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A signature lists what a function touches, so a test can hand it a fake

A function that prints, sends mail, or reads the clock usually looks like any other function: you learn what it touches by reading its body, and testing it means patching globals or adding a mocking framework. In hd a public signature lists what the function needs after $. $ Console says that remind_unpaid writes to the console, and nothing else. The caller provides the console: main passes on the real one, and the test uses $.with to put a recording MemoryConsole in its place for one block, then reads what was written. A test gets no real console at all, so a test that forgets the fake does not compile. Your own traits work the same way: a Mailer, a Clock, a PaymentGateway.

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pub fn remind_unpaid(invoices: List[Invoice]) -> void $ Console: # ← its whole reach for invoice in invoices: if !invoice.paid: println("Reminder: invoice ${invoice.id} is due")tests: it("reminds only the unpaid invoices"): console := MemoryConsole::new() $.with(Console=console): # ← a recording console, for this block only remind_unpaid(sample()) assert_equal(console.output(), ["Reminder: invoice INV-7 is due", "\n"], reason="one reminder in two writes")# Press Test to run the test. Then delete ` $ Console` from the# first line and Run. A public function can't hide what it touches:# missing-requirement: this needs `$ Console`, which the enclosing function's row does not name# ── plumbing ──use std.console.MemoryConsolepub struct Invoice: id: string paid: boolfn sample() -> List[Invoice]: [Invoice { id: "INV-7", paid: false }, Invoice { id: "INV-8", paid: true }]pub fn main() -> void $ Console: remind_unpaid(sample())