Watch.MakeSourceMake builds an implementation of watch helpers.
The type for watch handlers.
The type for watch state.
stats t is a tuple (k,a) represeting watch stats. k is the number of single key watchers for the store t and a the number of global watchers for t.
Notify all listeners in the given watch state that a key has changed, with the new value associated to this key. None means the key has been removed.
Watch a given key for changes. More efficient than watch.