haskell-jp / questions #82

GHCi, version 8.8.3:   :? for help
ghci> :set -XTypeFamilies
ghci> :set -XGADTs
ghci> type family F a
ghci> data T a b c = MkT b (F c)
ghci> :i T
type role T phantom representational nominal
data T a b c = MkT b (F c)
        -- Defined at <interactive>:4:1

daysUntilNext :: WeekDay -> Date -> Int
daysUntilNext nextWeekDay baseDate =
    if nextWeekDay > baseWeekDay
      then fromEnum nextWeekDay - fromEnum baseWeekDay
      else fromEnum nextWeekDay - fromEnum baseWeekDay + 7
    where
      baseWeekDay = getWeekDay baseDate

(fromEnum nextWeekDay - fromEnum baseWeekDay) `mod` 7

   enumFrom     x   = enumFromTo     x maxBound
   enumFromThen x y = enumFromThenTo x y bound
     where
       bound | fromEnum y >= fromEnum x = maxBound
             | otherwise                = minBound