interface

package main

import (
“errors”
“fmt”
“log”
)

func main() {
var e interface{}
e = func() error {
return errors.New(“err”)
}()
if e != nil {
fmt.Printf(“%T\n”, e)
log.Println(e)
}
fmt.Println(e)
}
输出内容：

*errors.errorString
2019/01/05 18:54:43 err
err

这边很容易将e的类型误认为是error，但是实际运行中却被转换成*errors.errorString。

src/builtin/builtin.go
// The error built-in interface type is the conventional interface for
// representing an error condition, with the nil value representing no error.
type error interface {
Error() string
}

src/errors/errors.go
// New returns an error that formats as the given text.
func New(text string) error {
return &errorString{text}
}

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