John Carmack on mutable variables

You're using really generic terms which I have to think is mostly because you're talking about it in the abstract. In most scenarios I find there are obvious non-generic names I can use for each step of a calculation.

I would say it's more than immutability - it's the "feel" of working with values. I've worked with at least 6 languages professionally and likely more for personal projects over last 20 years. I can say that Clojure was the most impactful language I learned.

I tried to learn Haskel before but I just got bogged down in the type system and formalization - that never sat with me (ironically in retrospect Monads are a trivial concept that they obfuscated in the community to oblivion, yet another Monad tutorial was a meme at the time).

I used F# as well but it is too multi paradigm and pragmatic, I literally wrote C# in F# syntax when I hit a wall and I didn't learn as much about FP when I played with it.

Clojure had the lisp weirdness to get over, but it's homoiconicty combined with the powerful semantics of core data structures - it was the first time where the concept of working with values vs objects 'clicked' for me. I would still never use it professionally, but I would recommend it to everyone who does not have a background in FP and/or lisp experience.

In the cases we're interested in here the variable does vary, what it doesn't do is mutate.

Suppose I have a function which sums up all the prices of products in a cart, the total so far will frequently mutate, that's fine. In Rust we need to mark this variable "mut" because it will be mutated as each product's price is added.

After calculating this total, we also add $10 shipping charge. That's a constant, we're (for this piece of code) always saying $10. That's not a variable it's a constant. In Rust we'd use `const` for this but in C you need to use the C pre-processor language instead to make constants, which is kinda wild.

However for each time this function runs we do also need to get the customer ID. The customer ID will vary each time this function runs, as different customers check out their purchases, but it does not mutate during function execution like that total earlier, in Rust these variables don't need an annotation, this is the default. In C you'd ideally want to label these "const" which is the confusing name C gives to immutable variables.

The term 'variable' is from mathematics. As others have said, the values of variables do vary but they do not mutate.

In python its common to see code like this:

  df = pd.concat(df,other_df)
  df = df.select(...)
  ...

My eyes hurts, when I see it. It makes me avoid python. I bet the reason for this mutable code is a missing simple pipes syntax. I love pipes. In R can do:

  df |> 
    rbind(other_df) |> 
    select(...)

It feels much better.