# ADR-0021: Parser-as-source-of-truth for H1a (per-command usage in parse errors)

## Status

Accepted.

Builds on ADR-0020 (tokenization layer). Addresses H1a from
`requirements.md` — the parse-error pedagogy gap that
ADR-0019's friendly-error layer left untouched.

Cross-references ADR-0019 (i18n catalog conventions; H1a's
output goes through the same catalog) and ADR-0009 (DSL
syntax conventions; usage templates render in the project's
documented surface form).

## Context

ADR-0019 dramatically improved engine-error wording.
Parse-error wording is now the visibly-weakest user surface —
the user-reported gap was concrete: typing `create` produces

```
parse error: after `create`, expected `table`
```

The error is *structurally* correct (chumsky has consumed
`create` and is now looking for the next required token) but
*pedagogically* silent. A learner who got this far typed
`create` because they'd been told that's how new tables are
made; what they need next is the shape of the command, not
a single missing-token pointer.

Comparable observations apply across the whole DSL surface:

- `add`  → expected `column` or `1` (uninformative; user
  needs the shape of `add column …` AND `add 1:n
  relationship …`).
- `update Customers`  → expected `set` (true; but `update`'s
  full grammar with `set …`, `where …`, `--all-rows` is what
  the user wants illustrated).
- `frobulate Customers`  → expected one of `create`, `drop`,
  `add`, `rename`, `change`, `show`, `insert`, `update`,
  `delete`, `replay` (true after ADR-0020; the available-commands
  list is now informative, but the no-prefix case wants its
  own framing — "available commands" rather than "expected").

H1a's job is to close that gap by surfacing the **grammar**
of the command at the point of error, not just the next
token.

### What ADR-0020 supplies

ADR-0020 lands the lexer + parser-over-tokens architecture.
What that buys H1a:

- **Aggregated `expected` sets at the failure point** (top-level
  `choice` failures now list every command-starting keyword,
  not just one). The user-visible "available commands" list
  becomes correct without any work in this ADR.
- **Token-kind error patterns** (`RichPattern<Token>` instead
  of `RichPattern<char>`). Each pattern renders via a stable
  catalog key — no per-character humanising.
- **A canonical entry-token for each command** (the first
  `Keyword(_)` consumed). H1a keys per-command usage
  templates off this token.

### What this ADR adds on top

- A registry of per-command **usage templates** (one
  declaration per command).
- A renderer that composes the parse error with: caret +
  structural error wording + matching usage template(s).
- New catalog keys under `parse.usage.*` (templates) and
  `parse.token.*` (single-token rendering for expected-set
  joins).
- A "no commands consumed" fallback that renders an
  available-commands list under a different prefix
  ("available commands:" rather than "expected:") for the
  zero-prefix case.

## Decision

### 1. Per-command usage template registry

Each command parser is paired with a `UsageEntry`:

```rust
pub struct UsageEntry {
    /// First keyword that distinguishes this command. Used
    /// as the registry key.
    pub entry: Keyword,
    /// Catalog key for the grammar template body (under
    /// `parse.usage.*`). One key per command.
    pub catalog_key: &'static str,
}
```

The registry is a `&'static [UsageEntry]` declared in one
place (`src/dsl/usage.rs`). Lookup: given a consumed entry
keyword, return all entries whose `entry == keyword`. For
`Keyword::Add` the registry returns the `add column` and
`add 1:n relationship` entries; for `Keyword::Drop` it
returns `drop table`, `drop column`, `drop relationship`;
for unique-entry keywords (e.g. `Keyword::Create` today) it
returns one.

The catalog key is what gets translated. Template bodies
live in `src/friendly/strings/en-US.yaml` under
`parse.usage.*`:

```yaml
parse:
  usage:
    create_table: "create table <Name> with pk [<col>:<type>[, ...]]"
    drop_table:   "drop table <Name>"
    add_column:   "add column [to] [table] <Table>: <Name> (<Type>)"
    add_relationship: |
      add 1:n relationship [as <Name>]
        from <Parent>.<col> to <Child>.<col>
        [on delete <action>] [on update <action>]
        [--create-fk]
    rename_column: "rename column [in] [table] <Table>: <Old> to <New>"
    change_column: |
      change column [in] [table] <Table>: <Name> (<Type>)
        [--force-conversion | --dont-convert]
    show_data:    "show data <Table>"
    show_table:   "show table <Table>"
    insert:       "insert into <Table> [(<col>[, ...])] [values] (<value>[, ...])"
    update:       "update <Table> set <col>=<value>[, ...] (where <col>=<value> | --all-rows)"
    delete:       "delete from <Table> (where <col>=<value> | --all-rows)"
    drop_column:  "drop column [from] [table] <Table>: <Name>"
    drop_relationship: |
      drop relationship <Name>
      drop relationship from <Parent>.<col> to <Child>.<col>
    replay:       "replay <path>  |  replay '<path with spaces>'"
```

(Wording is illustrative; exact phrasing settled at
implementation time. The bracket convention `[...]` for
optional parts and angle-bracket `<...>` for placeholders
matches ADR-0009's documentation surface.)

### 2. The renderer composes three blocks

A parse error renders as:

```
running: <user input>
        ^               ← caret (existing, unchanged)
parse error: <structural-or-content message>
usage: <template1>
       <template2>      ← when multiple entries share the entry keyword
```

Block 1 (the echo + caret) is unchanged from today.

Block 2 is the structural or content error. ADR-0020
guarantees the structural error is now properly aggregated
("expected `data` or `table`" not "expected `table`"). The
content errors (unknown type, mutually-exclusive flags) are
unchanged in voice.

Block 3 (usage:) is new. It is rendered if and only if **at
least one keyword token was consumed** before the parser
failed AND that keyword is a registered entry. If no keyword
was consumed (e.g., `frobulate Customers`, where `frobulate`
is an `Identifier`, not a `Keyword`), Block 3 is replaced
with the no-prefix fallback (§5).

If multiple entries match (e.g., the `add` family), all are
listed under a single `usage:` prefix, one per line.

### 3. Identifying the consumed entry keyword

The parser surfaces, alongside the `ParseError`, the
**deepest successfully-consumed keyword token**. Mechanism:

- `parse_tokens` returns `(Result<Command, ParseError>,
  ParseDiagnostics)` where `ParseDiagnostics` carries the
  furthest position chumsky reached AND a snapshot of the
  consumed prefix.
- The renderer walks the consumed prefix backward to find the
  first `Keyword(_)` token. (Almost always the first token,
  but a future grammar where a command starts with a
  literal — none today — would still resolve correctly.)

This logic lives in `src/dsl/usage.rs::matched_entry()` so
the registry and the lookup sit together.

### 4. `parse.token.*` — single-token catalog vocabulary

Chumsky's expected-set rendering needs a name for each token
kind. Today `humanise()` hand-codes these
(`describe_pattern` returns "`create`", "identifier", etc.).
ADR-0021 moves the vocabulary into the catalog:

```yaml
parse:
  token:
    # Keywords — one entry per Keyword enum variant.
    keyword.create: "`create`"
    keyword.table:  "`table`"
    keyword.with:   "`with`"
    # ... one per Keyword variant ...

    # Punctuation.
    punct.colon:        "`:`"
    punct.open_paren:   "`(`"
    punct.close_paren:  "`)`"
    punct.comma:        "`,`"
    punct.equals:       "`=`"
    punct.dot:          "`.`"

    # Token-class labels.
    identifier:     "identifier"
    number:         "number"
    string_literal: "string literal"
    flag:           "flag (--name)"
    end_of_input:   "end of input"

    # Lexer-error tokens.
    error.unterminated_string: "unterminated string starting at column {column}"
    error.unknown_char:        "unrecognised character {found}"
```

Joining ("`a`, `b`, or `c`") stays in code (`oxford_or` from
the current humanise machinery, lifted intact). Wording of
each token is in the catalog.

`parse.error` (existing wrapper key) stays. Its `{detail}`
placeholder is filled by:

```
{consumed_prefix} expected {oxford_or(expected)}, found {found_token}
```

— each piece sourced from the catalog, joined in code.

`parse.caret` (existing) and `parse.empty` (existing)
unchanged.

### 5. No-prefix fallback: "available commands"

When the parser fails with **no keyword consumed**, the
"expected" set lists every top-level command-starting
keyword. That's correct but the framing should be
"available commands" rather than "expected".

Renderer detects this case (consumed-keyword count == 0) and
substitutes Block 3 with:

```
available commands: create, drop, add, rename, change,
                    show, insert, update, delete, replay
```

via a new catalog key:

```yaml
parse:
  available_commands: "available commands: {commands}"
```

The list is the alphabetised set of `entry` keywords from
the usage registry, each rendered via its `parse.token.keyword.*`
catalog entry (so the strings are catalog-sourced, not
hard-coded).

This case only fires when the user typed something the
parser couldn't classify as any known command keyword — the
"frobulate Customers" case. It's both rarer and more useful
than the with-prefix case: a user this lost benefits more
from the full menu than from a missing-token pointer.

### 6. Anchor-phrase compliance (ADR-0019 §10)

ADR-0019's anchor-phrase list contains nine substrings the
catalog commits to keeping stable. None are parse-error-specific,
so this ADR doesn't add to the list. The existing parser
test that asserts on "unknown type" and "expected one of"
substrings stays — those come from `Type::from_str`'s custom
error message which ADR-0020 §4 keeps unchanged.

The current structural-error tests assert on substrings like
"after `show data`", "expected identifier", "found end of
input", "after `change column Rich`", "expected `:`". The
new render shape preserves all of these — the rendering
template is `{prefix} expected {set}, found {token}` and
the prefix / set / token come from the catalog with the same
wording. Tests should port unchanged or with at most minor
adjustments.

### 7. Catalog validator covers the new keys

ADR-0019 §8.6's `KEYS_AND_PLACEHOLDERS` validator extends
to cover:

- Every `parse.usage.<command>` key referenced from the
  registry exists.
- Every `parse.token.keyword.<variant>` key for every
  `Keyword` enum variant exists.
- Every `parse.token.punct.<variant>` key for every `Punct`
  variant exists.
- The `parse.token.{identifier, number, string_literal,
  flag, end_of_input}` keys exist.
- The `parse.token.error.*` keys exist for every
  `LexErrorKind` variant.
- The `parse.available_commands` key exists.
- No format specifiers (already enforced).
- No engine vocabulary (already enforced).

### 8. The `usage:` block respects the verbosity setting?

No. The `messages (short|verbose)` setting (ADR-0019)
governs *engine-error* verbosity (whether to render the
hint block of a `FriendlyError`). Parse errors don't go
through `FriendlyError`; they have their own render path,
and the usage block is always shown. Rationale: a learner
toggling to `messages short` is signalling they recognise
the engine-error patterns and want less explanation around
those — they're not signalling that they want less
parse-help. Parse errors mean the user couldn't even
formulate a runnable command; that's exactly the moment to
maximise pedagogical surface, regardless of the
engine-error verbosity preference.

If experience shows this is wrong, a future amendment can
gate the usage block on a separate setting. Doesn't need
to be designed now.

## Out of scope

1. **Tab completion (I3) and syntax highlighting (I4)**
   themselves. ADR-0020 §9-10 commits to the parser
   contract; ADR-0021 doesn't extend it.
2. **Schema-aware suggestions** ("did you mean `Customers`?"
   when the user typed `Customrs`). Useful but a separate
   feature; would land in I3 territory (completion + spell
   check share a candidate list).
3. **Suggested fixes** ("change `crete` to `create`"). Same
   bucket as schema-aware suggestions.
4. **Multi-error reporting.** Today and after this ADR, the
   parser reports the first error and stops. Recovery-based
   multi-error parsing is out of scope and re-opens with
   I3's ADR (ADR-0020 §11).
5. **Persisting the verbosity setting** (which doesn't
   affect parse errors anyway, per §8). ADR-0019 deferred it
   to a future settings ADR.

## Consequences

### Positive

- **Per-command usage at point of failure.** A learner who
  types `create` sees the full `create table` grammar
  instead of "expected `table`". The user-reported gap
  closes.
- **Aggregated `available commands` for cold starts.**
  `frobulate Customers` now lists the ten command-starting
  keywords under a sensible framing.
- **Vocabulary lives in the catalog, not in code.** Renaming
  a keyword's user-facing wording is one YAML edit. Adding
  a new keyword adds two lines (registry + token-name key);
  the validator catches both if forgotten.
- **The render path simplifies.** `humanise()` shrinks to a
  small composer over catalog lookups — no per-character
  description, no `RichPattern` walking, no
  prefer-custom-over-structural switching (the latter
  becomes "render the structural error and append the usage
  template").
- **Composes with ADR-0019's `FriendlyError`.** Engine
  errors and parse errors are rendered through different
  paths but both go through the catalog, so vocabulary
  drift between them is impossible.

### Costs

- **A second registry to keep in sync** with the parser. The
  validator (§7) catches missing usage entries / missing
  token keys at test time, but adding a new command means
  three steps (parser combinator, usage-registry entry,
  catalog YAML edit). Mitigation: a unit test asserts every
  command in the parser has a registry entry (catches
  forgotten entries; matches the friendly-module pattern).
- **Catalog grows by ~30-40 entries** (one usage template
  per command, one keyword name per `Keyword` variant, a
  handful of token-class names, a handful of error names).
  Each entry is one line of YAML; total catalog grows from
  ~170 entries to ~210. Within budget.
- **Wording iteration** on the usage templates will probably
  happen post-merge. This is normal for pedagogical text
  and the catalog makes it cheap.

### Neutral

- **Public parser API is unchanged.** `parse_command(&str)`
  signature stable. The new `lex` and `parse_tokens`
  functions exposed by ADR-0020 are the I3/I4 hook;
  ADR-0021 doesn't add to that surface.
- **`AppEvent` shape unchanged.** Parse errors continue to
  flow through `dispatch_dsl`'s existing path (push echo,
  push caret, push error). This ADR's render changes are
  internal to that function plus the `t!()` calls inside it.

## Implementation notes

### Order of operations (within the joint ADR-0020 + ADR-0021 implementation session)

1. Land ADR-0020 (lexer + parser refactor + minimal
   humaniser).
2. Add `src/dsl/usage.rs` with the registry struct, the
   static table, and `matched_entry()`.
3. Populate `parse.usage.*` and `parse.token.*` catalog
   sections.
4. Extend `friendly::keys::KEYS_AND_PLACEHOLDERS` with the
   new keys.
5. Rewrite `dispatch_dsl`'s error-render arm in `app.rs` to
   compose the three blocks per §2 (or §5 fallback).
6. Add tests:
   - Unit: every registered usage entry resolves through the
     catalog. Every `Keyword` variant has a `parse.token.keyword.*`
     entry.
   - Integration (`tests/parse_error_pedagogy.rs`, new):
     `create`, `add`, `update Customers`, `frobulate
     Customers`, `create table` (no PK clause), `insert into
     T` (no values), each producing the expected
     three-block output.
7. Update or port the two existing structural-error tests
   in `parser.rs::tests` to the new render shape.

### Things that interact subtly

- **The "deepest consumed keyword" mechanism** (§3) walks
  the prefix once per parse failure. Cheap; no perf concern.
  But it must not pick up keywords from inside content that
  is itself part of a partial AST (e.g. an identifier the
  user is typing that happens to be the first letters of a
  keyword); since the lexer commits to identifier-vs-keyword
  classification before the parser sees tokens, this isn't a
  real risk. Documented inline.
- **Multiple usage entries per `add` / `drop`** are rendered
  under one `usage:` prefix per §2. This is one of the
  pedagogically-best parts of the change: the user gets the
  full family rather than guessing which sibling they
  wanted.
- **`replay`'s special-case parsing** (ADR-0020 §6) is
  invisible to the usage layer. The user typing `replay`
  with no path gets the `parse.usage.replay` template.
- **`messages` is an app-level command, not a DSL command**,
  so it is not in the parser registry and doesn't appear in
  `available commands:`. Same posture as `mode`, `help`,
  `quit`. Documented in the registry's prelude.
