improvise/context/design-principles.md

# Improvise Design Principles

## 1. Functional-First Architecture

### Commands Are Pure, Effects Are Side-Effectful

Every user action flows through a two-phase pipeline:

1. **Command** (`Cmd` trait) — reads immutable context, returns a list of effects.
   The `CmdContext` is a read-only snapshot: model, layout, mode, cursor position.
   Commands never touch `&mut App`. All decision logic is pure.

2. **Effect** (`Effect` trait) — a small struct with an `apply(&self, app: &mut App)` method.
   Each effect is one discrete, debuggable state change. The app applies them in order.

This separation means:
- Commands are testable without a terminal or an `App` instance.
- Effects can be logged, replayed, or composed.
- The only place `App` is mutated is inside `Effect::apply`.

### Prefer Transformations to Mutation

Where possible, build new values rather than mutating in place:
- `CellKey::with(cat, item)` returns a new key with an added/replaced coordinate.
- `CellKey::without(cat)` returns a new key with a coordinate removed.
- Viewport positioning is computed as a pure function (`viewport_effects`) that
  returns a `Vec<Effect>`, not a method that pokes at scroll offsets directly.

### Compose Small Pieces

Commands compose via `Binding::Sequence` — a keymap entry can chain multiple
commands, each contributing effects independently. The `o` key (add row + begin
editing) is two commands composed at the binding level, not a monolithic handler.

### Decompose rather than early return

Early Returns usually are a signal of mixed responsibilities: if an early return 
would clarify a function, consider how the function could be decomposed for the 
same effect without the early return.

---

## 2. Polymorphism Over Conditionals

### Dispatch Through Traits and Registries, Not Match Blocks

- **Commands**: 40+ types each implement `Cmd`, organized by concern across
  submodules in `command/cmd/` (navigation, cell, commit, grid, mode, panel,
  search, text_buffer, tile, effect_cmds). A `CmdRegistry` maps names to
  constructor closures. Dispatching a key press looks up the binding, resolves
  the command name through the registry, and calls `execute`. No central
  `match command_name { ... }` block.

- **Effects**: 50+ types each implement `Effect`. Collected into a `Vec<Box<dyn Effect>>`
  and applied in order. No `match effect_kind { ... }`.

- **Keymaps**: Each mode has its own `Keymap` (a `HashMap<KeyPattern, Binding>`).
  Mode dispatch is one table lookup, not a nested `match (mode, key)`.

### Use Enums to Make Invalid States Unrepresentable

- `BinOp` is an enum (`Add | Sub | Mul | ...`), not a string. Invalid operators
  are caught at parse time, not silently ignored at eval time.

- `Axis` is `Row | Column | Page | None`. A category is on exactly one axis.
  Cycling is a four-state rotation — no boolean flags, no "row_or_column" ambiguity.

- `Binding` is `Cmd | Prefix | Sequence`. The keymap lookup returns one of these
  three shapes; dispatch pattern-matches exhaustively.

- `CategoryKind` is `Regular | VirtualIndex | VirtualDim | VirtualMeasure | Label`.
  Business rules (e.g., the 24-category limit counts only `Regular`) are
  enforced by matching on the enum, not by checking name prefixes. Virtual
  categories (`_Index`, `_Dim`, `_Measure`) always exist: `_Index` and `_Dim`
  support drill-down/records mode; `_Measure` holds numeric data fields and
  formula targets (added automatically by `add_formula`). Use
  `Model::regular_category_names()` when selecting a default category for
  prompts or other user-visible choices.

### When You Add a Variant, the Compiler Finds Every Call Site

Prefer exhaustive `match` over `if let` or `_ =>` wildcards. When a new `Axis`
variant or `AppMode` is added, non-exhaustive matches produce compile errors
that guide you to every place that needs updating.

---

## 3. Correctness by Construction

### Canonical Forms Prevent Equivalence Bugs

`CellKey::new()` sorts coordinates by category name. Two keys that name the same
intersection but in different order are identical after construction. Equality,
hashing, and storage all work correctly without callers needing to remember to
sort. Property tests verify this invariant.

### Smart Constructors Enforce Invariants

- `CellKey::new()` is the only way to build a key — it always sorts.
- `Category::add_item()` deduplicates by name and auto-assigns IDs via a private
  counter. External code cannot fabricate an `ItemId`.
- `Model::add_category()` checks the category limit (`MAX_CATEGORIES`, 24)
  before insertion.
- `Formula::new()` takes all required fields; there is no default/empty formula
  to accidentally leave half-initialized.

### Type-Safe Identifiers

`CategoryId` and `ItemId` are typed aliases. While they are `usize` underneath,
using named types signals intent and prevents accidentally passing an item count
where an item ID is expected.

### Symbol Interning for Data Integrity

`DataStore` interns category and item names into `Symbol` values (small copyable
handles). This means:
- String comparison is integer comparison — fast and allocation-free.
- A secondary index maps `(Symbol, Symbol)` pairs to cell sets, enabling O(1)
  lookups for aggregation queries.
- Symbols can only be created through the `SymbolTable`, so misspelled names
  produce a distinct symbol rather than silently matching a wrong cell.

### Parse-Time Validation

Formulas are parsed into a typed AST (`Expr` enum) at entry time. If the syntax
is invalid, the user gets an error immediately. The evaluator only sees
well-formed trees — it does not need to handle malformed input.

### Grammar-Defined File Format

The `.improv` file format is defined by a PEG grammar (`persistence/improv.pest`)
and parsed by pest. The grammar is the single source of truth — the parser is a
tree-walker over the grammar's parse tree, not an ad-hoc line scanner. This means:

- Adding a new format feature means updating the grammar first, then the walker.
- The grammar can be read as a specification independent of the Rust code.
- A grammar-walking test generator reads the grammar AST at test time (via
  `pest_meta`) and produces random valid files, ensuring the parser accepts
  everything the grammar describes.

### CL-Style Pipe Quoting for Names

Names in the `.improv` format use CL-style `|...|` pipe quoting. A name is bare
if it matches `[A-Za-z_][A-Za-z0-9_-]*`; everything else must be pipe-quoted.
Escapes inside pipes: `\|` (literal pipe), `\\` (backslash), `\n` (newline).
This convention is shared between the `.improv` persistence format and the
formula parser's identifier syntax.

### Formula Identifiers and Quoting

**Bare identifiers** are `(ASCII_ALPHA | "_") ~ (ASCII_ALPHANUMERIC | "_" | "-")*`
— no spaces, the same shape `improv.pest`'s `bare_name` has. A multi-word name
such as `Total Revenue` must be pipe-quoted.

**Keywords need a word boundary after them, not before.** `IS`, `WHERE` and the
aggregate names are spelled `^"…"` in `formula.pest`, and `is_expr`/`where_clause`/
`inline_where` are compound-atomic with `sp+` so that `Region ISLAND` and
`SUM(Rev WHEREAS x = y)` are parse errors rather than silently-wrong filters. A
`SUM`/`IF` without parens falls through to `ref_expr`, which is why a bare
keyword still names an item; the writer quotes it anyway (`quote::quote_name`).

**Pipe-quoted identifiers** (`|...|`) allow any characters — including spaces,
keywords, and operators — inside the delimiters. Use pipes when a category or
item name collides with a keyword or contains special characters:

```
|WHERE|             — category named "WHERE"
|Revenue (USD)|     — name with parens
|Cost + Tax|        — name with operator chars
SUM(|Net Revenue| WHERE |Region Name| = |East Coast|)
```

`identifier` is `pipe_quoted | bare_ident`, so a pipe-quoted name works
everywhere a bare one does: expressions, aggregate arguments, and a `WHERE`
clause's category name. A double-quoted string is a `filter_value` only — the
right-hand side of a `WHERE` or an `IS`, never an expression.

---

## 4. Separation of Concerns

### Four Layers

| Layer | Directory | Responsibility |
|-------|-----------|----------------|
| **Model** | `src/model/` | Categories, items, groups, cell data, formulas. Pure data, no rendering. |
| **View** | `src/view/` | Axis assignments, page selection, hidden items, layout computation. Derived from model. |
| **Command / Effect** | `src/command/`, `src/ui/effect.rs` | Intent (commands) and state mutation (effects). Bridges user input to model changes. |
| **Rendering** | `src/draw.rs`, `src/ui/` | Terminal drawing. Reads model + view, writes pixels. No mutation. |

### Formulas Are Data, Not Code

A formula is a serializable struct: raw text, target name, category, AST, optional
filter. It is stored in the model alongside regular data. The evaluator walks the
AST at read time. Formulas never become closures or runtime-generated code.

### Formula Evaluation Is Fixed-Point

`recompute_formulas(none_cats)` iterates formula evaluation until values
stabilize. Each pass evaluates all formula cells using the current cache
(for formula-derived values) and raw data aggregation (for data values).
This avoids recursive evaluation through `evaluate_aggregated` and
naturally handles chained formulas (`Margin = Profit / Revenue` where
`Profit = Revenue - Cost`). Circular references converge to
`CellValue::Error("circular")` after `MAX_EVAL_DEPTH` iterations.

### Display Rounding Is View-Only

Number formatting (`format_f64`) rounds for display. Formula evaluation always
operates on full `f64` precision. The rounding function is only called in
rendering paths — never in `eval_formula` or aggregation.

### Drill State Isolates Edits

When editing aggregated (drill-down) cells, a `DrillState` snapshot freezes the
current cell set. Pending edits accumulate in a staging map. On commit,
`ApplyAndClearDrill` writes them all atomically. On cancel, the snapshot is
discarded. No partial writes reach the model.

---

## 5. A Front End Displays State; It Does Not Reimplement Behaviour

**The goal is to minimise the amount of new code a new UI framework needs.**
Logic is centralised in a redux-like store, so a second or third front end is
mostly a function from state to pixels plus a wire for events — not a second
implementation of how the application behaves.

The pieces already exist and are worth naming as what they are:

| redux | here |
|---|---|
| action | `Command` / `ModelCommand` (`improvise-protocol`) |
| reducer | `reduce_view`, and `WidgetCache::apply` per widget |
| store | `ViewState` + the per-widget caches |
| dispatch | `Sessions::apply_checked` — the one door (gotcha 36) |

So the question to ask of any code in a front end is: **would a third front end
have to write this again?** If yes, it is behaviour and it belongs in the shared
layer. Three things make this concrete rather than aspirational:

**The measure is lines of framework-specific code, and it should trend to
"paint and wire".** `web/app.js` is the current benchmark: a few hundred lines
that know no protocol, reaching the application through a handful of string
calls. A front end that needs to understand `CellKey` coordinates, or which
category a row label belongs to, or what a keypress means, is one that has been
handed a job the store should have done.

**A derivation done twice is the defect, even when both copies are correct
today.** Two front ends deriving the same fact will diverge, and the divergence
is invisible until someone compares them side by side. `improvise-edp` is the
worked example — the category tree was flattened twice, two different ways, and
the fix put `viewmodel::flatten_cat_tree` where both ends call it. Gotcha 44
was the same defect and is now closed the same way (improvise-anr9): the grid's
*frame* — header levels, label tuples, group runs and their collapse state —
is `improvise_protocol::AxisFrame`, and both `src/ui/grid.rs` and
`improvise-wasm-client/src/render.rs` ask it rather than deriving one. Note
what the second cure cost that the first did not: a `Vec<String>` of labels
had to change **type**, and a shared derivation could not be added beside the
old one — `row_levels` next to `row_labels` would have been two answers to
"what are the labels", which is the disease and not the cure.

**Sharing the table beats testing two copies of it.** Where a front end must
turn input into actions, share the mapping rather than mirroring it: two
hand-kept copies held in step by a test are one definition waiting to be made
(`doctrine(a_consolidation_that_keeps_its_compatibility_test_is_unfinished)`).
That is why the keymap is shared rather than reimplemented per front end, and
why `doctrine(mirror_the_cmd_line_for_line)` is a rule about *resolvers* — the
narrow, unavoidable residue — and never a licence to restate a table.

**What legitimately stays per-framework.** Painting, event wiring, and the
mapping from a semantic value to a framework type: a `Face`, resolved by the
shared `Theme`, becomes a `ratatui::Style` in `draw.rs::face_style` (ADR 004). That mapping is the shape
to imitate — the *decision* is in the shared layer as a semantic enum, and only
the last translation is local. Anything larger than a lookup table is a
candidate for the store.

---

## 6. Guidelines for New Code

- **Add a command, not a special case.** If you need new behavior on a keypress,
  implement `Cmd`, register it, and bind it in the keymap. Do not add an
  `if key == 'x'` branch inside `handle_key`.

- **Return effects, do not mutate.** Your command's `execute` receives `&CmdContext`
  (immutable). Produce a `Vec<Box<dyn Effect>>`. If you need a new kind of state
  change, create a new `Effect` struct.

- **Use the type system.** If a value can only be one of N things, make it an enum.
  If an invariant must hold, enforce it in the constructor. If a field is
  optional, use `Option` — do not use sentinel values.

- **Test the logic, not the wiring.** Commands are pure functions of context;
  test them by building a `CmdContext` and asserting on the returned effects.
  You do not need a terminal.

- **Keep `Option`/`Result`/`Box` at the boundaries.** Core logic should work with
  concrete values. Wrap in `Option` at the edges (parsing, lookup, I/O) and
  unwrap early. Do not thread `Option` through deep call chains.

- **Reasoning lives in `context/`, not in comments.** A comment run is **at most
  two consecutive lines**, `///` and `//!` included, enforced by
  `scripts/check-comment-runs.py`. Where a reader would otherwise re-derive a
  rule wrongly, name the `context/` file that carries the argument — and only
  there, because a pointer on every function is noise. The prose lives in
  `context/code/<area>.md`; the rule exists because the arguments in this
  codebase are its most expensive artefact and a comment is the one place
  nothing checks them against the code (user, 2026-09-20; improvise-9c8c).
  A ``` fence in a doc comment is a doctest — a running test, not prose — and
  is exempt.

- **And an argument must not live only in a commit message.** A message is
  write-once and unauditable: it cannot be corrected without rewriting merged
  history, and no check in this repo can read one — the ID audit
  (`AGENTS.md`, "Cite a ticket ID you have read back") greps files in the
  tree, and a commit message is in none of them. So a reason recorded only there is a reason nothing can ever find wrong.
  When one turns out to be wrong, correct `context/` and say in `context/` that
  the message is superseded; that is the fix rather than a compromise. PR #51's
  `7d25762` is the worked example, corrected by #56.

---

## 7. Testing

### Coverage and ambition

Aim for **~80% line and branch coverage** on logic code. This is a quality floor —
go higher where the code is tricky or load-bearing, but don't pad coverage by
testing trivial getters or chasing 100% on rendering widgets. The test suite
should remain fast, and the rule is **two numbers** because the ambiguity
between them is what let the old single "under 2 seconds" drift: **no single
test binary may exceed 2 seconds**, and the workspace wall clock is the sum of
them. Excludes compilation, and excludes `harness/`, which is Playwright and
takes minutes in its own CI job. Slow tests erode the habit of running them.

**The threshold is a tripwire, and the seconds are not portable.** The same
commit measured `improvise-io`'s lib binary at 1.4s on one machine and
3.7–5.2s on another (improvise-8ec3), so a PR body citing "longest single
binary 1.47s" is checkable and *false* for the reader who checks it, which is
worse than no claim at all. Three consequences:

- **Re-measure; never re-assert.** `cargo test --workspace` prints a
  `finished in` per binary. Read your own run.
- **If you cite a figure, name the machine you took it on.** A number with no
  machine is a property of the suite, which it is not.
- **The portable signal is a ratio, not the seconds — and not the ratio to
  the median.** Most binaries print `0.00s` at two decimals, so the median is
  zero and a ratio to it is undefined (improvise-i5z6, 39 binaries). Compare
  the slowest binary with the next few slowest, across several runs: an
  outlier is one that stays well clear of them *in every run*. Both outliers so
  far were found that way and had one cause, a generator re-parsing its pest
  grammar per case: `improvise-io`'s lib (improvise-8shu) and
  `improvise-formula`'s lib (longest of 39 in all three i5z6 runs).

**improvise-8shu is closed, and its diagnosis was wrong** — which is the more
useful half. The ticket blamed an unbounded grammar walk and asked for a
constrained strategy. Instrumenting the two halves separately: `load_grammar()`
×1000 took **2.50s** against a **15.7ms** walk, so the walk was **0.6%** of the
cost and `improv_file()` was re-parsing `improv.pest` once per generated case.
Caching it in a `OnceLock` took `improvise-io`'s lib from **1.42s to 0.21s**
with the strategy's input space *unchanged*; constraining it would have moved
15.7ms and shipped as "the fix". Measured at `f152d10` on the orchestrator's
machine: **1445 tests over 34 binaries**, longest single binary **0.6–1.0s
across two runs** — the spread on one machine in one sitting being its own
argument for reading your own run. `improvise-io` is no longer the outlier.
Lowering the case count below 256 is still not on the table (see below).

### Demonstrate bugs before fixing them

Write a test that **fails on the current code** before writing the fix. Prefer a
small unit test targeting the broken function over an end-to-end test. After the
fix, the test stays as a regression guard.

**Record the bug in `context/test-evidence/`, keyed by test name** — what broke,
what fault was injected, and why the test can fail. It does not go in the test's
doc-comment: comment runs are capped at two lines (see below), and the evidence
is longer than that in every case worth having. The cost of the split is that a
renamed test orphans its entry. `scripts/check-comment-runs.py --check-evidence`
catches that, in two directions. Every `###`/`####` heading shaped like a test
name must name a test that **runs** — the names are diffed against
`cargo test --workspace -- --list` minus its `--ignored` twin, never scraped out
of the source, because text that looks like a test and a test are
indistinguishable to a scanner (improvise-rm4s). And a test whose doc comment
*cites* `context/test-evidence/` must have an entry there, or the pointer reads
as a promise the explanation exists.

Neither direction is "every test has evidence", deliberately: most tests have
none and should not be forced to have one. The reasoning and the residual holes
are in `context/code/tooling.md`.

### A check that looks like the answer, answering a neighbouring question

The characteristic defect in this repo is not a wrong line; it is a check that
is *adjacent* to the thing it names. Three instances landed in one campaign
(PRs #59/#60), in three different layers, which is why it is worth stating as a
shape rather than three gotchas:

- **A vacuous assertion.** `set_panel_open_and_cursor` asserted
  `category_panel_open` was true after an effect set it true — once the launch
  default supplied true, the effect could be replaced by a **no-op** and 1391
  tests passed. It answered *is the flag true* where it meant *did the effect
  set it*.
- **A vacuous repair of it.** The first fix folded `[open:false, open:true]`
  and asserted the final state — which a reducer that folds nothing also
  satisfies. It answered *does the text mention the other direction* where it
  meant *does the writer move the flag both ways*. **Reading the diff is what
  failed**, and it failed because the vacuous version contained `open: false`.
- **A clean auto-merge.** `git merge-tree` reporting no conflict answers *can
  git combine these texts*, where the question is *does the combination do what
  both sides meant*. Two PRs edited the same four files with no conflict and
  neither side CI-tested together.

Each has the same remedy shape: **name the question you actually mean, then
find an input that separates it from its neighbour.**

- For an assertion: **assert the direction the default cannot supply.** Closing
  the panel discriminates; opening it proves nothing.
- For a repair: inject a **one-directional** fault (`if open { set true }` —
  can raise, never lower). A vacuous repair cannot see it, because a test that
  only asserts the true direction is satisfied by a writer that only ever
  writes true.
- For a merge: **materialise it and run the gates**, and check that a count
  which should MOVE actually moved. Playwright going 16 → 17 is what showed the
  other side survived; "no conflict" and an unchanged total would both have
  been consistent with losing it.

This generalises `doctrine(fold_tests_must_test_order_not_the_axiom)` —
asserting `fold(prefix)+fold(rest) == fold(whole)` is the definition of a left
fold, so it answers a neighbouring question too. What the campaign adds is that
the pattern recurs in **test repairs** and in **merge hygiene**, not only in
production code and not only in fold tests.

### E2E pins the boundaries; unit tests work between them

**E2E and integration tests are rare and pin *boundary contracts*. Unit tests
*assume* those contracts and test between the boundaries** (user, 2026-09-20).

So the question to ask of a slow test is never "what feature does this cover" —
it is **"which boundary is this, and what is the contract at it?"** The answer
changes what you write. Four browser widgets had never worked — the tile bar,
the view panel, the formula panel, and the category sidebar ignoring
`category_panel_open` — and all four were **one** boundary failure: a
`RenderModel` field with no element painting it. Four per-widget Playwright
specs cost four slow tests and would still have missed the fifth; a single
contract assertion that *every field the render model exposes is painted*
catches all four and the next one. `harness/tests/opfs.spec.js` is the cleanest
instance of the right shape already in the tree: it runs the **storage
contract**, not a feature.

Three corollaries, in the order they bite:

- **A unit test may assume the contract rather than re-establishing it.** That
  assumption is what makes it cheap, and it is only sound because something
  slow pins the contract — so deleting the contract test silently invalidates
  every unit test that leaned on it.
- **Never weaken a contract to make it cheap.** If the honest contract needs a
  real browser, a real `Worker` or real OPFS, it stays slow. Moving it to a
  faster harness that cannot see the thing it names is how a suite goes green
  over a broken product.
- **Each slow test must say which contract it pins.** The difference between a
  contract test and a feature test is invisible from the code, so it has to be
  written down or the next person adds the fifth per-widget spec.

### Use property tests judiciously

Property tests (`proptest`) are for **invariants that must hold across all
inputs** — not a replacement for example-based tests. Good candidates:

- Structural invariants: CellKey is always sorted, each category lives on exactly
  one axis, toggle-collapse is involutive, hide/show roundtrips.
- Serialization roundtrips: save/load identity.
- Determinism: `evaluate` returns the same result for the same inputs.

Keep case counts at the default (256). Don't crank them to thousands — if a
property needs more cases to feel safe, constrain the input space with better
strategies rather than brute-forcing. Property tests that take hundreds of
milliseconds each are a sign something is wrong.

### What to test

- **Model, formula, view**: the core logic. Unit tests for each operation and
  edge case. Property tests for invariants. These are the highest-value tests.
- **Commands**: build a `CmdContext`, call `execute`, assert on the returned
  effects. Pure functions — no terminal needed. Tests are colocated in each
  command submodule (`command/cmd/<module>.rs` → `mod tests`), with shared
  test helpers in `command/cmd/mod.rs::test_helpers`.
- **Persistence**: round-trip tests (`save → load → save` produces identical
  output) plus grammar-driven property tests. The generator walks the pest
  grammar AST to produce random valid files; proptests verify
  `parse(generate())` succeeds and `parse(format(parse(generate())))` is
  stable. Cover groups, formulas, views, hidden items, pipe quoting edges.
- **Format**: boundary cases for comma placement, rounding, negative numbers.
- **Import**: field classification heuristics, CSV quoting, multi-file merge.

### What not to test

- Ratatui `Widget::render` implementations — pure drawing code that changes
  often. Test the data they consume (layout, cat_tree, format) instead.
- Trivial data definitions (`ast.rs`, `axis.rs`).
- Module re-export files.

### Test the property, not the implementation

A test like "calling `set_axis(cat, Row)` sets the internal map entry to `Row`"
is brittle — it mirrors the implementation and breaks if the storage changes.
Instead test the observable contract: "after `set_axis(cat, Row)`,
`axis_of(cat)` returns `Row` and `categories_on(Row)` includes `cat`." This
style survives refactoring and catches real bugs.