在Go中检查类型切换语句中的“ struct”类型会导致语法错误

Question

While learning about the type switch statement in Go, I tried to check for the "struct" type as follows:

package main

import "fmt"

func moo(i interface{}) {
    switch v := i.(type) {
    case string:
        fmt.Printf("Byte length of %T: %v 
", v, len(v))
    case int:
        fmt.Printf("Two times this %T: %v 
", v, v)
    case bool:
        fmt.Printf("Truthy guy is %v 
", v)
    case struct:
        fmt.Printf("Life is complicated with %v 
", v)
    default:
        fmt.Println("don't know")
    }
}

func main() {
    moo(21)
    moo("hello")
    moo(true)
}

However, this resulted in a syntax error related to the struct type (not seen if the case statement checking for the struct type is removed:

tmp/sandbox396439025/main.go:13: syntax error: unexpected :, expecting {
tmp/sandbox396439025/main.go:14: syntax error: unexpected (, expecting semicolon, newline, or }
tmp/sandbox396439025/main.go:17: syntax error: unexpected semicolon or newline, expecting :
tmp/sandbox396439025/main.go:20: syntax error: unexpected main, expecting (
tmp/sandbox396439025/main.go:21: syntax error: unexpected moo

Is there a reason why the struct type cannot be checked for here? Note that the func moo() is checking for i of type interface{}, an empty interface which should supposedly be implemented by every type, including struct

Go Playground full code:

https://play.golang.org/p/F820vMJRum

Answer 1

struct is not a type, it's a keyword.

This is a type for example (given using a type literal):

struct { i int }

Or Point:

type Point struct { X, Y int }

So the following code works:

switch v := i.(type) {
case struct{ i int }:
    fmt.Printf("Life is complicated with %v 
", v)
case Point:
    fmt.Printf("Point, X = %d, Y = %d 
", v.X, v.Y)
}

You may look at struct as being a kind of type, which you can check using reflection, for example:

var p Point
if reflect.TypeOf(p).Kind() == reflect.Struct {
    fmt.Println("It's a struct")
}

Try it on the Go Playground.