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

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/errors.Because 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/errors.Because

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:sha256:1dbac4684be42c226a7f0bf6d9f62b8f02d95c3d5d1e8e5438aaf36259b27f7b","contract":["errors.Because returns nil when err, cause and msg are zero","errors.Because returns error with cause's message when err is nil and msg is empty","errors.Because associates cause with error so errors.Cause returns cause","errors.Because wraps underlying error and adds custom message prefix","errors.Because preserves underlying error message when msg is empty"],"goal":"verify gopkg.in/errgo.v2/errors.Because 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/errors.Because"]},"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/errors.Because"],"verifierAdapter":"golang@1"}

module example.com/sample

go 1.23

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/errors"
)

var (
	ErrNotFound   = errors.New("item not found")
	ErrPermission = errors.New("access denied")
)

// FindRecord looks up a record and returns an annotated error using errors.Because.
func FindRecord(id string) error {
	if id == "" {
		// Annotating a sentinel cause with source location without wrapping an underlying error
		return errors.Because(nil, ErrNotFound, "")
	}
	if id == "forbidden" {
		// Wrapping an underlying low-level error with a cause and explanatory message
		underlying := errors.New("user lacks read permission")
		return errors.Because(underlying, ErrPermission, "cannot access record")
	}
	return nil
}

func main() {
	if err := FindRecord(""); err != nil {
		fmt.Printf("Error: %v\n", err)
		fmt.Printf("Cause: %v\n", errors.Cause(err))
	}

	if err := FindRecord("forbidden"); err != nil {
		fmt.Printf("Error: %v\n", err)
		fmt.Printf("Cause: %v\n", errors.Cause(err))
	}
}

{
  "schemaVersion": 1,
  "goal": "verify gopkg.in/errgo.v2/errors.Because 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/errors.Because"
  ]
}

package main

import (
	"fmt"
	"os"

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

func main() {
	sentinelCause := errors.New("sentinel cause")
	underlyingErr := errors.New("underlying failure")

	// 1. errors.Because returns nil when err, cause and msg are zero
	if err := errors.Because(nil, nil, ""); err != nil {
		fmt.Fprintf(os.Stderr, "assertion 1 failed: expected nil, got %v\n", err)
		os.Exit(1)
	}

	// 2. errors.Because returns error with cause's message when err is nil and msg is empty
	errNilMsg := errors.Because(nil, sentinelCause, "")
	if errNilMsg == nil {
		fmt.Fprintf(os.Stderr, "assertion 2 failed: expected non-nil error\n")
		os.Exit(1)
	}
	if errNilMsg.Error() != sentinelCause.Error() {
		fmt.Fprintf(os.Stderr, "assertion 2 failed: expected %q, got %q\n", sentinelCause.Error(), errNilMsg.Error())
		os.Exit(1)
	}

	// 3. errors.Because associates cause with error so errors.Cause returns cause
	if cause := errors.Cause(errNilMsg); cause != sentinelCause {
		fmt.Fprintf(os.Stderr, "assertion 3 failed: expected %v, got %v\n", sentinelCause, cause)
		os.Exit(1)
	}

	// 4. errors.Because wraps underlying error and adds custom message prefix
	wrappedErr := errors.Because(underlyingErr, sentinelCause, "operation failed")
	expectedMsg := "operation failed: " + underlyingErr.Error()
	if wrappedErr.Error() != expectedMsg {
		fmt.Fprintf(os.Stderr, "assertion 4 failed: expected %q, got %q\n", expectedMsg, wrappedErr.Error())
		os.Exit(1)
	}
	if cause := errors.Cause(wrappedErr); cause != sentinelCause {
		fmt.Fprintf(os.Stderr, "assertion 4 failed: expected cause %v, got %v\n", sentinelCause, cause)
		os.Exit(1)
	}

	// 5. errors.Because preserves underlying error message when msg is empty
	wrappedNoMsg := errors.Because(underlyingErr, sentinelCause, "")
	if wrappedNoMsg.Error() != underlyingErr.Error() {
		fmt.Fprintf(os.Stderr, "assertion 5 failed: expected %q, got %q\n", underlyingErr.Error(), wrappedNoMsg.Error())
		os.Exit(1)
	}
	if cause := errors.Cause(wrappedNoMsg); cause != sentinelCause {
		fmt.Fprintf(os.Stderr, "assertion 5 failed: expected cause %v, got %v\n", sentinelCause, cause)
		os.Exit(1)
	}

	fmt.Println("All contract assertions passed.")
}