David Roundy wrote:
Luke Palmer 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
If you mean what I think you mean by "dynamic", that these are runtime
permissions, then you're not going to get the type checker to check
them... of course. What did you mean by dynamic?
At the simplest (and stupidest) level, one could define
data CanReadA
data CanReadB
-- etc
data HavePermission perms where
HaveAPerm :: HavePermission CanReadA
HaveBPerm :: HavePermission CanReadB
and if you then restricted access to the constructors of HavePermission,
you could write code like
data RestrictedData permrequired a = Data a
-- constructor obviously not exported, or you'd lose any safety
readRestrictedData :: HavePermission perm -> RestrictedData perm a -> a
and now if you export readRestrictedData only, then only folks with the
proper permissions could access the data (and this could be done at
runtime).
At runtime, are you sure? I mean, if I want to use it like in
foo = ... readRestrictedData permission secret ...
I must know that the type of my permission matches the the type of
secret , either by knowing them in advance or by propagating this as
type variable into the type of foo. In any case, I may only write foo
if I know statically that permission is going to match the secret .
No runtime involved?
In other words, I fail to see how this GADT example is different from a
normal phantom type (modulo different naming)
data Permission p = Permitted -- not exported
low :: Permission Low -- same role as HaveAPerm
high :: Permission High -- a module which knows these constants has
-- corresponding permissions
data Restricted p a = Restricted a
readRestricted :: Permission p -> Restricted p a -> a
Regards,
apfelmus
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