days.hs

data Day = Monday | Tuesday | Wednesday | Thursday | Friday | Saturday | Sunday
  deriving (Eq, Ord, Show, Read, Bounded, Enum) 
  
-- and that's it. we have everything we need
-- david@mac:~% stack ghci
-- λ> Monday == Tuesday
-- False
-- λ> Monday == Monday
-- True
-- λ> Tuesday > Monday
-- True
-- λ> minBound :: Day
-- Monday
-- λ> maxBound :: Day
-- Sunday
-- λ> import qualified Data.Set as Set
-- λ> let orderedSet = Set.toList . foldl (flip Set.insert) Set.empty
-- λ> orderedSet [Wednesday, Wednesday, Monday, Tuesday, Friday]
-- [Monday,Tuesday,Wednesday,Friday]
-- λ> read "Monday" :: Day
-- Monday
-- λ> show Tuesday
-- "Tuesday"