data Day = Mon | Tue | Wed | Thu | Fri | Sat | Sun
deriving (Eq, Show)
fromDayToNum :: Day -> Int
fromDayToNum day = case (day) of
Mon-> 0
Tue -> 1
Wed -> 2
Thu -> 3
Fri -> 4
Sat -> 5
Sun -> 6
fromNumToDay :: Int -> Day
fromNumToDay number = case (number) of
0 -> Mon
1 -> Tue
2 -> Wed
3 -> Thu
4 -> Fri
5 -> Sat
6 -> Sun

isWorkDay :: Day -> Bool
isWorkDay day
| (fromDayToNum day >= 0 && fromDayToNum day <= 4) = True
| otherwise = False

isWorkDay' :: Day -> Bool
isWorkDay' day = case (day) of
(fromDayToNum day >= 0 && fromDayToNum day <= 4)-> True
_-> False
何故、isWorkDay'　はfromDayToNumを使えないのでしょうか？