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isReset is a function that identifies events with a special property: They reset the state to some value that only depends on this specific event and the initial state. That means no previous event has any bearing whatsoever on the state after the reset event has been applied.

When an event appears where isReset returns true, then the Pond will first reset the state to the initial state, and then apply the event.

When an event appears that is older than the most recent reset event, it is simply ignored, rather than causing time travel as it would normally do.

Performance Implications#

Suppose a stream of events recording the current value of something, like the temperature of a room. To get at the current value of the room, we write a very simple Fish:

// Just set the state to the new value.
const onEvent = (_previousTemperature: number, current: TemperatureEvent) => current.temperature
const currentTemperatureFish = {
initialState: NaN,
isReset: (current: TemperatureEvent) => true
// etc.

Without defining isReset, we would lose a lot of performance due to

  • going through all historic events

  • time traveling when events occur out of order

By defining isReset, we disregard historic events and avoid unnecessary time travel.