## Description
`iterator.Try` previously copied the entire iterator and synced it back on success, causing an unnecessary heap allocation on every call.
This PR simplifies `Try` to save and restore the index directly, and removes the now-unused `Copy` and `Sync` methods.
- Rewrite `ScanRune` and `ParseRawToken` as peek-then-advance, eliminating the need for `Try` at leaf level.
- Remove redundant `Try` wrappers from `parseExpression`, `parseAbstraction`, `parseApplication`, `parseLet`, and `parseToken`, which are already disambiguated by their callers.
- Keep `Try` only where true backtracking is needed: `parseStatement`, which must choose between `parseLet` and `parseDeclare`.
- Fix pre-existing panic in saccharine `parseExpression` when the iterator is exhausted (added `Done()` guard).
### Decisions
- `Try` now operates on the original iterator instead of a copy, removing the confusing pattern where the callback's `i` was a different object than the caller's `i`.
- Removed `parseSoftBreak` and `parseHardBreak` helper functions since `ParseRawToken` no longer needs `Try` wrapping.
## Benefits
- Eliminates a heap allocation per `Try` call.
- Reduces nesting and indirection in all parse functions.
- Makes the code easier to follow by removing the shadow-`i` pattern.
- `Try` is now only used at genuine choice points in the grammar.
## Checklist
- [x] Code follows conventional commit format.
- [x] Branch follows naming convention (`<type>/<description>`). Always use underscores.
- [x] Tests pass (if applicable).
- [x] Documentation updated (if applicable).
Reviewed-on: #47
Co-authored-by: M.V. Hutz <git@maximhutz.me>
Co-committed-by: M.V. Hutz <git@maximhutz.me>
## Description
Both the `saccharine` and `lambda` packages need tokenizing and parsing primitives.
This PR extracts shared token infrastructure into a new `pkg/token` package, then wires both languages up to use it.
- Add `pkg/token` with a generic `Token[T]` type, `Scan`, `ScanAtom`, `ScanRune`, `ScanCharacter`, `IsVariable`, `ParseRawToken`, and `ParseList`.
- Refactor `pkg/saccharine` to delegate to `pkg/token`, removing duplicated scanning and parsing helpers.
- Implement `Codec.Decode` for `pkg/lambda` (scanner + parser) using the shared token package.
- Add `iterator.While` for predicate-driven iteration.
- Rename `iterator.Do` to `iterator.Try` to better describe its rollback semantics.
### Decisions
- The `Type` constraint (`comparable` + `Name() string`) keeps the generic token flexible while ensuring every token type can produce readable error messages.
- `iterator.Do` was renamed to `iterator.Try` since it describes a try/rollback operation, not a side-effecting "do".
## Benefits
- Eliminates duplicated token, scanning, and parsing code between languages.
- Enables the `lambda` package to decode (parse) lambda calculus strings, which was previously unimplemented.
- Makes it straightforward to add new languages by reusing `pkg/token` primitives.
## Checklist
- [x] Code follows conventional commit format.
- [x] Branch follows naming convention (`<type>/<description>`). Always use underscores.
- [x] Tests pass (if applicable).
- [ ] Documentation updated (if applicable).
Reviewed-on: #46
Co-authored-by: M.V. Hutz <git@maximhutz.me>
Co-committed-by: M.V. Hutz <git@maximhutz.me>
