fold
Start with current value zero
and then apply the current and next value to the given function. When upstream completes, the current value is emitted downstream.
Description
Start with current value zero
and then apply the current and next value to the given function. When upstream completes, the current value is emitted downstream.
Note that the zero
value must be immutable.
emits when upstream completes
backpressures when downstream backpressures
completes when upstream completes