golang.org/x/xerrors v0.0.0-20191011141410-1b5146add898: Formatter

Clean-room public code sample — generation instructions

Write a brand-new, minimal, self-contained code sample in this clean-room directory.
Do not copy, paraphrase, or reference any existing project source. Work only from this spec.

A csx.json manifest scaffold already exists. Do not recreate it from memory. Preserve its case.goal, packages and symbols; fill its empty case.contract with exact assertions and correct its environment, commands and verifierAdapter for the files you generate.

Goal: verify golang.org/x/xerrors.Formatter in pkg:golang/golang.org/x/xerrors@v0.0.0-20191011141410-1b5146add898
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:golang/golang.org/x/xerrors@v0.0.0-20191011141410-1b5146add898
Demonstrate these symbols/APIs:
  - golang.org/x/xerrors.Formatter

Rules:
  - One focused purpose; the smallest project that proves the goal.
  - Include a contract test (test/contract.*) that runs OFFLINE and exits 0 exactly when the goal behavior works.
  - Pin every dependency with a lockfile so resolution is reproducible.
  - No secrets, credentials, or tokens. No real URLs (only example.com or localhost). No absolute paths.
  - No personal names, emails, company names, or project identifiers of any kind.
  - No binaries and no generated output (node_modules, dist, target, venv, .git, .env).
  - Keep it under 200 files and 256KB packed.

{"case":{"caseId":"case:sha256:43d7f79aa127c8621ee63383a4531902266ea72f5a9a4f8d687a6e190a4a64b7","contract":["Custom error type implements xerrors.Formatter via FormatError(xerrors.Printer) error","xerrors.FormatError formats the xerrors.Formatter chain for %v as 'operation failed: underlying cause'","xerrors.FormatError with %+v enables Printer.Detail() and outputs stack frames across the error chain"],"goal":"verify golang.org/x/xerrors.Formatter in pkg:golang/golang.org/x/xerrors@v0.0.0-20191011141410-1b5146add898","kind":"HOW","packages":["pkg:golang/golang.org/x/xerrors@v0.0.0-20191011141410-1b5146add898"],"schemaVersion":1,"symbols":["golang.org/x/xerrors.Formatter"]},"contractCommand":["go","run","./test"],"environment":{"arch":"x64","distro":"ubuntu","ecosystem":"golang","libc":"glibc","libcVersion":"2.39","os":"linux","osVersionBucket":"24","packageManager":"go","schemaVersion":1},"license":"MIT-0","packages":["pkg:golang/golang.org/x/xerrors@v0.0.0-20191011141410-1b5146add898"],"schemaVersion":1,"subject":"pkg:golang/golang.org/x/xerrors@v0.0.0-20191011141410-1b5146add898","symbols":["golang.org/x/xerrors.Formatter"],"verifierAdapter":"golang@1"}

module example.com/sample

go 1.21

require golang.org/x/xerrors v0.0.0-20191011141410-1b5146add898 // indirect

golang.org/x/xerrors v0.0.0-20191011141410-1b5146add898 h1:/atklqdjdhuosWIl6AIbOeHJjicWYPqR9bpxqxYG2pA=
golang.org/x/xerrors v0.0.0-20191011141410-1b5146add898/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=

{
  "schemaVersion": 1,
  "goal": "verify golang.org/x/xerrors.Formatter in pkg:golang/golang.org/x/xerrors@v0.0.0-20191011141410-1b5146add898",
  "kind": "HOW",
  "packages": [
    "pkg:golang/golang.org/x/xerrors@v0.0.0-20191011141410-1b5146add898"
  ],
  "symbols": [
    "golang.org/x/xerrors.Formatter"
  ]
}

package main

import (
	"fmt"
	"os"
	"strings"

	"golang.org/x/xerrors"
)

type customErr struct {
	msg   string
	frame xerrors.Frame
	next  error
}

func newCustomErr(msg string, next error) error {
	return &customErr{
		msg:   msg,
		frame: xerrors.Caller(1),
		next:  next,
	}
}

func (e *customErr) Error() string {
	return fmt.Sprint(e)
}

func (e *customErr) Format(s fmt.State, v rune) {
	xerrors.FormatError(e, s, v)
}

func (e *customErr) FormatError(p xerrors.Printer) error {
	p.Print(e.msg)
	if p.Detail() {
		e.frame.Format(p)
	}
	return e.next
}

func main() {
	base := xerrors.New("underlying cause")
	err := newCustomErr("operation failed", base)

	var f xerrors.Formatter = err.(*customErr)
	if f == nil {
		fmt.Fprintln(os.Stderr, "expected non-nil xerrors.Formatter")
		os.Exit(1)
	}

	plain := fmt.Sprintf("%v", err)
	if plain != "operation failed: underlying cause" {
		fmt.Fprintf(os.Stderr, "unexpected plain format: %q\n", plain)
		os.Exit(1)
	}

	detailed := fmt.Sprintf("%+v", err)
	if !strings.Contains(detailed, "operation failed:") || !strings.Contains(detailed, "main.main") || !strings.Contains(detailed, "underlying cause:") {
		fmt.Fprintf(os.Stderr, "unexpected detailed format: %q\n", detailed)
		os.Exit(1)
	}
}