使用字符串定义的类型值作为字符串

Question

I have a defined named type

type User string

and now I want to use a User value as an actual string like so

func SayHello(u User) string {
    return "Hello " + user + "!"
}

But I receive an error:

cannot use "Hello " + user + "!" (type User) as type string in return argument

How do I use a named type as its underlying type?

Answer 1

From Golang Spec

A value x is assignable to a variable of type T ("x is assignable to T") if:

• x's type is identical to T.
• x's type V and T have identical underlying types and at least one of V or T is not a defined type.

Variables of named type type User string and unnamed type var user string are absolutely different. We can check that using reflection for showing the underlying type.

package main

import (
    "fmt"
    "reflect"
    )

type User string

func main() {
    var name User = "User"
    var name2 string = "User"
    //fmt.Print(name==name2) // mismatched types User and string
    fmt.Println(reflect.TypeOf(name2))
    fmt.Println(reflect.TypeOf(name))
}

So check the underlying type of a variable and then type cast the type to primitive one to compare the value.

package main

import (
    "fmt"
)

type User string

func main() {
    var u User
    fmt.Print(SayHello(u))
}

func SayHello(u User) string {
    change := string(u)
    return "Hello " + change + "!"
}

Check on Go Playground