Go-datastore and return values

Datastore.Get returns its result as a return value, which means that the Datastore implementation needs to allocate it. But if one wishes to store arbitrary types of data into a datastore, how can Get know what type of value to allocate? The only ways I can think of are
- either the Datastore implementation needs to memoize the reflect.Type of every value as it is Put, so it can recall the type based on the key when it is Get, or
- the Datastore implementation takes on initialization a mapping from key prefixes/types/something else to reflect.Types, so it can again determine the correct type to allocate based on the key,

but these would bring considerable complexity to any Datastore implementation that wishes to allow storage and retrieval of arbitrarily typed data. Am I missing something obvious, or is this by design? Should any unmarshaling from datastore representations to Go objects (and vice versa) be the client code's responsibility?