Accessing a lazy property on a struct mutates the struct

I have a lazy property in a struct and every time I access it, it mutates the struct.

The ideal behaviour should be first time mutation only (since that's how lazy variables behave). Is this a bug in Swift or am I doing something wrong?

It's actually the semantic. MyStruct.myProperty is a mutating get due to it being lazy. So every time you access myProperty, the compiler assumes that you're mutating self whether you actually mutate it or not. You can try the adjustment below for the same effect:

struct MyStruct {
  var myProperty: Int {
    mutating get { 0 }
  }
}

The interface doesn't much care if it's using lazy or custom implementation.

2 Likes

That makes sense @Lantua, thanks!

I find myself avoiding lazy more and more recently, because it makes code harder to reason about and potentially behave in surprising ways -- you can't tell when exactly the code will run from simply looking at the code.

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