Leap.isLeapYear 2400)
, skip <|
test "y2k" <|
\() -> Expect.equal True (Leap.isLeapYear 2000)
]``````
``````module Leap exposing (isLeapYear)

isLeapYear : Int -> Bool
isLeapYear year =
modBy 4 year == 0 && not (modBy 100 year == 0 && modBy 400 year /= 0)``````

### What can you learn from this solution?

A huge amount can be learned from reading other peopleâ€™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?