[Haskell-cafe] Enum instantiation

[R J]

I'd like to make "Day" an instance of class "Enum," but the definition of toEnum below seems to be completely wrong, because integers seem not permit pattern matching.  How is toEnum defined?  Thanks.
data Day                   =  Sunday                           |  Monday                           |  Tuesday                           |  Wednesday                           |  Thursday                           |  Friday                           |  Saturday                           deriving (Eq, Ord, Show)
instance Enum Day where    fromEnum Sunday        =  0    fromEnum Monday        =  1    fromEnum Tuesday       =  2    fromEnum Wednesday     =  3    fromEnum Thursday      =  4    fromEnum Friday        =  5    fromEnum Saturday      =  6        toEnum 0               =  Sunday    toEnum 1               =  Monday    toEnum 2               =  Tuesday    toEnum 3               =  Wednesday    toEnum 4               =  Thursday    toEnum 5               =  Friday    toEnum 6               =  Saturday 		 	   		  
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