Go Error 嵌套实现创建方式

// src/builtin/builtin.go

type error interface {
	Error() string
}

package main

import (
	"errors"
	"fmt"
)

func main() {
	err := errors.New("这是 errors.New() 创建的错误")
	fmt.Printf("err 错误类型：%T，错误为：%v\n", err, err)
}

/* 输出
err 错误类型：*errors.errorString，错误为：这是 errors.New() 创建的错误
*/

// src/errors/errors.go

// New returns an error that formats as the given text.
// Each call to New returns a distinct error value even if the text is identical.
func New(text string) error {
	return &errorString{text}
}

// errorString is a trivial implementation of error.
type errorString struct {
	s string
}

func (e *errorString) Error() string {
	return e.s
}

package main
import (
	"database/sql"
	"fmt"
)
func foo() error {
	return sql.ErrNoRows
}
func bar() error {
	return foo()
}
func main() {
	err := bar()
	if err == sql.ErrNoRows {
		fmt.Printf("data not found, %+v\n", err)
		return
	}
	if err != nil {
		fmt.Println("Unknown error")
	}
}
/* 输出
data not found, sql: no rows in result set
*/

func foo() error {
   return fmt.Errorf("foo err, %v", sql.ErrNoRows)
}

package main
import (
	"fmt"
)
type myError struct{}
func (e myError) Error() string {
	return "Error happended"
}
func main() {
	e1 := myError{}
	e2 := fmt.Errorf("E2: %w", e1)
	e3 := fmt.Errorf("E3: %w", e2)
	fmt.Println(e2)
	fmt.Println(e3)
}
/* output
E2: Error happended
E3: E2: Error happended
*/

// src/fmt/errors.go

func Errorf(format string, a ...interface{}) error {
	p := newPrinter()
	p.wrapErrs = true
	p.doPrintf(format, a)
	s := string(p.buf)
	var err error
	if p.wrappedErr == nil {
		err = errors.New(s)
	} else {
		err = &wrapError{s, p.wrappedErr}
	}
	p.free()
	return err
}

// src/fmt/errors.go
type wrapError struct {
	msg string
	err error
}
func (e *wrapError) Error() string {
	return e.msg
}
func (e *wrapError) Unwrap() error {
	return e.err
}

// src/errors/wrap.go

func Unwrap(err error) error {
	u, ok := err.(interface {
		Unwrap() error
	})
	if !ok {
		return nil
	}
	return u.Unwrap()
}

package main
import (
	"database/sql"
	"errors"
	"fmt"
)
func bar() error {
	if err := foo(); err != nil {
		return fmt.Errorf("bar failed: %w", foo())
	}
	return nil
}
func foo() error {
	return fmt.Errorf("foo failed: %w", sql.ErrNoRows)
}
func main() {
	err := bar()
	if errors.Is(err, sql.ErrNoRows) {
		fmt.Printf("data not found,  %+v\n", err)
		return
	}
	if err != nil {
		fmt.Println("Unknown error")
	}
}
/* output
data not found,  bar failed: foo failed: sql: no rows in result set
*/