Lab

Pure functions

A calculation and its caller

Add the helper below beside remainingAt, then replace the block from const today through the second output with the shown calls. Run it and compare first and second: the same readDay function is supplied twice, with the service day advanced between calls. Inspect the audit counts too. Restore code returns to the sampled-day version.

// Add this helper beside the calculation:
function remainingUsing(trial: Trial, clock: () => number): number {
	return remainingAt(trial, clock());
}

// Replace the block from the sampled-day declaration
// through the second output with:
const first = remainingUsing(trial, readDay);
console.log(`first: ${first}`);
serviceDay = 41;
const second = remainingUsing(trial, readDay);
console.log(`second: ${second}`);

What to look forAn input value versus a way to obtain one

The sampled day stays 40, so both calculations return 3. The supplied clock can instead read day 40 and then day 41, producing 3 and 2. Neither calculation appends an audit entry; the shell does.

type Trial = { endsOn: number };
let serviceDay = 40;
const audit: string[] = [];
const readDay = () => serviceDay;

// #region core
function remainingAt(trial: Trial, today: number): number {
	const daysLeft = Math.max(0, trial.endsOn - today);
	return daysLeft;
}
// #endregion

function inspectTrial(trial: Trial): number {
	const today = readDay();
	const daysLeft = remainingAt(trial, today);
	audit.push(`inspected:${daysLeft}`);
	return daysLeft;
}

const trial: Trial = { endsOn: 43 };
const today = readDay();
const first = remainingAt(trial, today);
console.log(`first: ${first}`);
serviceDay = 41;
const second = remainingAt(trial, today);
console.log(`second: ${second}`);
console.log(`audit before shell: ${audit.length}`);
const inspected = inspectTrial(trial);
console.log(`shell: ${inspected}`);
console.log(`audit after shell: ${audit.length}`);

SERVICE DAY·

Time outside the calculation

The calendar can advance. A captured value stays put.

01First calculationWaiting for the call

·answer pending

02Repeat calculationWaiting for the call

·answer pending

An explicit inspection

This action has its own clock read and audit write.

Audit journal

Not read yetObserved local audit list

The caller has a trial to inspect. First, it needs a day to use.