gopkg.in/errgo.v2 v2.1.0: errors.Newf

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 gopkg.in/errgo.v2/fmt/errors.Newf in pkg:golang/gopkg.in/errgo.v2@v2.1.0
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:golang/gopkg.in/errgo.v2@v2.1.0
Demonstrate these symbols/APIs:
  - gopkg.in/errgo.v2/fmt/errors.Newf

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:sha256:825e8d7625b95343fe834f70984f650484b7fc67b419a0b3904ce51345df46be","contract":["errors.Newf formats error message with fmt specifiers","errors.Newf returns an error whose cause resolves to itself via errors.Cause","errors.Newf records caller file and line location implementing errors.Locator","errors.Newf returns an error implementing errors.Wrapper with nil underlying error"],"goal":"verify gopkg.in/errgo.v2/fmt/errors.Newf in pkg:golang/gopkg.in/errgo.v2@v2.1.0","kind":"HOW","packages":["pkg:golang/gopkg.in/errgo.v2@v2.1.0"],"schemaVersion":1,"symbols":["gopkg.in/errgo.v2/fmt/errors.Newf"]},"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/gopkg.in/errgo.v2@v2.1.0"],"schemaVersion":1,"subject":"pkg:golang/gopkg.in/errgo.v2@v2.1.0","symbols":["gopkg.in/errgo.v2/fmt/errors.Newf"],"verifierAdapter":"golang@1"}

module example.com/sample

go 1.26.6

require gopkg.in/errgo.v2 v2.1.0

github.com/kr/pretty v0.1.0/go.mod h1:dAy3ld7l9f0ibDNOQOHHMYYIIbhfbHSm3C4ZsoJORNo=
github.com/kr/pty v1.1.1/go.mod h1:pFQYn66WHrOpPYNljwOMqo10TkYh1fy3cYio2l3bCsQ=
github.com/kr/text v0.1.0/go.mod h1:4Jbv+DJW3UT/LiOwJeYQe1efqtUx/iVham/4vfdArNI=
gopkg.in/check.v1 v1.0.0-20180628173108-788fd7840127/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
gopkg.in/errgo.v2 v2.1.0 h1:0vLT13EuvQ0hNvakwLuFZ/jYrLp5F3kcWHXdRggjCE8=
gopkg.in/errgo.v2 v2.1.0/go.mod h1:hNsd1EY+bozCKY1Ytp96fpM3vjJbqLJn88ws8XvfDNI=

package main

import (
	"fmt"

	"gopkg.in/errgo.v2/fmt/errors"
)

func main() {
	err := errors.Newf("item %s not found (code: %d)", "user", 404)
	fmt.Printf("Error: %v\n", err)

	if loc, ok := err.(errors.Locator); ok {
		file, line := loc.Location()
		fmt.Printf("Location: %s:%d\n", file, line)
	}
}

{
  "schemaVersion": 1,
  "goal": "verify gopkg.in/errgo.v2/fmt/errors.Newf in pkg:golang/gopkg.in/errgo.v2@v2.1.0",
  "kind": "HOW",
  "packages": [
    "pkg:golang/gopkg.in/errgo.v2@v2.1.0"
  ],
  "symbols": [
    "gopkg.in/errgo.v2/fmt/errors.Newf"
  ]
}

package main

import (
	"fmt"
	"os"
	"strings"

	"gopkg.in/errgo.v2/fmt/errors"
)

func main() {
	if err := runContractTests(); err != nil {
		fmt.Fprintf(os.Stderr, "Contract test failed: %v\n", err)
		os.Exit(1)
	}
	fmt.Println("All contract assertions passed.")
}

func runContractTests() error {
	code := 404
	item := "resource"
	err := errors.Newf("item %s not found (code: %d)", item, code)
	if err == nil {
		return fmt.Errorf("expected non-nil error from errors.Newf")
	}

	// 1. errors.Newf formats error message with fmt specifiers
	expectedMsg := "item resource not found (code: 404)"
	if got := err.Error(); got != expectedMsg {
		return fmt.Errorf("expected error message %q, got %q", expectedMsg, got)
	}

	// 2. errors.Newf returns an error whose cause resolves to itself via errors.Cause
	cause := errors.Cause(err)
	if cause != err {
		return fmt.Errorf("expected cause to be identical to error, got %v", cause)
	}

	// 3. errors.Newf records caller file and line location implementing errors.Locator
	loc, ok := err.(errors.Locator)
	if !ok {
		return fmt.Errorf("expected error to implement errors.Locator")
	}
	file, line := loc.Location()
	if file == "" || line == 0 {
		return fmt.Errorf("expected non-empty location, got %s:%d", file, line)
	}
	if !strings.HasSuffix(file, "contract.go") {
		return fmt.Errorf("expected location file to end with contract.go, got %s", file)
	}

	// 4. errors.Newf returns an error implementing errors.Wrapper with nil underlying error
	wrap, ok := err.(errors.Wrapper)
	if !ok {
		return fmt.Errorf("expected error to implement errors.Wrapper")
	}
	if wrapMsg := wrap.Message(); wrapMsg != expectedMsg {
		return fmt.Errorf("expected wrapper message %q, got %q", expectedMsg, wrapMsg)
	}
	if under := wrap.Underlying(); under != nil {
		return fmt.Errorf("expected nil underlying error, got %v", under)
	}

	return nil
}