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A function that can wait says so with `!`, and so does every call to it

A call that waits, for a timer, a disk, or a server, is a point where other work can run and the world can change. In hd such a function's name ends in !, and so does each call that waits for it, so you can spot every pause point by reading. Like ?, the mark travels up: a function that makes a ! call must be a ! function itself. Writing the call without !, as in fetch_orders(user), does not run it yet; all! takes such calls, runs them as tasks, and waits until all are done. Reading a file or a line of input is such a wait, so a program that does I/O through a ! call starts at main!. A plain main that calls read_line!() is an error: only a ! function may make a ! call. The clock is provided, so a test can use a fake one.

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fn dashboard!(user: string) -> string $ Clock: let (orders, unpaid) = all!(fetch_orders(user), fetch_unpaid(user)) # ← wait for both "$user: $orders orders, $unpaid unpaid invoices"pub fn main!() -> void $ Console: clock := ManualClock::new(Timestamp::from_unix_milliseconds(0)) $.with(Clock=clock): println(dashboard!("ada"))# Delete the `!` from `fn dashboard!` and Run. A function that waits# can't pretend it doesn't:# bang-call-outside-suspension: a bang call needs a suspending function or closure# ── plumbing ──use std.task.alluse std.time.{Clock, ManualClock, Timestamp, ms}# Stand-ins for two services: each waits on the clock it is given.fn fetch_orders!(user: string) -> i32 $ Clock: $.use(Clock).sleep!(120ms) 3fn fetch_unpaid!(user: string) -> i32 $ Clock: $.use(Clock).sleep!(80ms) 1

A function that can wait says so with !, and so does every call to it

A function that can wait says so with `!`, and so does every call to it