On Oct 10, 2006, at 12:04 PM, Seth Gordon wrote:
data Secret a = Secret {password :: String, value :: a}
classify :: String -> a -> Secret a
classify = Secret
declassify :: String -> Secret a -> Maybe a
declassify guess (Secret pw v) | guess == pw = Just v
| otherwise = Nothing
Put that in a module, do not export the Secret data type, and you're
good to go. I'm unsure what a Monad is giving you....
I was just curious if I could do that within a monad.
If the answer to my question is "no, you can't", then I'll pick up the
shattered pieces of my life and move on. :-)
I think you can. Your original monad is just a little too
simplistic. Try something like this (untested):
import Control.Monad.State
type Password = String
type Secret s a = State (Password -> Maybe s) a
classify :: Password -> s -> Secret s ()
classify pw s = put (\x -> if x == pw then Just s else Nothing)
declassify :: Password -> Secret s (Maybe s)
declassify pw = get >>= \f -> return (f pw)
runSecret :: Secret s a -> a
runSecret m = runState m (const Nothing)
Note how this relies on "opaque" functions to hide the secret. This
wouldn't work if Haskell had intensional observation of functions,
although you could still use a newtype in that case.
Slightly related: I've sometimes wondered about a monadic API for
cryptographic primitives. With compiler support you could do nifty
things like make sure to use non-swappable memory for encryption keys
and use fancy special purpose hardware for cryptographic primitives,
if available. The API would give a nice way to ensure proper
information hiding policy. Has anything like this been done or studied?
Rob Dockins
Speak softly and drive a Sherman tank.
Laugh hard; it's a long way to the bank.
-- TMBG
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