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

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.Is 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.Is

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:sha256:8537db975625059dbec48b552ad9fdda3606a73a4c4de7c227ba254b777ab5cb","contract":["errors.Is returns a non-nil predicate function for a target error","errors.Is predicate returns true when tested against the identical target error","errors.Is predicate returns false when tested against a distinct error","errors.Is predicate preserves cause when used with errors.Note on matching cause","errors.Is predicate drops cause when used with errors.Note on non-matching cause"],"goal":"verify gopkg.in/errgo.v2/errors.Is 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.Is"]},"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.Is"],"verifierAdapter":"golang@1"}

module 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 sample

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

// Matcher returns a predicate function that tests equality against target using errors.Is.
func Matcher(target error) func(error) bool {
	return errors.Is(target)
}

// CheckIs tests whether candidate error matches target error using errors.Is.
func CheckIs(target, candidate error) bool {
	pred := errors.Is(target)
	return pred(candidate)
}

// NotePreservingCause wraps err with msg and preserves its cause only if cause matches target.
func NotePreservingCause(err, target error, msg string) error {
	return errors.Note(err, errors.Is(target), msg)
}

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

package main

import (
	"fmt"
	"os"

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

func main() {
	sentinel := errors.New("target error")
	otherErr := errors.New("other error")

	// Assertion 1: errors.Is returns a non-nil predicate function for a target error
	pred := sample.Matcher(sentinel)
	if pred == nil {
		fmt.Fprintf(os.Stderr, "FAIL: errors.Is returned nil predicate\n")
		os.Exit(1)
	}

	// Assertion 2: errors.Is predicate returns true when tested against the identical target error
	if !sample.CheckIs(sentinel, sentinel) {
		fmt.Fprintf(os.Stderr, "FAIL: errors.Is predicate expected true for target error\n")
		os.Exit(1)
	}

	// Assertion 3: errors.Is predicate returns false when tested against a distinct error
	if sample.CheckIs(sentinel, otherErr) {
		fmt.Fprintf(os.Stderr, "FAIL: errors.Is predicate expected false for distinct error\n")
		os.Exit(1)
	}

	// Assertion 4: errors.Is predicate preserves cause when used with errors.Note on matching cause
	notedMatching := sample.NotePreservingCause(sentinel, sentinel, "context added")
	if notedMatching == nil {
		fmt.Fprintf(os.Stderr, "FAIL: NotePreservingCause returned nil\n")
		os.Exit(1)
	}
	if cause := errors.Cause(notedMatching); cause != sentinel {
		fmt.Fprintf(os.Stderr, "FAIL: expected cause %v, got %v\n", sentinel, cause)
		os.Exit(1)
	}

	// Assertion 5: errors.Is predicate drops cause when used with errors.Note on non-matching cause
	notedMismatch := sample.NotePreservingCause(otherErr, sentinel, "context added")
	if notedMismatch == nil {
		fmt.Fprintf(os.Stderr, "FAIL: NotePreservingCause returned nil\n")
		os.Exit(1)
	}
	if cause := errors.Cause(notedMismatch); cause == otherErr {
		fmt.Fprintf(os.Stderr, "FAIL: expected cause to be masked, but still got otherErr\n")
		os.Exit(1)
	}

	fmt.Println("PASS: gopkg.in/errgo.v2/errors.Is contract verified")
}