John Carmack on mutable variables

Post by ErroneousBosh » Fri, Oct 31, 2025, 7:10 PM UTC

jayd16 wrote: post #341

ErroneousBosh wrote: post #250

Earlier quoted context omitted.

I guess I'm not that good a programmer, because I don't really understand why variables that can't be varied are useful, or why you'd use that. How do you write code that actually works?

If you need new values you just make new things. If you want to do an operation on fooA, you don't mutate fooA. You call fooB = MyFunc(fooA) and use fooB. The nice thing here is you can pass around pointers to fooA and never worry that anything is going to change it underneath you. You don't need to protect private variables because your internal workings cannot be mutated. Other code can copy it but not disrupt it.

Post by ErroneousBosh » Fri, Oct 31, 2025, 7:12 PM UTC

stickfigure wrote: post #377

ErroneousBosh wrote: post #262

Earlier quoted context omitted.

Okay, so for example I might set something like "this bunch of parameters" immutable, but "this 16kB or so of floats" are just ordinary variables which change all the time? Or then would the block of floats be "immutable but not from this bit"? So the code that processes a block of samples can write to it, the code that fills the sample buffer can write to it, but nothing else should?

Sounds like you have a data structure like `Array `. The immutable approach has methods on Array like: Array append(Float value); Array replace(int index, Float value); The methods don't mutate the array, they return a new array with the change. The trick is: How do you make this fast without copying a whole array? Clojure includes a variety of collection classes that "magically" make these operations fast, for a var…

Post by ErroneousBosh » Fri, Oct 31, 2025, 7:15 PM UTC

jimbokun wrote: post #393

ErroneousBosh wrote: post #250

Earlier quoted context omitted.

I guess I'm not that good a programmer, because I don't really understand why variables that can't be varied are useful, or why you'd use that. How do you write code that actually works?

The concept is actually pretty simple: instead of changing existing values, you create new values. The classic example is a list or array. You don't add a value to an existing list. You create a new list which consists of the old list plus the new value. [1] This is a subtle but important difference. It means any part of your program with a reference to the original list will not have it change unexpectedly. This eli…

Post by ape4 » Fri, Oct 31, 2025, 7:19 PM UTC

catlifeonmars wrote: post #335

ape4 wrote: post #277

Earlier quoted context omitted.

I would be nicer if you gave x1 and x2 meaningful names

What would those names be in this example?

    const pi = 3.1415926
    const 2pi = 2 * pi
    const circumference = 2pi * radius

Post by layer8 » Fri, Oct 31, 2025, 7:21 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?

  function circumference(radius)
      return 2 * PI * radius