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

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.Becausef 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.Becausef

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:da17eeb3e4030851f54bd384fdf0602f861a70be96956f5d82c811118f3aee9a","contract":["errors.Becausef formats message and wraps underlying error with cause","errors.Becausef formats message when underlying error is nil and associates cause","errors.Becausef returns nil when err, cause and format result are empty","errors.Becausef records caller file and line location implementing errors.Locator","errors.Becausef returns an error implementing errors.Wrapper with underlying error"],"goal":"verify gopkg.in/errgo.v2/fmt/errors.Becausef 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.Becausef"]},"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.Becausef"],"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"
)

var ErrRecordNotFound = errors.New("record not found")

// FindRecord looks up a record ID and wraps failures with Becausef.
func FindRecord(id int) error {
	if id <= 0 {
		return errors.Becausef(nil, ErrRecordNotFound, "query with invalid id %d", id)
	}
	lowLevelErr := errors.New("connection reset by peer")
	return errors.Becausef(lowLevelErr, ErrRecordNotFound, "failed to query record %d", id)
}

func main() {
	if err := FindRecord(-1); err != nil {
		fmt.Printf("Error: %v\n", err)
		fmt.Printf("Cause: %v\n", errors.Cause(err))
	}
}

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

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 {
	causeErr := errors.New("sentinel cause")
	underlyingErr := errors.New("underlying failure")

	// 1. errors.Becausef formats message and wraps underlying error with cause
	wrappedErr := errors.Becausef(underlyingErr, causeErr, "task %s step %d failed", "sync", 42)
	if wrappedErr == nil {
		return fmt.Errorf("expected non-nil error")
	}
	expectedMsg := "task sync step 42 failed: underlying failure"
	if got := wrappedErr.Error(); got != expectedMsg {
		return fmt.Errorf("expected message %q, got %q", expectedMsg, got)
	}
	if cause := errors.Cause(wrappedErr); cause != causeErr {
		return fmt.Errorf("expected cause %v, got %v", causeErr, cause)
	}

	// 2. errors.Becausef formats message when underlying error is nil and associates cause
	nilUnderlyingErr := errors.Becausef(nil, causeErr, "item %d missing", 7)
	if nilUnderlyingErr == nil {
		return fmt.Errorf("expected non-nil error")
	}
	expectedNilMsg := "item 7 missing"
	if got := nilUnderlyingErr.Error(); got != expectedNilMsg {
		return fmt.Errorf("expected message %q, got %q", expectedNilMsg, got)
	}
	if cause := errors.Cause(nilUnderlyingErr); cause != causeErr {
		return fmt.Errorf("expected cause %v, got %v", causeErr, cause)
	}

	// 3. errors.Becausef returns nil when err, cause and format result are empty
	if nilErr := errors.Becausef(nil, nil, ""); nilErr != nil {
		return fmt.Errorf("expected nil error, got %v", nilErr)
	}

	// 4. errors.Becausef records caller file and line location implementing errors.Locator
	loc, ok := wrappedErr.(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 to end with contract.go, got %s", file)
	}

	// 5. errors.Becausef returns an error implementing errors.Wrapper with underlying error
	wrap, ok := wrappedErr.(errors.Wrapper)
	if !ok {
		return fmt.Errorf("expected error to implement errors.Wrapper")
	}
	if wrap.Underlying() != underlyingErr {
		return fmt.Errorf("expected underlying error %v, got %v", underlyingErr, wrap.Underlying())
	}
	if wrap.Message() != "task sync step 42 failed" {
		return fmt.Errorf("expected wrapper message 'task sync step 42 failed', got %q", wrap.Message())
	}

	return nil
}