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Demonstration of incorrect (Enum x, Enum y) => Enum (x, y) instance
-- See http://stackoverflow.com/questions/9967790/instance-in-haskell
instance (Enum x, Enum y) => Enum (x, y) where
fromEnum (x,y) =
k^2 + 2*j + if permuted then 1 else 0
where
k = max (fromEnum x) (fromEnum y)
j = min (fromEnum x) (fromEnum y)
permuted = (fromEnum y) > (fromEnum x)
toEnum n =
let k = floor . sqrt $ fromIntegral n
(j, permdAdd) = (n-k) `divMod` 2
permute (x,y) | permdAdd>0 = (toEnum y, toEnum x)
| otherwise = (toEnum x, toEnum y)
in permute (k, j)
data A = A1 | A2 deriving (Show, Enum)
data B = B1 | B2 | B3 deriving (Show, Enum)
main = do
print [(A1,A1)..(A2,A2)]
print [(B1,B1)..(B3,B3)]
print [(A1,B1)..(A2,B3)]
print [(B1,A1)..(B3,A2)]
-- Program crashes, as expected, since creating such an instance is impossible.
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