Add a parser for a new file format

remindb's pkg/parser/ package turns raw source bytes into a slice of *ContextNode trees. Adding a format means routing one new extension through ParseBytes and emitting nodes that match the conventions every other format already follows.

Where the format lands

Five files. Skipping any one of them ships a half-wired format.

File	What changes
pkg/parser/<format>.go	New file — the parser itself
pkg/parser/parser.go	Add a case to the ParseBytes switch
pkg/parser/<format>_test.go	New file — table tests over fixture strings
pkg/parser/fuzz_test.go	Add 2–4 f.Add(...) seeds to FuzzParseBytes
pkg/parser/testdata/ (optional)	Drop a sample file if tests are easier with a real fixture

The parser file shape

Mirror pkg/parser/yaml.go exactly. The pattern is from .claude/rules/go-concise.md §4 ("Namespace prefix-sharing helpers via a struct"): an empty struct provides the namespace, a free function is the entry point, methods drop the prefix.

package parser

import "fmt"

type TomlParser struct{}

func parseToml(path string, data []byte) ([]*ContextNode, error) {
    return TomlParser{}.parse(path, data)
}

func (p TomlParser) parse(path string, data []byte) ([]*ContextNode, error) {
    // ... format-specific decode ...
    if err != nil {
        return nil, fmt.Errorf("failed to parse: toml %s: %w", path, err)
    }
    return []*ContextNode{buildNode(path, "", root, 1)}, nil
}

Initialism rule (memory + go-concise.md): TomlParser not TOMLParser, JsonParser not JSONParser. Pascal-case file-format initialisms.

Error message rule (go-concise.md §5): action errors take a failed to <verb>: prefix and wrap with %w. Validation errors carry no prefix.

Abort vs. skip on bad content: by default a parse failure aborts the whole-workspace compile (the errgroup fails fast). If a malformed file of your format should instead be skipped with a warning — like JSON config that routinely carries comments (#171) — wrap the decode failure with the ErrMalformed sentinel (fmt.Errorf("%w: <format> %s: %v", ErrMalformed, path, err)) instead of the failed to <verb>: form. pkg/compiler/compiler.go already skips ErrMalformed exactly as it skips ErrUnsupportedExt; no compiler change needed. Note this drops the loud error for genuine corruption, so reserve it for formats where unparseable input is expected noise, not a signal.

The switch entry

Open pkg/parser/parser.go, find ParseBytes, add the case in the alphabetic position you'd expect a reader to scan:

switch ext {
case ".md", ".markdown":
    return parseMarkdown(path, data)
case ".toml":
    return parseToml(path, data)         // <-- new line
case ".yml", ".yaml":
    return parseYaml(path, data)
// ...
default:
    return nil, fmt.Errorf("%w: %q", ErrUnsupportedExt, ext)
}

Multi-extension formats list every variant in one case line (see .md/.markdown, .yml/.yaml, .jsonl/.ndjson).

The unit test

pkg/parser/yaml_test.go is the cleanest template. Table-driven, exercises the happy path plus at least one each of: empty input, malformed input, deeply nested input, and the format's specific edge case (BOM, frontmatter delimiter, mixed-type collection, etc.).

The fuzz seed corpus — do not skip this

pkg/parser/fuzz_test.go defines FuzzParseBytes, which scripts/fuzz.sh auto-discovers. Add seeds covering your format's shape variety:

f.Add("data.toml", []byte("title = \"Hello\"\n[section]\nkey = 1"))
f.Add("data.toml", []byte(""))                    // empty
f.Add("data.toml", []byte("[unclosed"))           // malformed
f.Add("DATA.TOML", []byte("title = \"upper\""))   // extension case

The fuzz invariant is "must never panic regardless of input." If your parser deref's a nil from the decoder or indexes past a slice, fuzz will find it. Run scripts/fuzz.sh 30s locally before committing to confirm the new seeds don't regress.

Quick reference

1. Create pkg/parser/<format>.go        (empty struct + parseXxx + (p) parse)
2. Wire pkg/parser/parser.go            (one case in ParseBytes)
3. Create pkg/parser/<format>_test.go   (table tests)
4. Add seeds to pkg/parser/fuzz_test.go (FuzzParseBytes f.Add lines)
5. go test ./pkg/parser/...             (must pass)
6. scripts/fuzz.sh 30s                  (must pass, no panics)

Common mistakes

• Forgetting fuzz seeds. The fuzz test will still run and pass, but it'll never exercise the new format. The fuzz step is what catches "decoder returns nil and parser deref's it" before it ships.
• Returning (nil, nil) for empty input but the test asserts len(nodes) == 0. Both work since len(nil) == 0, but be explicit in the test about which one your parser actually returns. yaml.go short-circuits with nil, nil when the document is empty — mirror its style if your decoder gives you an obvious "no content" signal.
• Setting SourceFile to something other than the path arg. Downstream code (search ranking, MemoryFetch ancestor walks) keys on it. Pass path straight through to buildNode.
• Shadowing the package error sentinels. Use ErrInvalidUTF8, ErrUnsupportedExt, and ErrMalformed (skip-on-bad-content) from parser.go; don't redefine them per format.
• Stutter naming. parser.NewTomlParser() is wrong; the empty struct is exposed bare as TomlParser and constructed with TomlParser{}. See go-concise.md §7.

Cross-references

• .claude/rules/go-concise.md — error message format, naming, struct-with-methods pattern
• .claude/rules/git-versioning.md — commit each of (parser, switch, tests, fuzz seeds) as one logical chunk; feat(parser): add toml support covers all five files in one commit since they share the idea
• .claude/skills/add-fuzz-target/SKILL.md — for the broader fuzz discipline if you're adding a brand-new fuzz target rather than seeding an existing one