David Roundy wrote:
Try
module Secret (Secret, classify, declassify) where
data Secret a = Secret String a
classify :: String -> a -> Secret a classify pw x = Secret pw x
declassify :: Secret a -> String -> Maybe a declassify (Secret pw x) pw' | pw' == pw = Just x declassify (Secret _ _) _ = Nothing
instance Monad Secret where return = classify "" (Secret pw x) >>= f = case f x of Secret _ y -> Secret pw y
That's just the sort of thing I was looking for. Thanks! Arie Peterson wrote:
What should 'q >>= r' mean, when 'q' and 'r x' are secrets with different passwords? In the code above, the result is a secret with the same password as 'q'. This allows you to declassify any secret without knowing its password:
Yeah, but I think that's easy to fix: make classify and declassify take a set of strings rather than a single string, and then make >>= return a secret containing the union of passwords on both sides.