I recently experimented with that kinda stuff. See the following:
type family SelectPrivilege a :: Bool
type instance SelectPrivilege ReadTransaction = True
type instance SelectPrivilege WriteTransaction = True
type family UpdatePrivilege a :: Bool
type instance UpdatePrivilege ReadTransaction = False
type instance UpdatePrivilege WriteTransaction = True
data Read
data Write
data Transaction t r
executeUpdateTransaction :: UpdatePrivilege t ~ True => Transaction t r -> IO r
executeUpdateTransaction =
undefined
The above code ensures that transactions of type Transaction Read r cannot
be executed using the executeUpdateTransaction function. However then the
same type level logic can be encoded using an existence of a type class
instance:
class SelectPrivilege t
instance SelectPrivilege ReadTransaction
instance SelectPrivilege WriteTransaction
class UpdatePrivilege t
instance UpdatePrivilege WriteTransaction
data Read
data Write
data Transaction t r
executeUpdateTransaction :: UpdatePrivilege t => Transaction t r -> IO r
executeUpdateTransaction =
undefined
2014-10-16 14:58 GMT+04:00 Ivan Lazar Miljenovic
Using the Constraint type and the ConstraintKinds extension, is there any way we can determine if a Constraint is satisfied (i.e. a type-level function of type Constraint -> Bool using DataKinds)?
-- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com http://IvanMiljenovic.wordpress.com _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe