Atoms are lazily evaluated. If a state initializer function is used, it won't be executed until the
.value getter is read, or the atom is subscribed using
.watch for the first time.
Lazy evaluation in derived atoms
A derived atom can be created by using the callback argument of a state initializer function.
$sum atom is also lazily evaluated. Also, there's more to this. When the last listener of
$sum unsubscribes, the atom goes back to idle mode, and its state initializer stops running.
After the atom went back to idle, let's say we read the
.value getter. If the dependencies aren't changed, we can still avoid an extra execution of the state initializer, because even though the atom is idle, it's aware of its dependencies (
beta), because they are collected during the latest run.