如何使用Golang的反射实例化nil错误？

Question

I am writing a function using reflect.MakeFunc. That function can return an error. When it succeeds, I want it to return nil for its error-typed return value. How can I do that using reflect? Currently I have this:

package main

import (
    "fmt"
    "reflect"
    "errors"        
)

func main() {
    fmt.Println("Hello, playground")
    f := func() error {return nil}
    fn := reflect.MakeFunc(reflect.TypeOf(f), func(args []reflect.Value) []reflect.Value {
        return []reflect.Value{reflect.New(reflect.TypeOf(errors.New("")))}
    }).Interface().(func() error)
    fmt.Printf("err: %v", fn())
}

I get panic: reflect: function created by MakeFunc using closure returned wrong type: have **errors.errorString for error. I also tried adding a .Elem() after reflect.New(reflect.TypeOf(errors.New(""))), but I got panic: reflect: function created by MakeFunc using closure returned wrong type: have *errors.errorString for error. I tried .Elem().Elem(), and I got a segmentation fault.

How can I get a reflect.Value representing a nil error?

Answer 1

Use the following:

var nilError = reflect.Zero(reflect.TypeOf((*error)(nil)).Elem())

func main() {
    fmt.Println("Hello, playground")
    f := func() error { return nil }
    fn := reflect.MakeFunc(reflect.TypeOf(f), func(args []reflect.Value) []reflect.Value {
        return []reflect.Value{nilError}
    }).Interface().(func() error)
    fmt.Printf("err: %v", fn())
}

Let's break this down. The first step is to get a reflect.Type for error: reflect.TypeOf((*error)(nil)).Elem(). The simpler reflect.TypeOf((error)(nil)) does not work because the concrete value of the argument is nil. There's no type for nil and it's not the type we want anyway. The workaround is to pass a pointer to error and then call Elem() on the type to get the relfect.Type for error.

The second step is to create a zero value for the type.