5. Syntax

5.1 Lexical

• One line = one command. # begins a line comment.
• The line starts with a command keyword (positional). All remaining arguments MUST be key=value. Positional arguments require remembering argument order and are fragile to order hallucination and omission. Keys like mat= / side= act as attention anchors for an LLM and stabilize generation. Prefer deterministic generation over a small token saving.

window side=front mat_slot=glass offset=2 y=2 size=2x2 sym=true   # OK
window front G 2 2 2x2                                            # forbidden (positional args)

5.2 Nesting

Keep nesting shallow (struct / def / level / room / theme / site). Deep nesting increases LLM generation errors.

5.3 Headers (optional declarations)

Metadata MAY be placed in headers rather than in the semantic body:

@cairn 2026.06                           # optional. The Cairn language version it was written against (CalVer)
@requires version>=1.20                  # capability floor (optional). Conflict with the inferred value → E_REQUIRES_CONFLICT
@intended_targets ["1.20.4","1.21.4"]    # wish/hint. Not a verification record (the record lives in the lock)

• @cairn is the version of the Cairn language itself (see the README’s Versioning). It is a separate axis from @requires / @intended_targets (Minecraft versions). It is optional, and exists as provenance so a future compiler can parse/warn correctly.
• See Versioning and Editions for @requires.
• @intended_targets is a hint about “which Minecraft version it was designed for”, not a claim of being verified. The verified target is recorded only in the lock.

5.4 Selectors (P4)

• Wall selectors: front (+z) / back / left / right. offset runs along the wall; y is measured from the floor (= 0).
• offset origin. offset=0 sits at the wall’s left end viewed from outside that wall. Concretely: front and back walls anchor at low x (front from the +z viewpoint, back mirrored along x so a sym=true opening looks symmetric from either side of the building); left and right walls anchor at low z and mirror analogously. sym=true mirrors the opening across the wall’s midpoint (mirror_offset = wall_length - offset - size_w); a mirror that overlaps the primary rectangle is rejected with a W_DEFERRED_MEMBER and only the primary is painted.
• at= door anchors. A door’s wall-local column is taken from one of three named anchors: at=center picks the column at wall_length / 2 (round-half-up — odd-length walls have a unique geometric centre; even-length walls pick the column to the right of the midpoint so the choice is deterministic), at=left picks the wall-local axis origin (u = 0), and at=right picks the far corner (u = wall_length - 1). The same column resolves both the openings cut and any connect walkway anchored to this door (§9.3.5). Numeric offsets (at=N) are reserved for a future extension.
• Inside reference: prefixed, e.g. inside.front.
• Blocks, block entities, and entities all use the same selector grammar.

5.5 IDs, classes, addresses

• Important members MAY declare id=. class= groups members.
• Unspecified members are auto-assigned a stable, meaning-based address by the compiler (editing model in Components, Editing, and Multi-building).