2. Type Definitions

For all its faults, TypeScript does have a pretty incredible type system. Here's one of the things it can do that I often miss in Go:

type sendingChannel = "email" | "sms" | "phone";

function sendNotification(ch: sendingChannel, message: string) {
  // send the message
}

This sendingChannel type that we've created is a union type. It can only be one of the three strings that we've defined. That means when a developer calls sendNotification() they can't accidentally pass an invalid sendingChannel like "slack" or even a misspelled "emil". The TypeScript compiler will catch that mistake at compile time.

In Go, we don't have these nice things. We embrace the Grug mentality. The closest thing we have to a union type is a type definition:

type sendingChannel string

const (
    Email sendingChannel = "email"
    SMS   sendingChannel = "sms"
    Phone sendingChannel = "phone"
)

func sendNotification(ch sendingChannel, message string) {
    // send the message
}

It's a bit safer than using a plain old string in Go, but it's not completely safe. Go will stop us from doing this:

sendingCh := "slack"
sendNotification(sendingCh, "hello") // string is not sendingChannel

But it will not stop us from doing this:

// "slack" is automatically implied as a sendingChannel
sendNotification("slack", "hello")

And will also not stop us from doing this:

sendingCh := "slack"
convertedSendingCh := sendingChannel(sendingCh)
sendNotification(convertedSendingCh, "hello")

The sendingChannel type is just a wrapper for string, and because we made some constants of that type, most developers will just use those constants: we've made it easy to do the right thing. That said, Go still doesn't force us to do the right thing like TypeScript does.

Assignment

Use the following code to answer the questions:

type perm string

const (
    Read  perm = "read"
    Write perm = "write"
    Exec  perm = "execute"
)

var Admin = "admin"
var User = perm("user")

func checkPermission(p perm) {
    // check the permission
}