main.go

package main

import (
	"errors"
	"fmt"
	"log"

	pkgerrors "github.com/pkg/errors"
)

type FooError struct {
	Msg string
}

func (e *FooError) Error() string {
	return fmt.Sprintf("FOOO %s", e.Msg)
}

func main() {
	if err := run(); err != nil {
		log.Fatalf("!! %+v", err)
	}
}

func run() error {
	var fooErr *FooError
	fmt.Println("foo as Foo?", errors.As(foo(), &fooErr))               // true
	fmt.Println("withWrap-foo as Foo?", errors.As(withWrap(), &fooErr)) // true
	fmt.Println("withWrap-foo cause Foo?", ok(withWrap()))              // true
	fmt.Println("errorf-foo as Foo?", errors.As(withErrorf(), &fooErr)) // true
	fmt.Println("errorf-foo cause Foo?", ok(withErrorf()))              // false
	return nil
}

func ok(err error) bool {
	_, ok := pkgerrors.Cause(err).(*FooError)
	return ok
}

func foo() error {
	return &FooError{Msg: "hmm"}
}

func withWrap() error {
	if err := foo(); err != nil {
		return pkgerrors.Wrap(err, "with pkg.errors.Wrap")
	}
	return nil
}

func withErrorf() error {
	if err := foo(); err != nil {
		return fmt.Errorf("with errorf: %w", err)
	}
	return nil
}