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17. Fixture semantics: three distinct flags, one shared predicate, one derived table ​

  • Status: Accepted
  • Date: 2026-08 (v1.28.0 / v1.29.0)

Context ​

Until v1.28.0 the fixture catalogue knew 18 categories, eight of which drew a symbol. Everything else — bed, sofa, desk, wardrobe, dining_table, car, words the shipped examples had been writing for months — fell through to "unknown category": no footprint, no wall semantics, and a labelled rectangle where a drawing belonged.

Widening the catalogue to 59 words across 36 families forced a question the small catalogue had let us dodge. requiresWall was doing three jobs at once: it decided whether a piece must touch a wall (W_FIXTURE_FLOATING), whether its footprint may be derived from against wall … in <room>, and — through orientationMatters — whether the symbol takes a derived quarter-turn from the wall it backs onto. Those three coincide for a wc and a basin. They come apart the moment a sofa is catalogued: a sofa has a back worth turning to a wall, and no plumbing whatsoever.

Turning the flag on for the new room furniture proved it: 23 spurious floating-fixture warnings across nine shipped plans, the largest group being twelve on examples/library.arch, whose stacks are free-standing runs mid-floor. Those plans were correct. The flag was wrong.

Decision ​

1. requiresWall means SERVICES, and only services. It stays with the plumbed and vented goods, and W_FIXTURE_FLOATING stays keyed on it alone — because that rule's remedy prose is about supply and waste, and it has nothing to say to a sofa in the middle of a room.

2. A separate directional flag carries "this symbol has a back worth turning toward a wall, though it needs no services." orientationMatters becomes (requiresWall || directional) && !symmetric. Eleven categories carry it. sofa/chair/bench/desk deliberately do not: seating is arranged, not installed, and deriving a turn for it would assert a fact the plan never stated.

3. A third flag, underlay (v1.29.0), marks a piece that lies flat and is stood on — and it is read through exactly one predicate, solidFurniture(). Four consumers ask whether a piece is solid: the overlap rule, the clearance rule, the whole-plan nav grid and the per-room flood fill. Routing all four through one predicate is the whole point — two grids that answer "is a rug walkable?" differently is a defect nobody would see. The exemption is narrow, and proved in both directions: a rug under a sofa raises nothing, two ordinary pieces overlapping still raise W_FURNITURE_OVERLAP, and two rugs overlapping each other still raise it too. An underlay is not "overlap checking off".

4. The category list is a single derived table, never a switch. FIXTURE_FAMILIES (in src/elements/fixtures-glyphs.ts) is the one place a family and its aliases are written; FIXTURE_CATEGORIES, CANONICAL_FIXTURES, the manifest's fixtureCategories, the legend's hasFixtureGlyph filter and spec.llm.md's footprint list are all derived from it. Adding a family is a row plus a CATALOG entry — never a new element, never a switch arm.

5. A drawn symbol ignores its label. Long-standing for wc and basin, now true of twenty shipped examples whose fixture words vanish from the drawing while staying in the source and in describe(). The labelled rectangle remains the deliberate escape hatch for any word the catalogue does not know.

Rejected: a sofa_l_r category ​

sofa_l's return is always on the left, and place … mirror will not produce a right-handed one — a reflection transforms a resolved element's position, not the symbol drawn inside it. Adding a mirrored category was rejected rather than forgotten: it would put the fix in the vocabulary, where every future handed symbol then needs its own twin. The fix belongs in transformElement's det < 0 branch, which is already where the handed rules flip (see ADR 0016); it is recorded as an open item in docs/backlog.md.

Consequences ​

  • arch lint got quieter on nine plans and sharper on five: the 23 spurious warnings went, and directional surfaced six real defects the drawing could not previously show. A flag that means one thing can be believed.
  • Three flags cost more to keep straight than one, and the failure mode is a new category copying its neighbour's flags without asking what they claim. The mitigation is that each flag now has a one-sentence meaning that names its consumer, stated here and in src/fixtures-catalog.ts.
  • Any future rule that wants to exempt a class of furniture must extend solidFurniture() or state plainly why it needs a fourth notion of "solid". Two doorway rules (W_SWING_OBSTRUCTED, W_DOORWAY_BLOCKED) do not yet consult it, and that gap is recorded in docs/furniture.md rather than left to be discovered.
  • The derived table means a family added without a glyph is caught rather than shipped: the legend filters on hasFixtureGlyph, and the spec generator interpolates the list it documents instead of retyping it.