rdbms-playground/docs/adr/0053-contextual-hint-command-and-keybinding.md

# ADR-0053: Contextual `hint` — F1 live-input keybinding + `hint` command, with a tier-3 teaching corpus (H2)

## Status

Accepted — implementation pending. Revised after a `/runda` review
(2026-06-14): corrected the verbosity-default fact; re-keyed tier-3
content on a new `hint_id` (not `help_id`) so every command form — simple
and advanced-SQL — gets distinct, mode-correct content; split the
pre-submit-diagnostic and runtime-error paths; added a comprehensiveness
coverage test. The parallel question of whether the in-app `help` command
should likewise distinguish advanced-SQL forms is tracked **separately**
as Gitea issue #36 (it touches shipped, ADR-backed `help` behaviour).

Decided in conversation 2026-06-14. Closes the last open piece of **A1**
(the canonical app-command set, ADR-0003): every app command is
implemented except `hint`, which ADR-0003's command table listed as
*"Request a hint for the current input (ADR pending)."* This ADR is that
pending decision. Tracked as **H2** in `docs/requirements.md`.

References ADR-0003 (app-command set + the `:` escape), ADR-0019 (the
friendly error layer / H1), ADR-0021 (per-command usage templates / H1a),
ADR-0022 (ambient typing assistance — colour + hint panel + completion),
ADR-0027 (input validity indicator), ADR-0046 (sidebar navigation +
responsive input hint), ADR-0049 (input-field readline keymap), and
ADR-0051 (context/state-aware keybinding strip).

## Context

`hint` is the only unbuilt app command. The naive reading — "show a hint" —
hides a real subtlety, and a real cost.

**The subtlety: a submitted `hint` command cannot see live input.** App
commands are submitted with Enter, which empties the input buffer. By the
time `hint` dispatches, the partial command it was meant to help with is
gone. So "a hint for the current input" cannot be served by a submitted
command alone — it needs a *keybinding* that acts on the live buffer
without submitting. ADR-0003 said "current input"; `requirements.md`
broadened it to "current input **or the most recent error**." Both are
wanted; they map to two different trigger surfaces.

**The cost: the value of `hint` is content, not plumbing.** The app
already carries two tiers of contextual text:

- **Tier 1** — terse, always-on: syntax colour (ADR-0022); the error
  *headline* alone (ADR-0019, when `messages_verbosity: Short`).
- **Tier 2** — short contextual lines: the ambient typing prose /
  `expected` set, shown live while typing (ADR-0022, catalogue
  `hint.ambient_*` / `hint.value_slot_*`); and the error `hint:` field —
  which, because `Verbosity::Verbose` is the **default**
  (`src/friendly/translate.rs:46`), is shown **by default** beneath every
  error headline (`messages short` is the opt-*out*, not `messages
  verbose` the opt-in).

So the verbose error hint is **already on screen by default**. If `hint`
merely re-showed it, it would duplicate what the user can already see (and
the ambient panel). To justify itself, `hint` must add a **tier 3**: a
genuinely deeper, *teaching*-grade explanation — what the command/error
means, a worked example, and the underlying relational concept. That
corpus does not exist yet, and
authoring it (to the standard of a teaching tool, where "pedagogy wins
ties") is the bulk of the work.

The mechanism is small and reuses everything already present: the command
REGISTRY (`src/dsl/grammar/mod.rs`), the `AppCommand` enum
(`src/dsl/command.rs`), key dispatch (`App::handle_key`,
`src/app.rs:1155`), the `note_help`/`note_help_topic` renderers
(`src/app.rs:2982`/`3021`), the parser/walker expected-set
(`ParseError.expected`, `WalkResult.tail_expected`), the friendly
catalogue + `t!` macro + `keys.rs` validation, and the output styling
vocabulary (`OutputStyleClass::Hint`).

## Decision

### D1 — Two surfaces, no topic argument

`hint` is delivered through **two complementary surfaces**:

1. **F1 keybinding → live input.** Pressing **F1** while typing renders a
   tier-3 hint for the command currently in the buffer, into the output
   panel, **without submitting or altering the buffer**. This is the
   primary, most-valuable path (it serves the literal "current input").
2. **`hint` command → most recent error.** Submitting `hint` renders the
   tier-3 expansion of the most recent error. This is why the command
   exists despite the empty-buffer problem: the thing it helps with is
   the *last thing you tried*, not the now-empty buffer.

`hint` takes **no topic argument**. Explicit per-command reference is
already `help <topic>` (H3); `hint` is purely *contextual*, which keeps
the two cleanly distinct (`hint` = "help me with what I'm doing right
now"; `help insert` = "show me the insert reference").

F1 is a **read-only overlay**: it never alters the input buffer, the
cursor, or the live completion memo (ADR-0022) — it only emits a block
into the output journal. (It must therefore be handled in `handle_key`
*before* the "any other key clears the memo" fall-through.)

### D2 — Trigger matrix

| Trigger | Buffer / state | Result |
|---|---|---|
| **F1** | non-empty input | tier-3 hint for the command being typed, plus the live "expected next" (from the walker's `tail_expected` / parser `expected`) |
| **F1** | empty input, a recent error exists | tier-3 expansion of that error |
| **F1** | empty input, no recent error | a short "getting started" pointer (press F1 while typing a command; `help` for the full list) |
| **`hint`** (submitted) | a recent error exists | tier-3 expansion of that error (primary use) |
| **`hint`** (submitted) | no recent error | the same "getting started" pointer |

F1 is inert behind a modal and while a sidebar panel holds navigation
focus (consistent with the existing `handle_key` gates, ADR-0046); it is
active in the input context in both Simple and Advanced mode.

**Two error sources, one namespace.** Errors come in two kinds and reach
`hint` by different routes:

- **Pre-submit diagnostics** (the ~33 `diagnostic.*` classes — arity,
  type, unknown table/column) are computed *while typing* by the walker.
  The **F1 live-input path** reads the current under-cursor diagnostic
  directly from the walker (the same source the ambient panel uses) and
  renders its `hint.err.<class>` block — no stored state needed.
- **Runtime errors** (the 9 `translate_error` classes) occur *after*
  submit. The **`hint` command / empty-input F1** path reads them via the
  stored `last_error_hint_key` (D5).

Both render from the same `hint.err.*` namespace. **`:`-prefix handling:**
on the simple-mode one-shot escape (`: SELECT …`), command
identification for the F1 path strips the leading `:` first, so the
advanced form is matched.

### D3 — The tier-3 content model

Tier-3 blocks live in the friendly catalogue under the existing `hint:`
top-level namespace (where tier-2 ambient strings already live), in two
new sub-namespaces:

- **`hint.cmd.<hint_id>`** — one per command **form**, keyed by a **new
  `hint_id: Option<&'static str>`** field added to `CommandNode`
  (`src/dsl/grammar/mod.rs:512`, parallel to the existing `help_id` /
  `usage_ids`). The F1 live-input path resolves the current input to its
  command node and looks up `hint.cmd.<node.hint_id>`.

  **Why a new field, not `help_id`:** `help_id` is **not** 1:1 with
  command forms. The 7 advanced-mode SQL nodes (`SELECT`, `WITH`,
  `SQL_INSERT/UPDATE/DELETE`, `EXPLAIN_SQL`) carry `help_id: None` *purely
  to dedup the `help` command's printed list* (they share an entry word
  with a simple sibling — see `grammar/mod.rs:915-918`), not because they
  lack distinct content. Their SQL syntax differs from the simple-DSL
  sibling's, so they **must get their own tier-3 block**. A dedicated
  `hint_id` gives every one of the ~37 REGISTRY nodes — simple and
  advanced-SQL alike — its own key and its own mode-correct example, with
  no sharing or deferral. (The analogous gap in the `help` command is out
  of scope here — issue #36.)
- **`hint.err.<class>`** — one per error/diagnostic class, keyed by the
  friendly error/diagnostic key (e.g. `hint.err.foreign_key.child_side`,
  `hint.err.type_mismatch`, `hint.err.insert_arity_mismatch`). Used by
  both error routes (D2).

Each tier-3 block is a **structured entry with three labelled parts**, so
the voice stays consistent and the renderer can style them uniformly:

```yaml
hint.cmd.dsl.insert:
  what: "Add one or more rows to a table."
  example: "insert into Customers values ('Ann', 'ann@x.io')"
  concept: "A row is one record; each value lines up with a column, in
    order. Columns typed `serial`/`shortid` fill themselves — leave them out."
```

- **`what`** — one or two plain sentences: what this command does / what
  this error means.
- **`example`** — a single concrete, copyable line (rendered neutral, not
  muted, so it stands out as runnable).
- **`concept`** — the underlying relational idea, in teaching voice; the
  part that makes this tier-3 rather than tier-2.

`concept` is optional where there is genuinely no concept beyond the
mechanics (e.g. `quit`); `what` + `example` are always present.

### D4 — Rendering

Both surfaces render through one new renderer, `App::note_hint*` (sibling
of `note_help`/`note_help_topic`, `src/app.rs`), emitting a small framed
block into the `output` buffer as `OutputKind::System` with
`OutputStyleClass::Hint` on the `what`/`concept` prose and `Neutral` on
the `example` line. The block is **persistent** (scrolls in the journal),
unlike the transient ambient panel — pressing F1 is an explicit request
to *keep* the deeper guidance on screen. The bottom keybinding strip
(ADR-0051) advertises F1 in the editing/typing state.

### D5 — "Most recent (runtime) error" state

The **runtime-error route** (submitted `hint`, and empty-input F1) needs
to map the last runtime error back to its `hint.err.<class>` key. Runtime
errors today live only as rendered text in the `output` buffer. We add a
single small piece of `App` state — **`last_error_hint_key:
Option<String>`** — set at the `translate_error` call sites
(`runtime.rs:2615`, `app.rs:2424`) when a friendly error is rendered,
cleared when a later command succeeds. Absent → the "getting started"
pointer.

The **pre-submit-diagnostic route** (the F1 live-input path) needs no
stored state: it reads the current diagnostic from the walker at F1 time
(D2). This is the cleaner split the `/runda` pass surfaced — typing-time
diagnostics and post-submit runtime errors are genuinely different
sources and should not be funnelled through one stored key.

### D6 — Content scope: comprehensive for v1

v1 ships tier-3 content for the **whole inventory**, not a subset (the
graceful tier-2 fallback below is a safety net, not the plan):

- **~37 command forms** — every distinct node in `REGISTRY` gets its own
  `hint.cmd.<hint_id>` block (app + DSL + DDL + advanced-mode SQL forms),
  each with a **mode-correct example** (the advanced-SQL forms show SQL
  syntax, their simple siblings show DSL — no sharing).
- **9 runtime error classes** — `unique`, `foreign_key` (×4 sides),
  `not_null`, `check`, `type_mismatch`, `not_found`, `already_exists`,
  `generic`, `invalid_value` — each gets a `hint.err.*` block.
- **~33 `diagnostic.*` pre-submit classes** — arity, type, unknown
  table/column, etc. — each gets a `hint.err.*` block.

The full enumerated checklist is the implementation plan's tracking
artifact (see *Content inventory*, below).

**Fallback (safety net):** if a tier-3 key is ever missing at runtime,
the surface degrades to tier 2 — the ambient prose for the command path,
or the verbose error `hint:` for the error path — never to a blank or an
error. The `keys.rs` build-time validation keeps the corpus honest, so a
missing key is caught in tests, not in front of a student.

### D7 — Authoring process: exemplars-first

Because the corpus is large and its *voice* is a pedagogical decision the
maintainer owns, content is produced in two stages:

1. This ADR carries **2–3 worked exemplars** (below) as the canonical
   style reference. The `/runda` review of this ADR is where the voice and
   depth are approved.
2. Once approved, the remaining blocks are authored to that template in
   **reviewable batches** (grouped by area: DDL, DML, app commands,
   error classes), not one monolithic drop.

### Exemplars (the style reference to approve)

**Command (F1 live-input), `insert`:**

```
Hint — insert
  What:    Add one or more rows to a table.
  Example: insert into Customers values ('Ann', 'ann@x.io')
  Concept: A row is one record; each value lines up with a column, in
           order. Columns typed serial/shortid fill themselves — leave
           them out.
  Next:    a value list `(...)`, or `(col, ...) values (...)` to name columns
```
(The "Next:" line is the live expected-set from the walker, shown only on
the non-empty-input F1 path.)

**Error (`hint` command), foreign-key child-side violation:**

```
Hint — no parent row to point at
  What:    The value you inserted into Orders.customer_id doesn't match
           any Customers row, so the foreign key has nothing to point at.
  Example: First   insert into Customers values ('Ann', ...)
           Then    insert into Orders values (..., 'Ann')
  Concept: A foreign key is a promise that every child points at a real
           parent. The parent must exist first. To allow orphans on
           delete instead, set the relationship's `on delete` to
           `set null` or `cascade`.
```

**Command (F1 live-input), `add 1:n relationship`:**

```
Hint — add relationship
  What:    Link two tables so a parent row can own many child rows.
  Example: add 1:n relationship from Customers.id to Orders.customer_id
  Concept: The "1:n" means one parent, many children. The child column
           holds the foreign key; `--create-fk` adds it for you if it
           doesn't exist yet.
```

## Forks (all user-chosen, 2026-06-14)

- **Trigger model:** both a keybinding (live input) and a submitted
  command (last error), rather than command-only or keybinding-only — the
  live-input path is the most useful, but the command completes the A1
  slot and serves the error case.
- **Keybinding = F1:** the universal help convention; the key is
  genuinely free (no `KeyCode::F(1)` binding exists today — the `"F1"`
  strings in `input_render.rs`/tests are scenario labels, not the key, and
  ADR-0022 uses no `F1` requirement label). No collision with the ADR-0049
  readline keys, `Ctrl-O` (ADR-0046), `Esc`-clear, or the reserved
  `Ctrl-C` cancel (I5). Rejected: `?` (a typeable character — fiddly
  position-dependent handling) and a Ctrl/Alt chord (less discoverable, no
  advantage).
- **No topic argument:** contextual only; `help <topic>` already owns
  explicit reference lookup.
- **Comprehensive content for v1:** the full inventory, not a starter
  subset.
- **Exemplars-first authoring:** lock the voice on a few blocks, then
  mass-author to template.

## Consequences

- **A1 closes.** With `hint` registered and built, all 15 canonical
  app-level commands exist in both modes.
- **A third contextual tier exists.** Students get on-demand, teaching-
  grade guidance that is deeper than the always-on colour, the headline,
  the ambient one-liner, and the verbose error hint — without cluttering
  those terse defaults.
- **One new keybinding (F1)** joins the keymap and the ADR-0051 strip.
- **A new `hint_id` field on `CommandNode`** (parallel to `help_id`), one
  new field of `App` state (`last_error_hint_key`), and one new renderer
  family (`note_hint*`); the `AppCommand` enum gains `Hint`, the grammar a
  `HINT` node, the REGISTRY one entry.
- **A large, durable content corpus** (~37 command blocks + ~42 error/
  diagnostic blocks ≈ 80) enters the catalogue under `hint.cmd.*` /
  `hint.err.*`, validated by `keys.rs`. This is ongoing surface area: new
  commands/error classes should ship with their tier-3 hint (a checklist
  item for future feature ADRs).
- **Testing:** Tier-1 unit tests for the trigger matrix (F1 with
  empty/non-empty input; `hint` with/without a recent error;
  `last_error_hint_key` set on the `translate_error` sites and cleared on
  success; the pre-submit-diagnostic vs runtime-error routing; the `:`
  strip), the command-identification logic, and the tier-2 fallback;
  Tier-2 `insta` snapshots for a representative rendered hint block;
  Tier-3 integration tests for the end-to-end flows (type a partial
  command → F1 → block appears, **buffer and completion memo untouched**;
  run a failing command → `hint` → error expansion). **A
  comprehensiveness coverage test** (enforces D6): iterate the REGISTRY
  and assert every node has a `hint_id` resolving to a `hint.cmd.*` block,
  and every runtime-error/diagnostic class has a `hint.err.*` block —
  `keys.rs` only checks that *referenced* keys resolve, not that every
  command/error *has* one, so this test is what makes "comprehensive"
  enforceable rather than aspirational.

## Out of scope

- **Per-topic `hint <topic>`** — OOS (rejected): `help <topic>` already
  serves explicit lookup; a topic arg would overlap it and double the
  content-authoring surface.
- **Re-showing tier-3 inline as the always-on ambient hint** — OOS
  (rejected): the ambient panel stays terse by design (ADR-0022); tier-3
  is on-demand. Promoting it would defeat the tiering.
- **Localised tier-3 content beyond `en-US`** — OOS (deferred): the
  catalogue is structured for i18n (ADR-0019), but additional locales
  follow the project's English-only-for-v1 stance (requirements X2).
- **`hint` for a *successful* command's deeper teaching** (e.g. "you just
  created a table — here's what an index would add") — OOS (deferred): a
  plausible future tier-3 use, but v1 scopes the command path to errors
  and the F1 path to in-progress input.

## Content inventory (implementation tracking)

The implementation plan enumerates and checks off every block:

- **`hint.cmd.<hint_id>`** — one per distinct `REGISTRY` node (~37), each
  with its own `hint_id` and a mode-correct example: app (`save`, `save
  as`, `load`, `new`, `rebuild`, `export`, `import`, `replay`, `undo`,
  `redo`, `mode`, `messages`, `copy`, `help`, `hint`, `quit`); DDL
  (`create table`, `create m:n`, `add column`/`relationship`/`index`,
  `drop`, `rename`, `change column`); DML (`insert`, `update`, `delete`,
  `show`, `seed`, `explain`, `select`/`with`). The **7 advanced-mode SQL
  forms** (`SQL CREATE TABLE`, `ALTER TABLE`, `CREATE/DROP INDEX`, `DROP
  TABLE`, `SQL INSERT/UPDATE/DELETE`, `EXPLAIN SQL`, raw `SELECT`/`WITH`)
  each get their **own** block with SQL syntax — they do **not** reuse
  their simple sibling's (this is the `/runda` correction; the parallel
  `help`-side gap is issue #36).
- **`hint.err.*`** — one per runtime error class (`unique`,
  `foreign_key.{child,parent}_side`, `not_null`, `check`,
  `type_mismatch`, `not_found`, `already_exists`, `generic`,
  `invalid_value`) and per `diagnostic.*` pre-submit class.