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
For installation and learning resources, refer to the exercism help page.
To run the test suite, execute the following command:
stack test
No .cabal file found in directory
You are probably running an old stack version and need to upgrade it.
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
If you want to play with your solution in GHCi, just run the command:
stack ghci
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
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
where
test Case{..} = it explanation assertion
where
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) where
isLeapYear :: Integer -> Bool
isLeapYear year =
if (mod year 4) == 0
then do
if (mod year 100) == 0
then (mod year 400) == 0
else True
else False
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Community comments
In Haskell do is not used for code blocks but for monadic computations, see here. To sum it up: You can (and should) remove it from your code. :) Haskell just uses indentation to mark a code block (like you have it already).
Instead of a nested if-else you could consider using guards. They are often more readable.
Another idea that could be applied here: If you have an if-else expression that evaluates to a Bool you can transform it into just a Bool expression. For example if p1 then True else if p2 then True else False
can be rewritten as p1 || p2
Here is some more on this (sorry for the Java code, couldn't find something similar for Haskell).
And finally you might have a helper function to reduce the code duplication.
@abo64 Thanks for the input! Haskell and functional programming is mostly brand new to me. Will read more and revisit this problem.