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Haskell Quiz/DayRange/Solution Jethr0

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-- > dayRange [1,2,3,6,7]
-- "Mon-Wed, Sat, Sun"
data Weekday = Mon | Tue | Wed | Thu | Fri | Sat | Sun deriving (Show,Enum)
 
dayRange :: [Int] -> String
dayRange = sepComma . map range . map (map toWeekday) . groupAscend . sort
    where sepComma    = concat . intersperse ", "
          toWeekday x = show $ (toEnum (x-1) :: Weekday)
          range xs | length xs < 3 = sepComma xs
                   | otherwise     = head xs ++ "-" ++ last xs
 
-- group list of numbers into directly ascending subgroups
groupAscend :: [Int] -> [[Int]]
groupAscend (x:xs) = together $ foldl ascend ([],[x]) xs
    where ascend (done,curr) e = if e == (last curr)+1 then (done,         curr++[e])
                                                       else (done++[curr], [e])
          together (a,b) = a++[b]