improvise/context/code/core.md

improvise-core — the arguments

Written for a reader with the code open. This is where the reasoning that used to live in crates/improvise-core/**'s comments now lives; the comments themselves are capped at two lines (design-principles §6). Bug narratives keyed by test name are in context/test-evidence/core.md.

The crate is pure data: Model, View, GridLayout, Workbook, number formatting. It depends on improvise-formula for AST types and knows nothing about UI, IO or commands.

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1. CellKey is canonical by construction

CellKey(Vec<(category, item)>) is always sorted by category name, and the constructors are the only way to get one: CellKey::new(coords) sorts, with(cat, item) replaces-or-inserts-and-re-sorts, without(cat) filters and so preserves the order. Equality, hashing and storage all rely on the canonical form, so two keys naming the same intersection in different orders are the same key. Never build the inner Vec directly (gotcha 5).

That without preserves the sort is what lets record_of be a one-liner rather than a normalisation step — see §5.

2. Aggregating a cell's raw values

Three functions, deliberately with three different emptiness rules, and the differences are the whole content of the design (improvise-kdp):

• apply_agg(values: &[f64], func) — reduces numbers. None on empty: an empty cell reads as absent, not as zero, or every blank intersection in a pivot would render 0. Both aggregation paths (evaluate_aggregated and aggregate_raw) go through it so their semantics cannot drift.
• aggregate_cell_values(values: &[&CellValue], func) — reduces a mixed set. See below.
• eval_agg(...) — evaluates a formula-level SUM(x) / AVG(x WHERE …). Its emptiness rule is not apply_agg's: an aggregate over no rows is 0 for SUM/COUNT, which have an identity, and an error for AVG/MIN/MAX, which do not.

evaluate_aggregated takes Aggregation, not a list it cannot read

The parameter was none_cats: &[String], and the body consulted it exactly once, for emptiness — a boolean wearing a list's type, since the aggregation itself runs over key.0 through matching_values. It never filtered by that list, in any revision: git log -S back to the commit that introduced Axis::None shows the same is_empty() and nothing else. So the name promised a filter the body has never performed, and the next caller to build a careful list would have threaded it into a parameter that cannot use it.

Aggregation::{Exact, OverHidden} admits only the two states the body distinguishes, and Aggregation::of(none_cats) performs the reduction once, at the boundary that holds the list (improvise-5jm5, doctrine(an_invariant_is_made_unrepresentable_not_validated) — the same answer prune_axis_items and gp-tree's Row<T> got). It is an enum rather than a bool because the two states have names worth reading at the call site, and the regression guard is the signature: the thing the ticket describes — two different lists, same emptiness, same answer — is no longer constructible.

Note what does not change: recompute_formulas and aggregate_raw keep &[String], because they walk the elements (stripping each hidden coordinate off a stem). Two parameters that looked alike were doing two different jobs.

Non-numeric values reduce by unanimity, and the branch order is the rule

aggregate_cell_values checks the all-numeric branch first. If every value is a number it reduces through apply_agg; otherwise, if every value equals the first, the result is that value; otherwise it is CellValue::Error("mixed").

That ordering is the entire difference between this rule and "a single value passes through". Count is the only function that can tell them apart — every other function is the identity on a one-element set — so Count of a lone number must still be 1 and not the number itself. a_lone_number_still_counts_ as_one_rather_than_passing_through is the pin.

What the rule forbids is dropping. The old code did filter_map(as_f64), and since _Index/_Dim sit on Axis::None in every default view, none_cats is non-empty almost always and the aggregating path was almost always taken — so every CellValue::Text in every document rendered blank, including the pipe-quoted text .improv carries as a first-class value (Region=West, Measure=Revenue = |pending|). A user could type a text value, save it, load it, and never see it again (improvise-l4qx).

Both evaluate_aggregated and aggregate_raw share the helper, so a formula ref over mixed values now errors "mixed" rather than silently summing the numeric ones and reading as authoritative while under-reporting.

3. Formula evaluation

Fixed point. recompute_formulas(none_cats) clears formula_cache, collects the distinct coordinate stems (each data key with the formula's target category and every none_cat stripped), and iterates stem × formula target until nothing changes, bounded by MAX_EVAL_DEPTH = 16. Chained formulas (Margin = Profit / Revenue over Profit = Revenue - Cost) converge naturally; circular references converge to CellValue::Error("circular") rather than overflowing the stack.

A cell still moving when the bound runs out has not converged, and the cached path had to be told so (improvise-7iww). The direct evaluator gets this free: evaluate_depth bottoms out at depth == 0 and returns Error("circular"). The fixed point had no such arm — it simply stopped iterating and left the sixteenth iteration in formula_cache, which evaluate_aggregated then served as a valid number. Revenue = Revenue + 1 over a stored Revenue = 10 read Error("circular") directly and Number(26) through the cache, and the cached one is the displayed path, because the default views put the virtual categories on Axis::None. So a circular formula looked like a real calculated value. recompute_formulas now records the keys that changed on each pass and, if the last pass still changed any, writes Error("circular") over them.

Note what this costs and why it is the right cost: the fixed point can only confirm convergence with a pass that changes nothing, so a chain needing all sixteen passes is now reported circular. That is the same bound the direct evaluator already enforces — a sixteen-deep chain exhausts depth there too — so the two paths agree rather than diverge, which is the property §3 exists to protect. The anti-clamp guard is a_chained_formula_still_converges_after_the_circular_marker. The stems are an IndexSet: dedupe is O(1) per cell and insertion order is preserved (improvise-6os replaced an O(n²) Vec::contains).

There are two evaluators, and they must be changed together:

• eval_formula_depth / eval_expr — the direct, uncached path, recursion bounded by depth.
• eval_formula_with_cache / eval_expr_cached — the fixed-point path, which resolves a Expr::Ref through formula_cache first and falls back to aggregate_raw.

A rule stated in one and not the other is a live bug; improvise-0bf was exactly that (see §4).

find_item_category searches real categories first and then formula targets, because add_formula never inserts its target into _Measure (improvise-nrs). Without the fallback, a reference to a computed measure fails to resolve.

Display rounding never reaches eval. format_f64 is called only in rendering paths. Feeding a rounded value back into a formula is the bug formula_chain_preserves_full_precision exists to catch.

4. Float equality has one epsilon, and the div/0 guard shares it

FLOAT_EQ_EPSILON = 1e-10 is used by =/!= and by the division-by-zero guard, in both evaluators. The two must agree or IF(x = 0, a, y / x) is incoherent: the condition calls x zero and the else branch divides by it anyway. With x = 1e-11 the old exact rv == 0.0 guard returned 1e13 (improvise-0bf). 1e-10 is far below typical data magnitudes but absorbs accumulated f64 rounding noise from chained formulas.

Note the repo map's gotcha 17 (which called this "currently mixed") describes the pre-fix state; the guard and the comparison share the constant now.

5. Surrogate identities — model/record.rs (ADR 002)

A client sees a grid of label strings indexed by viewport position. It holds no Model, no category names and no page/filter constraints, so it cannot build a CellKey — which is why improvise-q08 found a thin client cannot compose SetCell. The answer is an opaque surrogate the server projects per visible cell and the client echoes back.

Two surrogates, because two kinds of thing are nameable and only one is settable. RecordHash names a record — the coordinate tuple minus _Measure (record_of). That unit is not chosen here: it is already ADR 001's unit of atomicity and already what the write counters are keyed by, because renaming a category coordinate in wide records mode moves every measure cell in the row (§6). One identity therefore serves addressing and staleness. AggregateHash names an aggregate's coordinate constraint — row and column coordinates plus the page and filter selections that decide which stored cells it sums (View::filtering_cats(), §7). An aggregate is not the absence of an identity; it is a different kind of identity, which is what makes Drill addressable at all. It is deliberately not normalised: a measure on Row or Column is part of what the aggregate sums, so dropping _Measure would give two visibly different totals one name.

The hash is specified here, not inherited. Three ways it breaks silently, each producing a wrong write rather than an error:

1. Not DefaultHasher, not RandomState. std's default HashMap hasher is randomly seeded per process, so a surrogate would not survive a restart or agree between two processes. This is SipHash-2-4 under a constant key.
2. Not #[derive(Hash)]. Rust gives no stability guarantee for a derived Hash across compiler versions or platforms. What is hashed is the explicit byte serialization in hash_coords, so the surrogate is defined by that format rather than by whatever the compiler did this week.
3. 128 bits, argued rather than defaulted. A collision is a silent write to the wrong record — precisely what got _Index rejected as a surrogate — and no later check catches it: a colliding surrogate resolves successfully to a record the client never saw. 64 bits reaches birthday risk around 2^32 records, which a pivot tool will not see by live record count; but the risk is over every record a long-lived document ever names, and the consequence is unbounded and undetectable. The other choice costs 8 more bytes per visible cell — 8 KB on a full 50×20 projection and far less on the diffs that carry steady-state traffic. Bounded cost against silent corruption is not close.

Serialization details that are load-bearing. The byte format is tag ‖ count ‖ (len(category) ‖ category ‖ len(item) ‖ item)*, every length a little-endian u64. The lengths are what make it injective: without them ("ab","c") and ("a","bc") serialize alike. Only Hasher::write is used, never write_u64/write_u8, which are free to feed the hasher in native byte order. The SipHash key constants are written as big-endian readings of b"improvis"/b"e-record" so the constant is the same on every platform. A domain tag byte (0x01 record, 0x02 aggregate) is hashed first, because a record and an aggregate may be built from the same coordinates — an aggregate over a single record is the degenerate case — and they must not hash alike.

Hex on the wire, not a JSON number. serde_json writes a u128 as a bare number and a JavaScript host parses every JSON number as f64, so anything above 2^53 comes back changed — which for a surrogate means resolving to a different record or to none. The wire form is a fixed 32-character lowercase hex string.

6. Records mode is wide format (view/layout.rs)

A row is one coordinate tuple — the cells sharing everything but _Measure — and each raw measure gets its own editable column. The columns are [coordinates…, +cat, measures…, +]: one per regular category, then the column that creates a new category (improvise-2zq), then one per raw measure, then the column that creates a new measure (improvise-c5v). Each + sits adjacent to what it adds to; two trailing columns would have made the user read a label to tell them apart, and position says it instead. Formula targets are not columns — they are computed, so there is nothing to type into them.

Consequences: two measures on one coordinate are one row; editing a category column moves every measure cell in the row, not just the one under the cursor; and _Dim in the synthetic key carries the column index, not a label, because a category and a measure item may share a name and a label could not say which column an edit landed on. Resolve the index through GridLayout::record_col into a RecordCol. Drill pending edits are keyed by (row, RecordCol).

RecordRow::group collapses loose cells into wide rows in first-appearance order, so the result follows the store's order rather than a hash walk (§8). A cell with no _Measure coordinate still anchors a row — that is how add-record-row seeds one — but contributes no column value.

Records mode is a property of the view's axes, and View owns the one definition. View::is_records_layout() (_Index on Row and _Dim on Column) is what GridLayout::new asks and what on_category_added asks; two derivations would drift (improvise-edp, the disease this codebase keeps curing).

prune_axis_items takes a predicate, not a flag vector

prune_empty hands it impl Fn(usize) -> bool over the data index — the position a row/col argument means, which group headers do not consume. The predicate's domain is derived from items itself, so there is no parallel length a caller can get wrong and debug and release builds cannot disagree about an out-of-range flag. The earlier &[bool] + debug_assert pair could have, had a third caller ever sized keep from something other than row_count()/col_count() (improvise-n3wt, doctrine(an_invariant_is_made_unrepresentable_not_validated)).

A group header survives only when a data item that follows it survives. In prune_empty the row pass does not touch col_items or has_value, so the column pass still reads the original grid. Records mode is exempt entirely — the user drilled in to see all the raw data.

7. Axes: Page and Filter are two different questions

Axis is Row | Column | Page | Filter | None. Filter names a coordinate the drill fixed (improvise-dz8): like Page it constrains which records are visible, but unlike Page the user cannot navigate it. Axis::next() maps Filter to itself and no navigable axis rotates into it, so tile select cannot hand one out, and is_user_assignable() is false for it.

Picking the wrong question is the trap:

• "what data is visible" → View::filtering_cats() (Page ∪ Filter). Used by GridLayout, by AggregateHash, and by add-record-row when it builds a key from the current constraints.
• "what can the user act on" → categories_on(Axis::Page) for [/] page cycling, or View::tileable_cats() for all three tile-select sites.

Leaving the constraining axes forgets the page selection (improvise-z336)

set_axis used to leave page_selections alone; only on_category_removed and on_item_removed ever cleared one. So cycling a category off Page merely hid its filter, and cycling back to Page silently reinstated it — the same item, unasked. Measured in the browser: Pag(Espresso) → None → Row → Col → Page and Espresso is back.

That mattered more than it reads, because there is no clear-a-page-selection operation anywhere in this workspace — not in ModelCommand, not in the View API, and not in the TUI; [ and ] cycle among the category's items with no "All" to land on. So a user who had applied a filter had no way out of it at either front end, which is a product gap at parity rather than a browser regression. Forgetting the selection on the way out closes it with no new command, no new wire variant and no new key: cycling the axis round is now a way out.

The predicate is Axis::filters_data() — Page ∪ Filter — and not "is it Page". That is this section's own trap, seen from the writing side: a drill fixes its coordinates on Filter and .improv round-trips a selection under it, so clearing on everything-but-Page would pass a test written for the browser symptom and silently break drill.

Axis also owns four presentation vocabularies — plain_label(), glyph() (which Display appends, giving "Row ↕"), short_label() (three columns for the tile bar; None is "·", not a truncation of "None") and initial() (one letter for the view panel; None is empty) — plus color() -> AxisColor. AxisColor is a semantic palette because improvise-core must not name a ratatui type; draw.rs::axis_fg does the one mapping, exactly as minibuf_fg does for MinibufColor (improvise-rml). The tile bar and the category panel each carried a copy of the colour table before, with nothing keeping them in agreement.

on_category_added: a virtual on an axis is a configuration, not a vacancy

Virtual (underscore) categories seat on Axis::None. A regular category takes the first slot no regular category holds — Row, then Column, then Page.

This used to bump a virtual off Row or Column to make room, assuming a virtual sitting there was a placeholder. It never is, and the assumption cost two live bugs (improvise-2zq):

• _Index on Row together with _Dim on Column is records mode, and the only production writers of those axes are the records and drill setups. Bumping _Index ejected the user from the grid they were standing in: naming a category with N in records mode silently returned them to a pivot.
• _Measure on Column is an ordinary user configuration — it is what web/demo.improv ships. Bumping it made every measure vanish from the grid the moment a second regular category was added.

So a virtual is never moved. A records view seats the new category on Axis::None, which is where records mode keeps every regular category and where records_columns — which filters on kind.is_regular() and never looks at an axis — picks it up as a column immediately. Elsewhere the new category simply joins whatever virtual is already there, nesting a header rather than destroying one.

The negative arm is load-bearing and must stay tested: without a_pivot_view_still_seats_categories_row_then_column_then_page, "always seat on None" would pass every assertion about records mode. Probing confirmed the asymmetry — seating everything on None fails 12 tests, while removing the virtual bump failed none at all, which is how the bump came to be wrong in two directions with nothing noticing.

Known defect: two answers to "is this name a virtual"

on_category_added tests cat_name.starts_with('_') (twice) while is_records_layout() four lines away asks CategoryKind::of_name. That is improvise-bidq and it is open; do not assume either spelling is the intended one when editing near it.

8. Ordering is correctness, not tidiness (model/cell.rs)

DataStore holds cells in an IndexMap keyed by InternedKey, plus a secondary index HashMap<(Symbol, Symbol), HashSet<InternedKey>>.

An index whose values are HashSets is for O(1) lookup, never for iteration. matching_cells/matching_values used to walk the index's HashSet directly, so their order was hash order — different in every process, and different between two stores in one process, because RandomState keys each HashMap afresh. That defeated the whole reason cells is an IndexMap, and it was not tidiness (improvise-shyt). It caused, in ascending order of seriousness:

• records-mode rows reordering between sessions;
• SUM/AVG drifting in the last f64 bits, because f64 addition is not associative (Min/Max/Count and the text-unanimity rule of §2 are order-free, so they were never affected);
• worst, the drill snapshot freezing its record list in hash order while drill_state.pending_edits are keyed by row index (§6) — so the same drill opened in a second process addressed different records. Silent misdirected writes, and nothing in the tracker knew.

matching_positions is the fix: the index still narrows the candidates, and the primary map orders them — each surviving candidate resolves to its position with one hash lookup and the positions are sorted. Shape stays O(smallest set × |partial|) plus O(m log m) over the matches, not a scan. Measured at 18,000 cells: +20% worst case on matching_values, and matching_cells 7% faster, sorted positions resolving CellKeys with better locality. The alternatives both move the cost — an ordered index value type pays on every set; scanning the primary is O(store) per query.

An empty partial matches every cell, vacuously, and matching_positions is where that is answered (improvise-hkt6). It used to index sets[0] and panic; both public callers happened to guard, so it was unreachable — but the guard lived one level away from the indexing, which is how this class of bug returns. The guards could not simply be pushed down as vec![]: "no coordinates constrain anything" means all positions, and answering "none" here would have made removing a caller's fast path change behaviour silently. The callers keep theirs as a fast path, not as a guard.

live_records has a decided order, not an inherited one (improvise-2yc2). It walks cells and projects insertion order onto records: a record sits where its first cell sits. It deliberately pays O(cells) rather than O(records), because cells' order is already the answer matching_positions gives and a second answer to "what order is this store in" is the disease. It is derived per call rather than memoised on RecordEntry, since a stored position is a copy and a record deleted and re-created would hold its old one while its cells sit at the end of the store. Note what is not on offer: an unordered return. Every iterator has an order, and a type that merely disclaims one still hands the caller hash order — the guard living one level away from the hazard, which is the shape this whole class of bug returns through. The function had no callers when this was decided, so the order was still free to decide rather than to discover; with no caller to benchmark against, correctness is the defensible default, and a hot-path caller can argue for a memoised position with numbers.

The record index: RecordEntry

DataStore::records: HashMap<RecordHash, RecordEntry> answers both of a client surrogate's questions — "which record do you mean?" (ADR 002) and "is your basis stale?" (ADR 001) — from one entry in one lookup, because both are keyed by the record and both are invalidated at the same moment.

It lives in the store because this is where the same job is already done: the store already interns every name and already maintains a secondary index updated at exactly these two points (set, remove), and it is that existing (Symbol, Symbol) index which makes invalidation tractable — when a category or item disappears, the cells naming it are found without a scan, so the records built from them are too.

It is deliberately not regenerated from the layout on demand. That would be O(visible cells) per resolve and would only ever cover the visible window, so a record scrolled out of view would look identical to a record that no longer exists — conflating "reject, nothing to write to" with "apply".

It is not serialized: Deserialize rebuilds it through set, and every version starts at zero, which is correct because a freshly loaded document is a new document_generation and no client holds a basis against it.

cells == 0 is a tombstone, not a reason to forget the entry. A record that is deleted and then re-created must not hand a client holding the old basis its own version number back; dropping the entry would reset the counter to zero and make a stale basis look current, which is the silent accept the basis check exists to prevent. The counter tracks liveness, not writes — set bumps cells only for a genuinely new cell, and version is moved by the observed diff in App::apply_effects, never from set.

resolve_record returning None means the record is gone — never stored, or every cell removed, which is what happens when a category or item it named is deleted. That is one arm of staleness and needs no version comparison: there is nothing to write to. record_version reporting 0 for an unknown surrogate is deliberate: a basis composed against an absent record and one composed against an untouched record are the same claim.

9. Categories

CategoryKind is Regular | VirtualIndex | VirtualDim | VirtualMeasure | Label. Business rules match on the enum rather than sniffing name prefixes.

CategoryKind::category_name() is the one place the name↔kind mapping is written down, so adding a variant forces a decision there (improvise-2lh). of_name(name) is safe to use in place of looking the Category up, because Model::add_category hands back the existing virtual category rather than shadowing a reserved name — stored_virtual_categories_match_their_names is what makes that sound. Label categories are user-named and so read as Regular from of_name; consult the stored Category::kind when the distinction matters.

VirtualMeasure items come from two sources: numeric data fields listed in the file, and formula targets added implicitly. They are virtual because formula-derived items are implied by the formula definitions — listing them explicitly would be redundant in the file format and confusing in the UI. add_formula does not add items to _Measure; use measure_item_names() / effective_item_names("_Measure").

Label is a high-cardinality per-row field (description, id, note): stored alongside the data so it shows up in record/drill views, but defaulting to Axis::None and excluded from the MAX_CATEGORIES = 12 pivot limit and from auto Row/Column assignment.

Category::add_item deduplicates by name and assigns IDs from a private counter, so external code cannot fabricate an ItemId.

Removing a name removes the formulas targeting it (improvise-g3no)

Model::remove_category has always ended by dropping every formula whose target_category is the category — that is why Dirty::of(LayoutGeometry) reports formulas: true (gotcha 35). remove_item did not do the same one level down, and the asymmetry was a live bug rather than an omission of symmetry for its own sake.

_Measure's items are explicit items ∪ formula targets, so a Profit that exists only because a Formula does is not in the category's own item list. remove_item called Category::remove_item, which no-opped, then swept the cells naming it, of which there are none because the value is derived. effective_item_names re-synthesised the row from the surviving formula and it came straight back — while both front ends reported success, each having composed an optimistic past-tense status beside the mutation.

Two things about the shape of the answer. The lie was a symptom, not the defect. The obvious reading is gotcha 20's ordering rule violated — the fallible effect first, the optimistic status second — and the obvious repair is to give remove_item an answer the front ends let win. That repair is unavailable on the browser side, in two independent ways: SetStatus is ViewOnly and never crosses the wire, so a host-side refusal cannot reach a client; and a client cannot decide for itself, because nothing in the projection stream says which _Measure rows are derived — FormulaCache is Vec<String> of Formula::raw, and recovering a target from that is a second home for the unquoting §34 says has exactly one. Restoring the sweep makes the status both ends already compose true, with no wire change and no second classification.

And the sweep is not restricted to _Measure. A non-measure target is a real item (add_formula creates it), so removing that item used to leave a formula aimed at nothing. The predicate is target_category and target: deleting Cost must not delete Profit = Revenue - Cost, which references it but does not target it.

A required virtual category cannot be removed (improvise-3b2b)

The category panel deliberately lists _Index/_Dim/_Measure (improvise-vjms — a decision about visibility), and its delete key treated them as ordinary removable categories. d at the panel's default cursor 0 therefore deleted _Index from a fresh session, and GridLayout's records-mode detection keys off _Index on Row, so records mode silently stopped working while the status line said it was on.

Model::remove_category refuses when CategoryKind::of_name(name).is_virtual(). The guard is at the model rather than in the two Cmds because hiding a key leaves ModelCommand::RemoveCategory unguarded on the wire. It reads of_name, the one home of the mapping, so a Label category — user-named, and Regular to of_name — stays removable.

10. Workbook exists to break the Model ↔ View cycle

Model knows nothing about views; View depends on Model, one direction. Cross-slice operations — adding or removing a category must update both the model's categories and every view's axis map — live on Workbook, so Model stays pure data and improvise-core can be extracted without pulling view code along. That is also why AddCategory/RemoveCategory are LayoutGeometry effects rather than CellData ones (gotcha 3).

Workbook::new seeds a Default view, registers the three virtual categories through on_category_added (so all three land on Axis::None), then bumps _Measure to Axis::Page so aggregated pivot views show a single measure at a time (improvise-kos, commit 709f2df). Leaving _Index/_Dim on None keeps pivot mode the default — records mode activates only when the user moves both onto axes.

Workbook::add_item errors on an unknown category so callers do not reach through model.category_mut(..) and invent their own handling (improvise-tvt). There is no Model::add_item.

11. Virtual views

_Records and _Drill (view::VIRTUAL_VIEW_NAMES) are scratch views the app builds on demand. Both put the virtual _Index/_Dim categories on axes, and those categories are never persisted — so these views are rebuilt from scratch each time rather than saved (improvise-60z).

12. axis_of panics, try_axis_of does not

Registration is guaranteed for every model category: Workbook::new, add_category, add_label_category and create_view all call View::on_category_added for every view. So any name obtained from Model::categories or from the view's own category_axes is safe for axis_of. Use try_axis_of when the name comes from anywhere else — cycle_axis does, and no-ops on an unknown category to match set_axis (improvise-t8s).

record_cell_key is a match, and the match is the guard (improvise-ir7l)

GridLayout::record_cell_key(row, col) is the one mapping from a records position to a model cell, and it answers Some for RecordCol::Measure alone. The other three arms are spelled out rather than swept into a _, and that exhaustive match is the whole guard: a new RecordCol variant fails the build until someone decides whether it names a stored cell.

There is no test for the Category arm, and the reason is not that the wrong answer would be harmless. It would be a real fault, and a subtle one worth naming so nobody re-derives it wrongly. RecordCol::Category(name) carries a category name, while RecordRow::cell_key(arg) puts its argument under _Measure. So answering Some there yields coords + _Measure=<category name>, and Model::formula_for would match it whenever a _Measure item happens to share a name with a regular category — projecting CellId::Derived for a coordinate column, which is never a formula target and which the pivot path does not have at all. The two front ends would then disagree, which is the design-principles §5 defect this ticket exists to close. RecordCol exists for exactly that collision: its doc says the enum resolves a column from its index "so a category and a measure sharing a name stay distinguishable".

What earns the decision to leave it untested is narrower: the reachable version of that fault is a new variant answered carelessly, and the exhaustive match makes that unrepresentable rather than merely unlikely (doctrine(an_invariant_is_made_unrepresentable_not_validated)). A test would pin behaviour the type system already pins, and would not survive the arm being renamed. Found in review, and the first recorded reason for declining it was wrong — it claimed the injected behaviour was defensible by analogy with a formula targeting a regular category, which is a different key entirely.

13. Route then aggregate: SUM(<expr>) folds over the member records

improvise-4zl7, and the one place the evaluator reaches past the cell it was asked about.

The problem. A formula is evaluated at a CellKey. In an aggregating cell evaluate_aggregated aggregates each Expr::Ref first and evaluates the expression once on the totals. That is exactly right for a linear formula — SUM(Revenue) - SUM(Cost) and SUM(Revenue - Cost) are the same number — and exactly wrong for anything that routes. IF(Cost < 50, 1, 10) over records costing 10 and 100 answers 1 + 10 = 11 per record and 10 on the total, because aggregate-then-route is not route-then-aggregate. Every fitted tree (improvise-8xhy, improvise-vx3) is such a formula, so it meant one thing on a record row and something else one row up. The user's ruling was to aggregate the rule over the members.

The mechanism, and why it needed no new vocabulary. SUM(e) already means "fold e over the members". It simply did not: eval_agg specialised Expr::Ref and, for every other inner expression, summed the raw cells matching the partial key and ignored the expression entirely. It now folds — one entry per member record, the expression evaluated at that record's own coordinates:

partial   = context.without(target_category)    (plus any inline WHERE)
records   = distinct(matching_cells(partial).map(key.without("_Measure")))
values    = records.map(|r| eval(inner, context.with(every coord of r)))

Three consequences worth stating.

The rule is written once, and so is the evaluator it folds with. eval_agg is shared by eval_expr and eval_expr_cached, and both hand it the same eval_at: the record-level eval_expr. That is not a convenience, it is the correctness. A member key is a record, and formula_cache is keyed by stems with none_cats stripped — so the cached evaluator's Expr::Ref can never hit the cache at a member key, falls through to aggregate_raw, finds no raw cell behind a formula target, and answers Err("ref:…"). A first version did exactly that, and the two evaluators then disagreed on any routing formula reading a derived measure: Number(4.0) uncached, Error("ref:Profit") cached, which is the very divergence this section exists to remove (§3: "a rule stated in one and not the other is a live bug").

And making the lookup "work" would have been worse than the error. The only way to hit the cache from a member key is to strip none_cats off it — which returns the group's value for that formula and hands it to every member. That is aggregate-then-route again, laundered through a cache lookup, and it would have been silent. The cache cannot answer a per-record question, so the fold does not ask it. Only Expr::Is needed an arm in each evaluator, and since the fold routes through eval_expr, the cached arm is now reached only by a bare Is outside a SUM — which is what the_cached_evaluator_has_its_own_working_coordinate_predicate pins.

The Expr::Ref fast path stays, and it is the same fold specialised: a record's Ref is that record's own cell, so summing them is summing the matching values. Two of its premises were unstated and both were false (improvise-s08t, improvise-s2w3), which is worth recording as a shape: a specialisation defended by an argument is only as good as the conditions the argument assumed, and neither condition was being checked.

The name must resolve. The narrowing was written if let Expr::Ref(name) = inner && let Some(cat) = find_item_category(name), so a name that resolved to nothing left the partial key unconstrained — SUM(Typo) aggregated every numeric measure matching the context and answered a plausible total. A missing identifier now fails before the fast path is reached, with the same ref:{name} the record-level evaluator uses.

The name must be stored. A formula target is a _Measure item that nothing wrote (improvise-nrs), so there are no raw cells behind it and the raw scan returns nothing: SUM(Double) answered 0 where SUM(Double + 0) folded and answered 20. The premise of the specialisation — "a record's Ref is that record's own cell" — is exactly what fails there, so the answer is to ask whether it holds rather than to delete the path. formula_for is the one home of "is this cell derived", so the check is that call and not a second derivation; a derived ref falls into the fold, where eval_expr's own Ref arm routes through evaluate_depth and sees the formula. A target that also has stored cells reads as the formula, because formula_for wins there too.

It differs in exactly one place — a record with no such cell is skipped there and an error in the fold. That is deliberate: a bare SUM(Revenue) asks for the values that exist, while SUM(<expr>) asks the expression of each member, and an expression that cannot be evaluated is an error rather than a zero. Silently dropping members is how a routed sum would quietly answer for a subset.

The asymmetry's blast radius is wider than it sounds, and worth stating: gotcha 7 makes a pipe-quoted text cell a first-class .improv value, so one text cell anywhere in a member set turns the whole aggregated fitted cell into Error("ref:Cost"), where the bare-ref path would have dropped it and returned a number. That is the loud failure this repo prefers to a quietly wrong sum, but it is a real behaviour difference and not a detail.

Records come out in DataStore insertion order, deduplicated by an IndexSet, for §8's reason — hash order here would make a fitted cell drift in its last bits between processes.

Interaction with the fixed point (§3) and formula_cache. None of this is new state. The fold reads raw data through the same evaluate_depth / aggregate_raw the non-folding path uses, so a formula cell still depends only on raw cells and on other formula cells' cached values; recompute_formulas iterates the same stems to the same fixed point, and formula_cache still holds exactly the aggregated formula cells it held before. The fold does not cache and does not memoise: it is O(members) per evaluation per pass.

Measured, because the cost was a blocker and not a footnote (improvise-9iyv; crates/improvise-core/examples/fold_bench.rs, five runs per point, best–worst). At 18,050 cells with one routing formula and Tag on Axis::None, recompute_formulas takes 18.1–19.2 ms release and 132.9–135.1 ms debug, against 7.1–9.0 ms / 71.7–74.6 ms for a linear Rev - Cost. That is 2.3× a linear formula and linear in cells: 400 → 0.4 ms, 2,000 → 2.0 ms, 18,050 → 18.1 ms, so 45× the cells costs 45× the time.

It was not always linear. Routing the fold through eval_expr_cached — the first version, above — made every member's Ref miss the cache and fall into aggregate_raw, one poorly-narrowed scan per member: 51.1–53.3 ms at the same 18,050 cells and superlinear with it (45× the cells cost 85× the time). So the divergence and the stall were one defect, and fixing the first fixed the second.

And "18,050 cells" was not a sufficient description of that workload. At the same cell count the old code cost 51 ms at a 25×361 shape and 244 ms at 95×95 — a 4.8× spread from group count alone, which is what a per-member scan predicts and a constant factor does not. After the fix the two shapes are 16.9–17.2 ms and 16.4–17.5 ms: indistinguishable. The shape-dependence being gone is the confirmation of the cause, more than the absolute number is. Re-measuring this means stating the shape, not just the cells.

The fixed point runs exactly 2 passes (measured, not assumed), so hoisting the member set out of the loop has a 2× ceiling on the fold's share and was declined: a memo is state, and a memo keyed by anything less than the whole partial is a second answer to "what does this cell depend on".

One asymmetry not to tidy: the cached path hands eval_expr a fresh MAX_EVAL_DEPTH at each Agg, while the uncached Agg threads its own depth through. The second is what bounds a chain of folded formulas; making them agree by resetting both would unbound it.

Expr::Is(Filter) — a coordinate predicate as a number. Cat IS Item is 1.0 when the cell's own Cat coordinate is Item and 0.0 otherwise. It keeps the evaluator f64-only — no string comparison enters eval_expr — and it reuses the primitive Formula::filter already had, context.get(&cat), including its unwrap_or(false): a cell with no Cat coordinate is not that item. Under route-then-aggregate that arm is reached only where a record genuinely lacks the coordinate, because the fold hands the expression a record key. Evaluated bare, outside a SUM, it degrades exactly as Formula::filter does — which is why a fitted tree is always emitted wrapped.

PageAxis — what constrains, and what the user may turn (improvise-p1ym)

GridLayout::page_coords answers two questions that gotcha 27 says must not be confused. The bar at the top of a view names every category constraining the visible records, which is View::filtering_cats() — Page ∪ Filter — because a drill fixes its coordinates on Filter and the user should still see them. [/] cycle only what the user may act on, which is categories_on(Axis::Page).

Those were two walks before: GridLayout did the first, navigation.rs:: page_cat_data did the second, and each spelled the Page-vs-Filter rule itself. Two spellings of one rule is the shape design-principles §5 calls the disease, and here it had already leaked into the protocol — GridCache::page_coords was a verbatim copy of the layout's list, so a browser handed it would have tried to page a Filter axis.

So page_coords is now Vec<PageAxis>, and items is Some exactly when the axis is navigable. The rule lives in the type: a Filter axis carries no item list, so no caller can compose a cycle for one, and PageAxis::cycle() is the one place that pairs a list with the index its selection sits at. page_cat_data and the browser's page_cycle both read it rather than deriving it.

Two consequences worth stating. The list is Model::effective_item_names, the same call that already chose the default selection, so the selection and the list it indexes into come from one place by construction — page_cat_data used ordered_item_names before, which for a _Measure page axis meant the default selection could be a formula target [/] could not reach. That incoherence is gone; the cost is that [/] over a _Measure page axis now visits formula targets, which is what the column list shows anyway.

And the list is not filtered to visible items. Hidden items are still cycled, because that is what the TUI has always done, and making the browser differ would be the divergence rather than the fix. Changing it is a change to both, together.

The number format — format.rs (improvise-711n)

A view's number_format is a printf-style picture, and only that. A , anywhere turns on thousands grouping; a . introduces the precision, which is the leading digit run after the last .; anything following those digits — a conversion letter — is ignored. So ,.0, ,.2, .2, ,.2f and %.2f all name what they look like they name.

Why the trailing f is accepted rather than refused. It round-trips into documents today and always has. improv.pest spells the line format_line = { "format: " ~ rest_of_line ~ NEWLINE ~ blank_lines } — the grammar takes the format string as an opaque line, so format: ,.2f is a document the grammar accepts; :set-number-format stores whatever the user typed verbatim (cmd/effect_cmds.rs), and format_md writes view.number_format back out unchanged. Refusing the spelling at load would therefore reject documents this repo's own grammar admits and its own .improv example spells, to no benefit: by gotcha 8 the format is display-only, so misreading it costs a wrong pixel and can never cost a wrong number. A refusal here buys nothing and costs a document.

Why printf and not Excel. The two conventions disagree on exactly the spelling that distinguishes them: .00 is precision zero to printf and two places to Excel, so a parser that tried to honour both would have to guess. One is picked, and it is the one every spelling in this repository already uses. Consequences worth stating rather than discovering: 0.00 reads as (false, 0), and an Excel picture such as #,##0.00 reads as (true, 0) — grouped, no decimals.

Nothing is validated, deliberately. A spelling the rule cannot read names no precision and formats with none; there is no error channel and no Result, because the reader is display-only and four call sites would have to thread one.

What the bug was, and why six tests sat on top of it. The precision was read as fmt[i+1..].parse::<u8>() over the whole tail, so "2f" failed to parse and fell through to unwrap_or(0) — ,.2f displayed zero decimals, in the TUI, in the browser projection and in CSV export. ",.2f" and its ,.3f/,.4f siblings are what this workspace's own fixtures reach for, in about ten tests, and none could see it: they assert the stored string round-trips or use it as an opaque value to dirty a widget, or — in export_csv_quotes_commas_per_rfc4180 — they assert the rendered value starts_with("1,200"), which 1,200 and 1,200.00 both satisfy. A prefix check over a value whose suffix is the thing in question answers a neighbouring question, which is why that one is now an exact equality.

Only two of the three consumers are guarded. Reverting the fix fails format.rs's own unit tests and export_csv_quotes_commas_per_rfc4180, and nothing else in the workspace: ui/project.rs::grid_of (the browser projection) and ui/app.rs both call parse_number_format and neither has a test that renders a cell and asserts its digits. That gap is named rather than closed — those files are improvise-command, which was another wave's ground when this landed — and it is the more useful half of this entry, because it says where the next regression would go unseen.