Created

Embed URL

HTTPS clone URL

SSH clone URL

You can clone with HTTPS or SSH.

Download Gist

Demonstration of incorrect (Enum x, Enum y) => Enum (x, y) instance

View Enum.hs
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
-- 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.
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment
Something went wrong with that request. Please try again.