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Default boundaries

koyu's notation is short not because there is little to learn but because there is much that goes unwritten. Read the syntax tables without knowing what is left out, and what leaving it out means, and you cannot tell whether a file has too few lines or too many.

There are three kinds of "not writing", and they mean different things.

Kind of contactWhat silence meansSo what is a declaration for
Two spaces with regions touching in plan on the same levela wall (derived)exceptions (type:open / air:1), and attributes and openings
Spaces on adjacent levels overlapping in plana floor (derived)exceptions (stair / shaft / void)
Contact with a space that has no region (exterior and the like)nothing at allthe envelope itself

The rules as written are in Default boundaries. What this page explains is why the three tiers are shaped this way.

The first two are symmetrical​

Horizontally a wall, vertically a floor. Neither is written.

grid X 0 3600 7200
grid Y 0 4000
level L1 0 h:2400 slab:150
space /L1/a room X1..X2 Y1..Y2 name:居室A
space /L1/b room X2..X3 Y1..Y2 name:居室B
✔ Consistent — 2 spaces / 1 boundary
  Structural consistency only — architectural validity is what koyu validate says, separately

There is a wall between the two rooms. Where storeys overlap, there is a floor between them. Both are the architectural default; the exception is the rare case.

Invert the default and the reason for this choice becomes visible. If a wall between two touching rooms had to be written, a file would need roughly as many boundary lines as it has spaces — 1,364 of them for a 425-space building. That is not description but dictation, and it carries not one design decision.

A declaration is needed only to depart from the default, or to give the defaulted substance a value.

boundary /L1/a /L1/b type:open          # exception — open, not a wall
boundary /L1/a /L1/b t:120 spec:PW1     # give the defaulted wall a thickness and a spec
boundary /L1/hall /L2/bed type:stair    # exception — a stair, not a floor

There used to be a warning for "these touch but no boundary is declared". Once the default became a wall it had no work left to do, and it was retired. Where silence carries positive meaning, it is not an omission.

Only the third is different​

Internal walls are automatic; external walls are manual. This asymmetry does not show up in a table of default values. Miss it and the drawing breaks.

The file above passes check, and yet it has no external walls at all. The only thing drawn in black is the one line in the middle — the derived default wall. The perimeter has nothing.

An envelope appears only once an exterior space and boundaries to it are written.

grid X 0 3600 7200
grid Y 0 4000
level L1 0 h:2400 slab:150
space /L1/a room X1..X2 Y1..Y2 name:居室A
space /L1/b room X2..X3 Y1..Y2 name:居室B
space /out exterior name:外部
boundary /L1/a /out t:150 spec:EW
boundary /L1/b /out t:150 spec:EW

There is a reason. Which exterior — street, neighbouring plot, garden, common corridor — is itself information, and no default can derive it. Whether the exterior is one monolithic /out or is split by orientation is a design decision. Road frontage is measured against exterior spaces declared as roads, so how you split it changes the numbers.

The dividing line is not "is it exterior" but "does it have a region". An exterior space that does have a region and a level — space /out/garden exterior X2..X3 Y1..Y2 level:L1 — gets a derived default wall against the rooms it touches.

A missing envelope is not caught by green — it is caught by judgement​

check does not look at envelope gaps, because a gap is not a contradiction in the composition.

Judgement does.

npx tsx src/cli.ts validate gap.muro
⚠ [envelope.gap] gap.muro:line 6: Perimeter not faced by any envelope: /L1/b — S 3600mm / E 4000mm / N 3600mm (11200mm over 3 run(s)). Write a boundary to the exterior

The rule is deliberately coarse — it looks only at levels where at least one boundary to the exterior has been written. It will not call a storey whose envelope has not been modelled yet "full of holes". It demands "if you started, finish"; it does not demand completeness. Coarseness is allowed because judgement lives in a domain that does not freeze (check and validate). The rule is envelope.gap.

Silence generates derivation — semi-outdoor​

The three tiers pay off most clearly in semi-outdoor space. Semi-outdoor cannot be declared. It is derived.

grid X 0 3600 5400
grid Y 0 4000
level L1 0 h:2400 slab:150
space /L1/room ldk X1..X2 Y1..Y2
space /L1/balcony balcony X2..X3 Y1..Y2
space /out exterior
boundary /L1/room /L1/balcony t:150
  window w:1600 h:2000
boundary /L1/balcony /out type:open

The last line is what makes the balcony semi-outdoor. Writing type:terrace, or type:balcony, does not — it is the boundary that makes it so.

L1
  /L1/room	room	ldk	14.40 m2
  /L1/balcony	balcony	balcony	7.20 m2 (semi-outdoor, reported separately)
  Subtotal 14.40 m2
Total 14.40 m2 (indoor floor area)
Semi-outdoor 7.20 m2 (balconies, external stairs and the like — whether they count is a matter of regulatory detail, so it is reported separately)

A space with a region that meets the exterior across an open or air:1 boundary is semi-outdoor. A terrace enclosed by a railing is written with air:1 — there is a thing, but it blocks neither air nor light — and that alone satisfies the condition. You do not declare the property; you write the composition that produces it.

Silence is not licence to be empty​

Not having to write something is different from getting a form without it. If information needed to make a form is missing, check errors.

If no ceiling height can be determined, no wall and no column stands on that level, so this is an error (SUF01). A space that has a region but no determinable level is an error too (SUF02). koyu does not invent a default and press on — if a thing cannot be made, it is not made, and the fact is put into words.

The list is in SUF — sufficiency.

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