John Carmack on mutable variables • Hacker News Board

Re: John Carmack on mutable variables

#281

Post by andsoitis » Fri, Oct 31, 2025, 1:49 PM UTC

mcv wrote: post #254

I use Javscript mostly. Or Typescript actually, these days. I remember when ES2015 introduced `let` because `var` had weird scoping issues. But ever since, I barely use either of them. Everything is `const` these days, as it should.

> I use Javscript mostly. Or Typescript actually, these days. I remember when ES2015 introduced `let` because `var` had weird scoping issues. But ever since, I barely use either of them. Everything is `const` these days, as it should. reply

const prevents reassignment of the variable but it does not make the object the variable points to immutable.

To do the latter, you have to use Object.freeze (prevent modification of an object’s properties, but it is shallow only so for nested objects you need to recurse) and Object.seal (prevent adding or removing properties, but not changing them).

May people use immutable.js or Immer for ergonomic immutable data structures.

Re: John Carmack on mutable variables

#282

Post by nixpulvis » Fri, Oct 31, 2025, 1:49 PM UTC

I try to keep deeper mutation to where it belongs, but I'll admit to shadowing variables pretty often.

If I have a `result` and I need to post-process it. I'm generally much happier doing `result = result.process()` rather than having something like `preresult`. Works nicely in cases where you end up moving it into a condition, or commenting it out to test an assumption while developing. If there's an obvious name for the intermediate result, I'll give it that, but I'm not over here naming things `result_without_processing`. You can read the code.

Re: John Carmack on mutable variables

#283

Post by anymouse123456 » Fri, Oct 31, 2025, 1:50 PM UTC

I completely agree with the assertion and the benefits that ensue, but my attention is always snagged by the nomenclature.

I know there are alternate names available to us, but even in the context of this very conversation (and headline), the thing is being called a "variable."

What is a "variable" if not something that varies?

Re: John Carmack on mutable variables

#284

Post by nixpulvis » Fri, Oct 31, 2025, 1:51 PM UTC

agentultra wrote: post #204

Agree. After working seriously on a large production Haskell codebase for several years I definitely took it for granted. Now that I’m writing stuff in C again I do think immutability should be the default. const isn’t really it though. It could go further.

Are Rust's defaults far enough?

Re: John Carmack on mutable variables

#285

Post by wodenokoto » Fri, Oct 31, 2025, 1:51 PM UTC

Mutability was by far the most difficult thing when learning Python and mutating objects by iterating over its items do get confusing, even as a senior.

When I was first learning I thought all methods would mutate. It has a certain logic to it

Re: John Carmack on mutable variables

#286

Post by beeflet » Fri, Oct 31, 2025, 1:52 PM UTC

xd1936 wrote: post #238

Wouldn't this allocate wasteful amounts of RAM unnecessarily for every step in a calculation?

not if it gets optimized out

Re: John Carmack on mutable variables

#287

Post by rlander » Fri, Oct 31, 2025, 1:54 PM UTC

mcv wrote: post #254

I use Javscript mostly. Or Typescript actually, these days. I remember when ES2015 introduced `let` because `var` had weird scoping issues. But ever since, I barely use either of them. Everything is `const` these days, as it should.

Except const is not sufficient. It will prevent the reference from being reassigned but the const can still reference a mutable object.

Re: John Carmack on mutable variables

#288

Post by dragonwriter » Fri, Oct 31, 2025, 1:54 PM UTC

hn_throw2025 wrote: post #249

sunrunner wrote: post #137

Earlier quoted context omitted.

As a Python programmer at day job, that is Clojure-curious and sadly only gets to use it for personal projects, and is currently threatened by an obnoxious TypeScript take over, I feel this.

In the context of the original discussion, TypeScript (and ES6) has const and let.

Neither let nor even const are immutable (const prevents reassignment but not mutation if the value is of a mutable type like object or array).

Re: John Carmack on mutable variables

#289

Post by [deleted] » Fri, Oct 31, 2025, 1:55 PM UTC

anymouse123456 wrote: post #283

I completely agree with the assertion and the benefits that ensue, but my attention is always snagged by the nomenclature. I know there are alternate names available to us, but even in the context of this very conversation (and headline), the thing is being called a "variable." What is a "variable" if not something that varies?

[deleted]

Re: John Carmack on mutable variables

#290

Post by DrScientist » Fri, Oct 31, 2025, 1:55 PM UTC

emil0r wrote: post #185

EastLondonCoder wrote: post #128

After a 2 year Clojure stint I find it very hard to explain the clarity that comes with immutability for programmers used to trigger effects with a mutation. I think it may be one of those things you have to see in order to understand.

The way I like to think about is that with immutable data as default and pure functions, you get to treat the pure functions as black boxes. You don't need to know what's going on inside, and the function doesn't need to know what's going on in the outside world. The data shape becomes the contract. As such, localized context, everywhere, is perhaps the best way to explain it from the point of view of a mutable world…

Sure modularity, encapsulation etc are great tools for making components understandable and maintainable.

However, don't you still need to understand the entire program as ultimately that's what you are trying to build.

And if the state of the entire programme doesn't change - then nothing has happened. ie there still has to be mutable state somewhere - so where is it moved to?