--- title: Concepts description: Where Yarunoka comes from, and why the Yrnk language is shaped the way it is. sidebar: order: 2 --- ## The question in the name Yarunoka is the Japanese question **やるのか？** (*yaru no ka?*) — roughly "so, do we do it?". That is the question the whole project exists to answer: here is a schedule, here is a moment — do we do it? **Yarunoka** is the project; **Yrnk** is the notation it defines. ## Calendar rules, not clock rules Real-world schedules are calendar rules. "Payday is the 25th — moved up to the previous business day when it falls on a day off." "Collection day is the first and third Friday, skipped on holidays." "The poller runs every 90 minutes, but only within business hours." None of these is a clock rule: they lean on what a *business day*, a *holiday*, an *ordinal weekday* mean, and those meanings belong to calendars and organizations, not to arithmetic on timestamps. Cron expressions and plain timestamps cannot carry such rules, so the rules end up as application code — a shift rule here, a holiday lookup there. Code in that position is hard to store, hard to display back to the person who asked for the rule, and impossible for them to edit safely. Yarunoka began inside a larger application whose schedules were exactly these ordinary calendar rules, and nothing about them was specific to that application — so the language was extracted and specified on its own. ## A description of a set of occurrences A Yrnk document is a pure description of a **set of occurrences** — points in time, or whole days. It is data, not behavior: nothing in the language can start a job, remember a last run, or catch up on missed work. "Should this fire", "last ran at", "run it twice to make up" are simply not words this language has. That line is deliberate, and it is what keeps an engine **pure**. A document describes a set of points; answers appear only when a caller asks a **query**. The evaluation model defines three: is this instant an occurrence (a judgment at a point)? was there an occurrence after the last run, through now (a judgment over a period)? which occurrences lie in this range (an enumeration)? Everything stateful — firing, retries, throttling — stays on the caller's side of the line, where it belongs. ## Definitions and expressions A business day is not a weekday. Weekdays are determined by the calendar alone; business days are decided by an organization — its holidays, its own closures, its weekly pattern. So a document is split in two: - the **calendar** holds the definitions — holidays, the organization's own closures and extra working days, the weekly pattern, business hours, and named date sets - the **schedules** are pure expressions — they refer to that vocabulary by name and carry no definitions of their own The same schedule — `{"days": [25], "shift": ["prev", "or_same", "business_day"], "times": ["10:00"]}` — means the right thing in every organization, because *whose* business days it runs on is the calendar's decision. The vocabulary is answered by stacked layers (`business_days` over `business_holidays` over `holidays` over the `workweek`), so the one question "is this a working day?" has one place to be decided. ## The document is authoritative A document declares how to read it. The timezone is written in the document — the host's locale or default timezone never changes what a schedule means. The spec version is written in the document, and an implementation that does not know that version must reject the document rather than guess. What a document cannot contain — this year's public holidays as computed by a library, closures kept in a database — it declares instead: **resolvers** name what the host must supply. The list is complete by rule: every name that is used and not defined must be declared, so what a document needs is readable from the document alone. Dynamic data flows in, while the document keeps the *intent* — "the days this holiday source says are holidays", not the days that happened to be true when it was written. ## One spelling, closed sets Yrnk is strict on purpose: - every meaning has exactly **one spelling** — no scalar shorthand for one-element lists, zero-padded times, a single way to write "requires nothing" - every vocabulary is a **closed set** — unknown keys and unknown words are errors, never silently ignored One spelling means round-tripping a valid document is the identity, which gives implementations a canonical form to be checked against, mechanically. Closed sets mean a mistake cannot be quietly read as something else — and they widen compatibly: additions raise the minor version, and every existing document keeps its meaning. ## Two planes that never cross A schedule with days and times is a product of two orthogonal planes: days are selected with no knowledge of times, and times are laid on the selected days without moving them. Nothing in the language can make a day selection depend on a time, or a time depend on how its day was chosen. Rules that would cross the planes ("two hours after the same time on the previous business day") are deliberately not expressible — the orthogonality is a design decision, and the specification keeps an explicit list of such deliberate omissions. ## A specification above the implementations Yarunoka puts the specification at the top: the language is defined here, language implementations conform to it, and their agreement is checked by a shared conformance suite rather than by comparing implementations to each other. Two programs in different languages reading the same document reach the same set of occurrences because both answer to this specification — and that is what lets a schedule written once be stored, shipped, and judged anywhere.