[Haskell-cafe] a monad for secret information
cgibbard at gmail.com
Mon Oct 9 23:41:00 EDT 2006
On 09/10/06, Seth Gordon <sethg at ropine.com> wrote:
> I finally (think I) understand monads well enough to make one up:
> > module Secret (Secret, classify, declassify)
> > where
> > data Secret a = Secret a
> > classify :: a -> Secret a
> > classify x = Secret x
> > declassify :: Secret a -> String -> Maybe a
> > declassify (Secret x) "xyzzy" = Just x
> > declassify (Secret x) _ = Nothing
> > instance Monad Secret where
> > return = classify
> > (Secret x) >>= f = f x
> The nice thing about this is that (correct me if I'm wrong) clients of
> the module can't sneak information out of the monad without either using
> the right password or by using one of the unsafe*IO methods. (Or by
> running the program in a debugger. But you get the idea.)
> The not-so-nice thing is that the literal text of the password is baked
> into the data definition. I'd like to have a more general version of
> Secret that allows someone to pass the password in when constructing a
> secret, and preserves that password when "return" is used, but doesn't
> let the second argument of (>>=) see the password. Something like this:...
> > data Classification pw a = Classification pw a
> > declassify (Classification pw a) pw' = case pw' of
> > pw -> Just a
> > _ -> Nothing
> > type Secret = Classification "xyzzy"
> ...but that doesn't parse.
> Is it even possible to have a type like this that still observes the
> monad rules? Is this the sort of thing that I need to understand arrows
> to pull off?
Why not just:
secret :: a -> Classification String a
secret = Classification "xyzzy"
The password string isn't part of the type, it doesn't even
necessarily exist at compile time. You might have just got confused
between type and data constructors for a moment.
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