On 8 March 2012 18:32, Anthony Cowley
Perhaps Data.Key meets your needs?
http://hackage.haskell.org/packages/archive/keys/2.1.2/doc/html/Data-Key.htm...
Ah, perhaps indeed. Thanks!
On 8 March 2012 19:12, Francesco Mazzoli
The type signature that you wrote is very generic and doesn't help in introducing effects while retrieving the indexed value, which I imagine is what you wanted to do.
Because Maybe is already a monad and it's nice to fail in the monad of choice, e.g. if I'm in the list monad I get empty list instead, or if I'm in the Result monad from JSON it'll fail in there. ‘Course "fail" is suboptimal and MonadError might be better.
I guess you could define a type family for the monad type as well, e.g.:
type family Index f type family IndexMonad f :: * -> *
class Functor f => Indexed f where index :: Index f -> f a -> (IndexMonad f) (Maybe a)
Right, that sounds interesting, similar to Data.Key above! It seems like a type family is a good approach. I'll try this "keys" library out. Grazie mille, a dopo… ;-)