[Haskell-cafe] Re: a monad for secret information

2006-10-10 Thread Arie Peterson 

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

 Now return itself doesn't assign a password, but you can classify
 something
 manually, and then perform computations on that data in a safe manner.
 It's just as safe as your code, because the constructor of secret is
 hidden
 which hides the password just as well as the data.

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:

  break :: Secret a - a
  break q = fromJust $ declassify (classify bloep ()  q) bloep

.

-- 

Mr. Pelican Shit may be Willy.

  ^
 /e\
 ---


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Re: [Haskell-cafe] Re: a monad for secret information

2006-10-10 Thread Seth Gordon 

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.


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