[Haskell-cafe] Re: Role based access control via monads or arrows or... something

[apfelmus]

David Roundy wrote:
> apfelmus wrote:
>> David Roundy wrote:
>>> porrifolius wrote:
>>>>  (7) ideally required permissions would appear (and accumulate) in
>>>> type signatures via inference so application code knows which are
>>>> required and type checker can reject static/dynamic role constraint
>>>> violations
>>
>> In other words, I fail to see how this GADT example is different from a 
>> normal phantom type (modulo different naming)
> 
> The difference is that I can inspect at runtime what permissions I have.  I
> see that I didn't demonstrate this.  You can introduce a function
> 
> checkPerms :: HavePermission p -> HavePermission p' -> EqCheck
> checkPerms HaveAPerm HaveAPerm = IsEq
> checkPerms HaveBPerm HaveBPerm = IsEq
> checkPerms _ _ = NotEq
> 
> data EqCheck a b where
>   IsEq :: EqCheck a a
>   NotEq :: EqCheck a b
> 
> which allows you to compare permissions at runtime and make use of them.

Ah, so you are able to use case expressions

    casePerm :: (Permission Low -> a) -> (Permission High -> a)
             -> Permission any -> a

which is not possible with a plain phantom type approach. One example 
use would be

    foo :: Permission p -> Either String Bar
    foo = casePerm (const $ Left "foo: permission too low")
                   (\p -> readRestricted p ... )


Of course, you may not export  HaveAPerm  and  HaveBPerm  (at least not 
for construction, only for pattern matching), so you probably need such 
a special function  casePerm  anyway.


Regards,
apfelmus