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

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.Cause 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.Cause

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:sha256:6bf59e6251b27e9cee5e700d4ba14dd17a861f76a9254e245893955dae9048a4","contract":["errors.Cause returns nil when input error is nil","errors.Cause returns the error itself when error does not implement Causer","errors.Cause extracts the cause from an error created with errors.Because","errors.Cause extracts the cause from an error created with errors.Becausef","errors.Cause returns the wrapped error itself when error was wrapped with errors.Wrap which hides the cause","errors.Cause extracts the cause from an error created with errors.Note preserving the cause"],"goal":"verify gopkg.in/errgo.v2/fmt/errors.Cause 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.Cause"]},"contractCommand":["go","run","./test"],"environment":{"arch":"x64","distro":"ubuntu","ecosystem":"golang","language":"go","libc":"glibc","libcVersion":"2.39","os":"linux","osVersionBucket":"24","packageManager":"go","packageManagerVersion":"1.26.6","runtime":"go","runtimeVersion":"1.26.6","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.Cause"],"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 h1:L/CwN0zerZDmRFUapSPitk6f+Q3+0za1rQkzVuMiMFI=
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 h1:45sCR5RtlFHMR4UwH9sdQ5TC8v0qDQCHnXt+kaKSTVE=
github.com/kr/text v0.1.0/go.mod h1:4Jbv+DJW3UT/LiOwJeYQe1efqtUx/iVham/4vfdArNI=
gopkg.in/check.v1 v1.0.0-20180628173108-788fd7840127 h1:qIbj1fsPNlZgppZ+VLlY7N33q108Sa+fhmuc+sWQYwY=
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"
	"io"

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

// ErrNotFound is a sentinel domain error.
var ErrNotFound = errgoFmt.New("resource not found")

// findRecord simulates finding a record that fails with a formatted underlying error and explicit cause.
func findRecord(id string) error {
	if id == "" {
		return errgoFmt.Becausef(io.ErrUnexpectedEOF, ErrNotFound, "empty id provided for item %s", id)
	}
	return nil
}

func main() {
	err := findRecord("")
	if err != nil {
		cause := errgoFmt.Cause(err)
		fmt.Printf("Original error: %v\n", err)
		fmt.Printf("Diagnosed cause: %v\n", cause)
		if cause == ErrNotFound {
			fmt.Println("Handled: matched ErrNotFound sentinel cause")
		}
	}
}

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

package main

import (
	"errors"
	"fmt"
	"os"

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

func main() {
	// Assertion 1: errors.Cause returns nil when input error is nil
	if cause := errgoFmt.Cause(nil); cause != nil {
		fmt.Fprintf(os.Stderr, "FAIL: Cause(nil) expected nil, got %v\n", cause)
		os.Exit(1)
	}

	// Assertion 2: errors.Cause returns the error itself when error does not implement Causer
	stdErr := errors.New("standard error")
	if cause := errgoFmt.Cause(stdErr); cause != stdErr {
		fmt.Fprintf(os.Stderr, "FAIL: Cause(stdErr) expected %v, got %v\n", stdErr, cause)
		os.Exit(1)
	}

	// Assertion 3: errors.Cause extracts the cause from an error created with errors.Because
	rootCause := errors.New("root failure cause")
	underlyingErr := errors.New("network socket timeout")
	wrappedWithCause := errgoFmt.Because(underlyingErr, rootCause, "operation failed")
	if cause := errgoFmt.Cause(wrappedWithCause); cause != rootCause {
		fmt.Fprintf(os.Stderr, "FAIL: Cause(wrappedWithCause) expected %v, got %v\n", rootCause, cause)
		os.Exit(1)
	}

	// Assertion 4: errors.Cause extracts the cause from an error created with errors.Becausef
	wrappedWithCausef := errgoFmt.Becausef(underlyingErr, rootCause, "operation failed with code %d", 500)
	if cause := errgoFmt.Cause(wrappedWithCausef); cause != rootCause {
		fmt.Fprintf(os.Stderr, "FAIL: Cause(wrappedWithCausef) expected %v, got %v\n", rootCause, cause)
		os.Exit(1)
	}

	// Assertion 5: errors.Cause returns the wrapped error itself when error was wrapped with errors.Wrap which hides the cause
	baseErr := errors.New("base database error")
	wrappedHidden := errgoFmt.Wrap(baseErr)
	if cause := errgoFmt.Cause(wrappedHidden); cause != wrappedHidden {
		fmt.Fprintf(os.Stderr, "FAIL: Cause(wrappedHidden) expected %v, got %v\n", wrappedHidden, cause)
		os.Exit(1)
	}

	// Assertion 6: errors.Cause extracts the cause from an error created with errors.Note preserving the cause
	preservedErr := errgoFmt.Note(wrappedWithCause, errgoFmt.Is(rootCause), "outer annotation")
	if cause := errgoFmt.Cause(preservedErr); cause != rootCause {
		fmt.Fprintf(os.Stderr, "FAIL: Cause(preservedErr) expected %v, got %v\n", rootCause, cause)
		os.Exit(1)
	}

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