Specified, not built. No compiler in this repository accepts rxdl. The parser has three profiles, typl, ridl and rsdl, so the loader reports a .rxdl file in a package directory with the warning RIDL-417 (unsupported-source-file) and does not compile it, and refuses a lone .rxdl file as the entry. This reference becomes normative when rxdl is implemented, and ridl's boundary-model core (E3) is its precondition. Nothing below can be compiled today. See what is built.

rxdl Language Reference

The unrestricted profile and the domain spellings — the layer of the RIDL family that lifts restrictions rather than adding semantics. A .rxdl file may carry any layer and any interaction domain; the domain spellings give the person and world boundaries readable keywords over ridl's kinds.

Version: 0.1.0 — Draft

Status: draft. No compiler accepts rxdl yet, and part of this reference waits for rmdl; the scope note below says which part.

Provenance. rxdl replaces uxdl. ADR-0012 retired uxdl as a family member on the finding that its content was ridl's and its shape was not: at every boundary with the non-software world the datum and its referent come apart, and the obligations that follow are the same four at every boundary, so they belong in one place. That place is ridl. What is left over — readable words for the combinations ridl already understands, and a file kind that forbids nothing — is this document. Its precursor lineage runs uxil → uxdl → rxdl; uxil's eleven interaction verbs and its corpus-registry discipline are mined in Appendix B.

This document is deliberately thin, and that is the point. ADR-0012 decision 7 gives an extension no grammar, no IR nodes, and no semantics. Everything normative about families, obligations, availability, timing, evolution, and errors lives in the ridl reference and is cited, never restated. uxdl's defect was a document that was mostly pointers while claiming to be a language; rxdl claims to be a spelling table, so pointers are the correct content. If a rule appears here that is not in ridl, it is a bug in this document.

Specified, not built. No compiler in this repository accepts rxdl. This reference lands as normative with the unrestricted profile; ridl's boundary-model core is its precondition. Part of that core is parked; see docs/ROADMAP.md.

Scope, as of 2026-09-12. rxdl stays a member of the family, and the part of it the plan carries is narrower than this document. The unrestricted profile covers types, interfaces and wiring in one file, with the per-package tightening of §9, which the current release plan sequences. The model layer of that profile, the domain spellings of §3 to §5, and the bindings they generate wait for rmdl: the roadmap parks the ecosystem half of rxdl and the spellings with their bindings as one block, reopened by rmdl. Nothing here is retired, only partly deferred. Recorded in docs/wip/2026-09-12-release-scope-and-plugin-system-design.md §3.2.

Table of Contents

1. What rxdl Is
2. The Unrestricted Profile
3. Domains and Families
4. The hmi Domain
5. The env Domain
6. Lowering
7. Admission Tests
8. What rxdl Is Not
9. Package Tightening
10. Diagnostics
11. Open Questions

• Appendix A — Worked Example
• Appendix B — Prior Art: uxil and uxdl

1. What rxdl Is

Two things, and nothing else:

1. A file kind that restricts nothing (§2). Every other profile is a restriction — .typl accepts type declarations, .ridl accepts dispatch-family interactions, .rmdl accepts behaviour. .rxdl accepts all of it.
2. Spelling tables for the non-dispatch families (§4, §5). present is not a new kind; it is a readable way to write the combination ridl already describes as a continuous state value at the person boundary.

The x reads both ways at once — the wildcard over the family pattern (r·i·dl, r·m·dl, r·s·dl) and extended. One rule covers both: .rxdl is the profile with no restrictions.

2. The Unrestricted Profile

Two restrictions are lifted, independently:

• the layer restriction — types, interfaces, behaviour, and wiring may share one file. This is the adoption gradient: a demo, a getting-started guide, or a solo developer writes one file (concept note §4).
• the domain restriction — the person and world spellings of §4 and §5 are available. In a .ridl file only the dispatch family is spelled.

Consequence, recorded. Because a production package that uses the hmi spellings must be .rxdl, the file extension no longer answers "which files contain executable behaviour?" on its own. Package tightening (§9) is the mitigation, and it is where that question is answered.

3. Domains and Families

A family is a property of a declaration and is ridl's (ADR-0012 decision 2). A domain is a packaging unit: which spellings a file may use, and which backends consume them. Domains do not appear in the IR — code generation selects on family.

The four correspondence obligations each family carries, what they mean, and how they compose along a path are ridl §3.3, not restated here. The families themselves, and which kinds each admits, are ridl §3.2.

4. The hmi Domain

The person boundary. presentation is what the system offers to be perceived; intent is what the person means and the system captures.

present  clusterSpeed: IndicatedSpeed @[50ms..200ms]
present  nowPlaying: TrackInfo
notify   overspeedChime: Alert
activate playButton
toggle   muteButton
select   trackItem(id: TrackId)
adjust   volumeSlider(level: Ratio)
dismiss  errorBanner

Payload rules on the operation shapes — none for activate, toggle, and dismiss; exactly one key for select; exactly one ranged value for adjust — are ridl §3.2.1 (RIDL-502), and are what earns those five their keywords under §7.

The set is closed and has no generic. A person performs one act at a time; a multi-parameter operation is a function call, not a gesture. Sending a message is three interactions, not one. Drag-and-drop, stripped of modality, is select then adjust.

The empty cells are consequences, not gaps. Nothing can be invoked on an agent, so presentation has no operations. A person's continuous state is knowable only by measuring a physical proxy, so a pedal is measure (§5), not an intent spelling. A system cannot block on a human, so intent has no query: "are you sure?" is present plus activate/dismiss.

5. The env Domain

The physical boundary. acquisition is what the system takes from the world; control is what the system causes in it.

measure wheelSpeed: Speed @10ms
measure pedalPosition: Ratio @20ms
detect  wheelSlip: SlipEvent
actuate targetAngle: Angle
trigger airbagDeploy(zone: Zone)

Both words are metrology's, not this document's choice: VIM (JCGM 200 / ISO-IEC Guide 99) gives measurement as the process term, and defines a detector as a device indicating the presence of a phenomenon when a threshold is exceeded — which is detect exactly. acquire and sense were weighed and rejected; ADR-0012 decision 4 records why. The obligations those words carry — measurand, calibration, measurement uncertainty, metrological traceability — are ridl's, and VIM is the reference for naming anything further in that layer.

Note pedalPosition. A pedal is measured, not intended: there is no interpretation gap between a foot's position and its meaning, and all four obligations instantiate exactly as they do for a wheel-speed sensor. A person appears at both boundaries — as an agent that means things (hmi) and as a physical object that is measured (env) — and which one applies is decided by whether the value carries meaning or magnitude.

The empty cells, again as consequences. Commanding a device to recalibrate is system → world, so it is control, not acquisition. Nothing leaves the system uninvited, so control has no occurrence. Effect is observed by measuring back and never returned, so control has no query.

6. Lowering

Every spelling resolves to a triple ridl already understands. The mapping is bijective.

fixed is family-neutral: a provisioned constant carries no correspondence at any boundary.

Three properties follow from bijectivity, and each is load-bearing:

• Round-tripping. ridl fmt, IR rendering, and diagnostics reproduce the authored spelling. A present never renders back as a signal.
• One way to say it. The family is an IR field, not an attribute, so [ presentation ] is not an alternative spelling anyone can hand-write.
• No drift. There is no desugaring that could change between releases and make a stored diff baseline disagree with a fresh snapshot on identical source.

(command, intent, no shape) is not a valid triple — §4's closed-set rule expressed structurally rather than as a lint.

7. Admission Tests

Two tests bind every future spelling. Both are ADR-0012 decision 4.

Modality independence. If two realisations afford the same capability they must produce the same declaration. A word naming how is rejected. This is what rules out scroll and drag (gestures — one of several ways to express "show me more" or "put this elsewhere"), scan (a sensing mechanism — the decisive case being a mechanical lidar swapped for a solid-state one: same capability, no sweep, and the contract must not change), and display (visual — a speaker does not display, and a haptic seat has no view).

Each rejected mechanism word named something real one layer down, in the obligation layer rather than here: sample names the sample instant, and scan the acquisition span — a swept frame corresponds to the world over an interval, not an instant. Both are ridl's to carry (ADR-0012 decision 3), not spellings.

Constraint bundling. A spelling is warranted if and only if it bundles constraints that would otherwise be re-declared by hand at every use. A word adding nothing over its core kind is rejected — which is precisely the test uxdl failed.

8. What rxdl Is Not

• Not a language. No grammar of its own. A .rxdl file is parsed by the one family grammar, with no profile restriction applied.
• Not semantics. Families (ridl §3.2), the four obligations (§3.3), availability and its five sources (§3.4), timing, init and invalid channels, errors, ordinals and evolution are all ridl's. This document defines none of them and may not.
• Not IR. family and shape are ridl's IR fields. rxdl adds no node, no field, and no attribute key.
• Not layout, styling, navigation, or localisation. A toggle may scaffold a switch; nothing here says switch, checkbox, or voice command. Screen flow is a graph over interactions and has no home yet (§11).
• Not a second evolution model. ridl-diff classifies a lowered declaration by its core kind, so a spelling cannot weaken the gate.

9. Package Tightening

.rxdl forbids nothing, so strictness is a package decision, declared in ridl.toml (ADR-0002's profile-purity mechanism, not a new one). A production package narrows what its files may contain — permitted layers, permitted domains — and the compiler enforces the narrowing.

This is where "which files contain executable behaviour?" is answered once .rxdl is in production use (§2).

10. Diagnostics

No RXDL- code family exists, and none should. Every rule a spelling can violate is a ridl rule, reported under RIDL-: a payload rule on select, an invalid (kind, family, shape) triple, an availability predicate a consumer cannot evaluate, an obligation missing where its family requires one.

The rules a spelling can violate are allocated: RIDL-501 (kind not admitted by its family), RIDL-502 (shape parameter rule), RIDL-503 (intent operation with no shape), RIDL-504 (non-dispatch interaction with no obligation), RIDL-505 (availability a consumer cannot evaluate), RIDL-506 (obligation on a dispatch interaction), RIDL-507 (derives across an impossible direction), RIDL-508 (control with no paired acquisition).

Two diagnostics are genuinely rxdl's, and both concern the profile rather than the spellings — a domain spelling used in a file whose profile does not permit it, and a layer used in a package that has tightened against it. Their codes are allocated with E7a.

11. Open Questions

1. The intent occurrence keyword. The person-supplies-content spelling — typing, scanning, dictation. Event-shaped: debounce, TTL, no acknowledgment. supply is the working candidate; provide and tell remain live. enter was rejected for modality, submit for implying a commit and an acknowledgment an occurrence does not have.
2. The acquisition/query cell. An explicit one-time read. Deferred to ridl v0.3 with ridl §17.10, which owns it — the cell is reserved in ridl §3.2, and a spelling here cannot land before the cell does. What separates it from measure is maintenance, not duration: measure obliges the provider to hold a current value, and forcing an explicit read into it would make the binding poll forever for something wanted only on request. Periodic and on-change acquisition need no new cell — @[min..max] already distinguishes them, rate floor against staleness bound. read is the candidate spelling. Reopened by a world-boundary contract that needs an on-request read of a quantity too expensive to hold current, which is the observation ridl §17.10 names.
3. Interaction citation paths. The projection from a declaration to the stable identifier that specifications, journeys, tests, and telemetry cite — uxil's founding problem. Not rxdl's: the observability semantic conventions need it for every interaction, so it is ridl's, and it is recorded here only because uxil's readers will look for it.
4. Journeys and screen flow. A sequence of interactions across surfaces. Neither ridl nor rsdl expresses a graph over interfaces. Likely a requirements-and-traceability artifact rather than a language.
5. Accessibility metadata. Roles, labels, reading order. The operation shapes already carry the machine-readable gesture semantics such tooling consumes; whether more is needed is unproven.

Appendix A — Worked Example

package veh.cockpit.cluster

import veh.common.Speed
import veh.common.Angle
import veh.common.Ratio

// --- vocabulary (typl) ---

type IndicatedSpeed: km/h [0.0..250.0 step 1.0]

// --- one interface, four families ---

interface Cluster {

  /// The sensor's reading of the world.
  measure wheelSpeed: Speed @10ms

  /// What the driver reads. Legally may never indicate below true speed.
  present clusterSpeed: IndicatedSpeed @[50ms..200ms]

  /// The driver's foot — measured, not intended: position is meaning.
  measure pedalPosition: Ratio @20ms

  /// Audible warning; momentary, no init value.
  notify overspeedChime: Alert

  /// The setpoint the steering actuator tracks.
  actuate targetAngle: Angle

  /// The driver's acts.
  activate resetTrip
  toggle   headlamps
  adjust   brightness(level: Ratio)
}

Obligation attributes are omitted above; ridl §3.3 carries the working spellings, which settle with the boundary model. The point of the example is that four families sit in one interface, each declaring what it corresponds to, and that the first word of every line says which.

Appendix B — Prior Art: uxil and uxdl

The full uxdl v0.1 reference is preserved at ../archive/uxdl-language-reference-v0.1.md. Read it as prior work, never as current design.

End of rxdl Language Reference v0.1.0 — Draft.