# Xo v0.8.0

Spec draft 0.8.

- Modules (decision 0086 steps 1 and 2): `require <path> <version>` of Xo
  modules, minimal version selection, git fetch into a read only module
  cache, `xo.lock` hashes verified on every build, `xo get`, `xo mod
  tidy`, `xo vendor`, `xo mod clean`, `--offline`, and `xo.work`
  workspaces (`--workspace=off`). Diagnostics XO1301 to XO1308.
- Codex r2 first run failures: `XO0175` for several match arms on one
  line has a split fix and no longer cascades into a false `XO0301`;
  `.display()` works on every `Display` type (derived, std errors,
  builtins); a closure that never completes infers `Never` instead of
  `XO0402`.
- Effect ceilings and checked semver (decision 0086 steps 3 and 4):
  `require <path> <version> uses none` in xo.mod and `use <dir> uses ...`
  in xo.work (XO1309 at a call above the ceiling), API and effect diffs
  from `xo get` (and `xo get -u`), `xo publish [--check]` against the
  previous tag (XO1310, XO1311; prints the tag command, never pushes),
  `xo init`, `/vN` major version module paths, and `xo serve` noticing a
  new or changed xo.work above the directories.
- `std/time` (decision 0094, Go backend): zones with per use embedded
  data (`--tzdata=system`), civil dates and times with explicit daylight
  saving gaps and overlaps, layouts with named fields checked at compile
  time (XO0420; XO0177 for `{yyyy}`, `%Y`, `2006-01-02` habits).
- Import direction rules (decision 0095): `deny <pkg> -> <pkg>, ...`
  lines in xo.mod with `/...` patterns, checked at every `use` line
  (new code XO1312; a rule naming no package is XO1301), kept by `xo mod
  tidy`; `xo query imports` and `xo query deps` list the `use` lines
  (also in the MCP `query` tool).
- std/time error text (std.md 13): `ZoneErr` and `CivilErr` display as
  sentences (`unknown time zone "Mars/Olympus"`, `2026-03-08T02:30:00
  America/New_York does not exist (skipped by a daylight saving
  change)`) instead of the derived `Skipped(at: ...)` form, matching
  `ParseErr` and `LayoutErr`.
- Newtypes (decision 0097, spec audit gap 4): `e.0` reads the underlying
  value and `Email(s)` destructures it in `let` and `match`.
- `xo check --fix` (decision 0096): applies the unambiguous machine fixes
  (as `xo fix`), reformats, then reports what remains plus an `applied`
  list; the MCP `check` tool takes `fix`, and the bench harness agent
  loop uses it while still scoring first runs as written and after fix.
- Go backend output (decision 0098): hello world 7.9 to 3.6 MB, Go build
  cache growth per program 34 to 2.9 MB, compile memory 286 to 123 MB.
  Binaries carry no DWARF or symbol table (`XO_GO_DWARF=1` keeps them for
  a debugger); programs that do not import std/http do not link net/http.
- Spec audit second pass: `x.debug()` works on every value (std.md 1;
  was XO0601 on concrete types); new tests for single field positional
  variants, `Secret` redaction in Debug and log fields, `target` at run
  time, `Stdio.write`, `Fs.cwd`, `Fs.case_sensitive`, a real `Proc.run`,
  `XO-F005`, `xo test --real`, and byte identical rebuilds (spec 12.1 now
  says "in the same directory").
- Modules in repository subdirectories (decision 0086): a module path
  `github.com/acme/tools/cli` is the directory `cli/` of the repository
  `github.com/acme/tools`, versioned by tags `cli/vX.Y.Z` (as Go);
  fetch, MVS, xo.lock, `xo vendor`, `xo get`, and `xo publish [--check]`
  (prefixed tag command) handle it; nested modules are left out of a
  module's tree and hash.
- Go calls in record and replay (decision 0093 stage 1): `--record`
  records `use go` calls whose arguments and results are values (an
  argument hash and the results) and `xo replay` answers them without
  calling Go; calls with a Go handle or a callback run again and the
  replay reports ``re executed foreign call `<name>` ``; pure std
  packages and xo.mod `replay rerun go <path> [names]` lines are not
  recorded. Binding format v5; traces gain `"foreign": 1`, and older
  traces still replay (every Go call runs again).
- Release builds (decision 0099): `xo build --release` compiles out
  `requires(debug)` and `ensures(debug)` on every backend and reports
  XO1308 for workspace overrides; `--ci` does not imply it.
- Lazy error frames (decision 0092, Go backend): frames keep copies of
  the parameter values and format them, bounded to 64 bytes, only when a
  trace is printed. 1,000,000 errors through 5 frames: 0.93 s to 0.10 s
  with short parameters, 2.78 s to 0.10 s with 90 byte ones; error output
  unchanged. `xo build --release --error-args=false` drops the values.