Streams provide a simple but powerful streaming abstraction. They consist of two concepts: Sinks and Sources.

Sinks

Sinks are stream consumers. More concretely, they describe how to reduce a stream, by providing three actions:

• onStart: () => Task<State>
• onData: (s: State, i: Input) => Task<State>
• onEnd: (s: State) => Task<Result>

Those three actions are called in order during the lifecycle of the stream. When a stream starts, onStart is called, returning a task that results in an initial state. As data flows through the stream, onData is called multiple times with the current state and a bit of input, providing the next state. The next onData won't be called unless the task returned from the previous call terminates, providing a way to handle back-pressure. At the point where the stream is exhausted, onEnd will be called once with the last state, finalizing it into the end result.

Sources

Sources are the objects that take a sink and run it. Two sources of the same type can be combined into a new source, which provides a simple way to build up sources from lots of different parts, concatenating many effectful data fetches.