In fact I think these Typesafe MRef's are exactly equivalent to dynamic types. In other words, if you've got one, you've got the other. Ralf Hinze has just shown that if you have dynamic types you can implement Typesafe MRef. The reverse implementation would be something like

data Dynamic = FM ()

toDyn :: a -> Dynamic toDyn a = insert empty () a

fromDynamic :: Dynamic -> Maybe a fromDynamic fm = let bogus :: Key () a (_,bogus) = insert empty () undefined in lookup fm bogus