let is a lie (sometimes)

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“let is a lie, sometimes”

This is quite a bold statement, so let me explain!

I’m not talking about when you use let to declare a local constant:

In that case, let is absolutely not a lie and try as you might you won’t be able to set another value to this constant.

But how about when you use let to declare a property inside a struct?

Let’s create a value of that struct and store it in a variable:

If we try to set a new value to the property, we do get an error just as we expect:

But how about a method call?

Would it be possible for a method to change the value inside that let property?

As it turns out, the answer is yes!

Because if the method of the struct is mutating, then it is allowed to assign a new value to self, which in turns allows it to set a new value to the let property!

So in that case, the fact that the property was marked as let actually gave us a false sense of security.

And the only way to be sure that the value of the property cannot be changed is to declare the variable holding the struct itself as a let:

Because, as we just saw, if you only declare the property as a let then you will have made it harder to change its value, but you won’t have made it impossible!

That’s all for this article, I hope you’ve enjoyed discovering this very tricky behaviour of let properties within structs!

I have a big thank you to Chris Eidhof of the website objc.io because I heard him say the words « let is a lie » at WWDC and it only took me 5 months to realise it would make a great title for an article!

Here’s the code if you want to experiment with it:

struct MyStruct {
  let value: String

  mutating func doSomething() {
    self = MyStruct(value: "Something else 😏")
  }
}

var myStruct = MyStruct(value: "Hello, World!")

myStruct.doSomething()

print(myStruct.value) // "Something else 😏"