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

[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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