Add isLeapYear function (#47)

Author: MichaelXavier

src/Data/Date.purs

@@ -7,6 +7,7 @@ module Data.Date
   , day
   , weekday
   , diff
+  , isLeapYear
   , module Data.Date.Component
   ) where
 
@@ -76,6 +77,12 @@ diff :: forall d. Duration d => Date -> Date -> d
 diff (Date y1 m1 d1) (Date y2 m2 d2) =
   toDuration $ runFn6 calcDiff y1 (fromEnum m1) d1 y2 (fromEnum m2) d2
 
+-- | Is this year a leap year according to the proleptic Gregorian calendar?
+isLeapYear :: Year -> Boolean
+isLeapYear y = (mod y' 4 == 0) && ((mod y' 400 == 0) || not (mod y' 100 == 0))
+  where
+  y' = fromEnum y
+
 -- TODO: these could (and probably should) be implemented in PS
 foreign import canonicalDateImpl :: Fn4 (Year -> Int -> Day -> Date) Year Int Day Date
 foreign import calcWeekday :: Fn3 Year Int Day Int

test/Test/Main.purs

@@ -98,6 +98,12 @@ main = do
   log "Check that diff behaves as expected"
   assert $ Date.diff d2 d1 == Duration.Days 29.0
 
+  let unsafeYear = unsafePartial fromJust <<< toEnum
+  log "Check that isLeapYear behaves as expected"
+  assert $ not $ Date.isLeapYear (unsafeYear 2017)
+  assert $ Date.isLeapYear (unsafeYear 2016)
+
+
   -- datetime ----------------------------------------------------------------
 
   let dt1 = DateTime.DateTime d1 t1