John Carmack on mutable variables

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.

I think the explanation is: When you mutate variables it implicitly creates an ordering dependency - later uses of the variable rely on previous mutations. However, this is an implicit dependency that isn't modeled by the language so reordering won't cause any errors.

With a very basic concrete example:

x = 7

x = x + 3

x = x / 2

Vs

x = 7

x1 = x + 3

x2 = x1 / 2

Reordering the first will have no error, but you'll get the wrong result. The second will produce an error if you try to reorder the statements.

Another way to look at it is that in the first example, the 3rd calculation doesn't have "x" as a dependency but rather "x in the state where addition has already been completed" (i.e. it's 3 different x's that all share the same name). Doing single assignment is just making this explicit.

That example is too simple for me to grasp it. How would you code a function that iterates over an array to compute its sum. No cheating with a built-in sum function. If you had to code each addition, how would that work? Curious to learn (I probably could google this or ask Claude to write me the code).

Carmack gives updating in a loop as the one exception:

> You should strive to never reassign or update a variable outside of true iterative calculations in loops.

If you want a completely immutable setup for this, you'd likely have to use a recursive function. This pattern is well supported and optimized in immutable languages like the ML family, but is not super practical in a standard imperative language. Something like

  def sum(l):
    if not l: return 0
    return l[0] + sum(l[1:])

Of course this is also mostly insensitive to ordering guarantees (the compiler would be fine with the last line being `return l[-1] + sum(l[:-1])`), but immutability can remain useful in cases like this to ensure no concurrent mutation of a given object, for instance.

While your example of `sum` is a nice, pure function, it'll unfortunately blow up in python on even moderately sized inputs (we're talking thousands of elements, not millions) due to lack of tail calls in Python (currently) and the restrictions on recursion depth. The CPython interpreter as of 3.14 [0] is now capable of using tail calls in the interpreter itself, but it's not yet in Python, proper.

[0]: https://docs.python.org/3/whatsnew/3.14.html#a-new-type-of-i...