(Though I seem to recall the monadic return value being frowned upon
but I don't recall why.)
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.
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)
Francesco.
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