Hacker News Reader

Comment by bigwheels | Hacker News Reader

Written a few hundred kloc of go and I'd never heard of double nil. Turns out it's nasty.

    var i interface{} // (Type: nil, Value: nil) -> i == nil is TRUE
    var p *int = nil  // p is nil
    i = p             // (Type: *int, Value: nil) -> i == nil is FALSE!

View on HN

While this behavior can be counterintuitive, it's easy to understand once you have the right mental model of interface values. A nil interface is an empty interface. Once p is assigned to i, then i is no longer empty: it holds a nil pointer to an int. Whether the interface is empty or not is one question, answered by comparing it against nil. Whether the value the interface holds is a nil value is another question, answered by downcasting and then checking the resulting value.

I don't blame people for being confused by this the first time they encounter it. However, anyone who finds it persistently challenging to understand is not going to have much luck with the subtler language features of C++, Rust, Typescript, etc.

Just because you can easily understand something doesn't mean it isn't a footgun.

That seems like the reasoning that given the amount of UNIX and C code in the industry, memory corruption bugs only happen to those that don't understand their job.

Never used go, but I assume interface is some kind of indirection? If so, this doesn't seem anymore confusing than:

    void *p=0;
    void **pp=0;
    assert(pp == 0);
    pp=&p;
    assert(pp != 0);
    assert(*pp == 0);

Reply on news.ycombinator.com