Choose a primitive

No primitive covers every case, and the essay argues none should. Answer a few questions about your data to find the primitive that fits. The matrix shows how they differ.

Bringing all of these reactive concepts into a single framework gives us an opportunity to tell a coherent story about reactive programming, promotes a better understanding about what tool is right for the job, and obviates the debate over whether any single primitive is a silver bullet.

How many values?

	Number	Axis	Cast	Cancel	Info	Control	Lossless
Value	singular	spatial	—	no	one way	—	yes
Iterator	plural	spatial	unicast	yes	two way	pull	yes
Promise	singular	temporal	broadcast	no	one way	push	yes
Task	singular	temporal	unicast	yes	two way	push	yes
Stream	plural	temporal	unicast	yes	two way	pressure	yes
Signal	plural	temporal	broadcast	no	one way	push	no
Behavior	plural	temporal	broadcast	no	one way	poll	no

Passing one primitive to another

Promise → Task

coerce either way; a task forks into a promise

Signal → Behavior

an observable's next() polls the latest value

Behavior → Signal

sample on a clock, push the samples

Signal → Stream

buffer pushed values so none are lost

Stream → Signal

drop pressure, broadcast each value

Iterator → Stream

Stream.from(iterable) lifts it onto the time axis

Stream → Promise

all(), join(), read(): collect into one value

why it mattersThere is a tension between cancelability and robustness. Promises guarantee that consumers and producers can't interfere; tasks and streams cooperate, so they can cancel.
why it mattersUse pressure to handle resource contention when every value matters. Use push (signals) or poll (behaviors) to skip states that don't.
why it mattersThe primitives are related, so converting between them is cheap. Promises and tasks make a good channel for plural signals and behaviors.