Swift: If Let vs. Guard Let

This is a brief explanation for newbies. It’s because when I was still a newbie, I didn’t really understand how or when to use either of these two. I never touched them at all. If you want a more thorough explanation on this, check out the Swift Language Guide.

if john.residence?.printNumberOfRooms() != nil {
    print("good")
} else {
    print("bad")
}

Both if and guard checks whether the value of a variable could be nil. They are interchangeable depending on the implementation. Let’s start with the code above. This is a primitive way of checking whether a variable/method is or returns nil. It is closer to how guard works than if let.

if let firstRoomName = john.residence?[0].name {
    print(firstRoomName)
} else {
    print("bad.")
}

It is important to note here that if let only allows you to acces the new variable firstRoomName inside the first bracket. That is if john.residence?[0].name optional assigns a non-nil value in our new variable.

Outside this if let conditional, firstRoomName cannot be accessed.

guard let firstRoomName = john.residence?[0].name else {
    print("bad.")
    return
}

print(firstRoomName)

guard let is just a smart way of making sure a new variable, firstRoomName, will not be accessible anywhere if it is nil. If firstRoomName happens to be assigned a nil value, then “bad” will print and will escape the function where this piece of conditional is located, never allowing to pass firstRoomName to anyone who wants to access its value. The code will not be able to reach the print line anymore.

However, if john.residence?[0].name hold say a string value, then all is good. firstRoomName will now have an unwrapped value of john.residence?[0].name.