Heinrich Apfelmus wrote:
Job Vranish wrote:
I've been in a situation a lot lately where I need to keep a collection of values, and keep track of them by a persistent index.
module Store (Key, Store, empty, add, delete, lookup) where
newtype Key = Key { int :: Int }
empty :: Store a add :: a -> Store a -> (Key, Store a) delete :: Key -> Store a -> Store a lookup :: Key -> Store a -> Maybe a
This way, the user doesn't know and care how Key is implemented.
Last but not least, there is the issue that trying to use an already deleted key might yield a wrong result instead of an error. That shouldn't happen if used correctly, but might give a headache when debugging.
There is even a very simple way to prevent at least some cases of misuse, when one key is accidentally used on stores of different type. A phantom parameter will do the trick: newtype Key a = Key { int :: Int } add :: a -> Store a -> (Key a , Store a) delete :: Key a -> Store a -> Store a lookup :: Key a -> Store a -> Maybe a Regards, apfelmus -- http://apfelmus.nfshost.com