Chapter 8

Parallel and Watch

Bounded parallel work, cancellation, and filesystem event streams.

Bounded parallelism

let urls = [
    "https://example.com",
    "https://example.org"
]

urls
    |> parallel(2) { url ->
    http.get(url)
        |> timeout(5s)
        |> send
}
    |> print

parallel(n) runs at most n isolated workers and emits results in input order. One failure cancels the remaining tasks. Values crossing worker boundaries must pass Sendable validation.

Watch filesystem changes

watch(path("./src"))
    |> where { event -> event.kind == "write" }
    |> debounce(300ms)
    |> for_each { event ->
    print(event.path)
}

watch returns an infinite FileEvent Stream. Compose automation with debounce, where, take, and other Flow operators; Ctrl+C uses the same cancellation path.