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Comment by banyaaa | Hacker News Reader

What's an example of a bug that would be caused by having an unused variable?

I honestly can't think of one.

Store an error code forget to check it, store a future but fail to poll it, etc.

Which is absolutely not solved by assigning it to _ neither, if anything it will just make the variable appear used in syntax highlighting and that will make me forget to properly use it!

IDEs just gray out the unused variables and that is an 1000x better way of handling this issue.

I think you misunderstand the purpose of assigning to _.

Extremely common bug 1: a function returns an error/future, but you don’t check/poll it “foo();”. May happen because you copy some tutorial code that’s not rigid about error checking, or you don’t realize that this language doesn’t have futures that run without being polled.

Quite common bug 2: you store the error/future, but don’t check/poll it “let a = foo();”. May happen when moving code, certain branches, copy paste error, not enough coffee.

Opt-in: it doesn’t matter if the call succeeded or the future is polled, leave me alone “let _ = foo()”. May happen in test code for example. This is not the common case, so the opt-in annoyance is justified to improve the common case.

You misunderstand me - I’m talking about my original problem of an unused variable that happened due to me commenting out something for example. Temporarily unused variable if you will.

Without recursively commenting out any further variable that have also become unused by my action (which I hope we can all agree is extreme tedious and error prone), I am left with no choice but assigning it to _, which as you mentioned already have a normal usage, making it later very hard to discern which is just a temporarily unused variable, or a deliberately ignored one. The very “feature” can cause bugs, besides being annoying as hell.

I agree. I gave Zig a fair shake, even went so far as to find the compiler PR that automatically adds `_ = foo` to your source code while compiling[1] and set up the VSCode extension with autofix. I couldn't get used to seeing the lines with `_ =` appear and disappear all over my code while I was typing.

With the usual warnings approach, you can rely on compiler/IDE/pre-commit tooling to find unused variables - they'll all nag you until it's fixed. With Zig's autofix approach, those problems are immediately silenced and you don't have any help from the compiler or tooling to find unused variables. It's quite an ironic outcome if you think about it.

[1]: https://github.com/ziglang/zig/pull/12803

That is something a decent type system should solve - make it impossible to pass 'incomplete values' on, so any state further on which depends on the error handled/not handled will expect the appropriate type and compiler will error at that call site.

An unused variable means you weren't passing it on anywhere so there is no code which depends on its value, so how can it be a bug?

future = x.do_async(); return;

should not error out because of 'unused variable', it should give a error message concerning the lifetime of the future object.

As someone who works at a company with an old horrible code base, for me its not so much of it can cause bugs, but it presents people from doing stupid things. In parts of a codebase we have a long legacy function that contains a string that tries to show what "state" a process is in, but the string is only ever used for that. Never is it actually used by the system. So we have code that looks like this

  public void processTransaction() {
    string state = "start";
    getCardDetails();
    state = "serialize";
    serializeCardData();
    state = "send data";
    sendData();
    state = "check return code";
    bool success = checkReturnCode();
    if (!success) {
      state = "failed";
      doFailThing();
      return;
    }
    state = "store transaction record";
    storeTheThing();
    state = "complete";
  }

First, this code is a simple example, in reality, the code has bunch of branching and doesn't actually call out to functions to do things like "getCardDetails," so just replace that function in the example above with some parsing logic of a string to parse Track1 and Track2 data. The equivalent method we actually have in our code base to do that is ~1500 lines of code. But that string is doing nothing that a comment couldn't accomplish. Or more importantly, what the code could describe itself if it actually adhered to good design principles. Ideally, I would refactor this, but the owner of the company is adament on keeping this 1500 line abomination untouched.

For me, I often unused variables in production code are usually filling the void a comment or good design would have filled.

The good news is that although intuitively that feels horrible, mechanically those are string literals, so, that doesn't actually do very much, just re-assigning a pointer. Moreover, there's no way an optimising compiler can't see those assignments are futile and elide them, whereupon in release builds it might as well be a comment.

And yes, lots of people who don't like Zig's choice agree unused variables are bad and shouldn't survive into your release code. They just don't agree with Andrew that it's a fatal error and the program shouldn't build.

using the wrong variable in repeated code:

value1 <- f()

value2 <- f()

g(value1)

or other similar cases

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