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▲ tombert 5 days ago

I'm saying that if I do a regular loop, something needs to explicitly mutate, either a reference or a value itself.

`for (var i = 0; i < n, i++)`, for example, requires that `i` mutate in order to keep track of the value so that the loop can eventually terminate. You could also do something like:

    var i = new Obj(); 
    while (!i.isDone()) {
        //do some stuff
        i = new Obj(); 
    }
There, the reference to `i` is being mutated.

For tail recursion, it's a little different. If I did something like Factorial:

    function factorial(n, acc) {
        if (n <= 1) return acc;
        return factorial(n - 1, acc * n);
    }
Doing this, there's no explicit mutation. It might be happening behind the scenes, but everything there from the code perspective is creating a new value.

In something like Clojure, you might do something like

    (defn vrange [n]
      (loop [i 0 v []]
        (if (< i n)
          (recur (inc i) (conj v i))
          v)))
(stolen from [1])

In this case, we're creating "new" structures on every iteration of the loop, so our code is "more pure", in that there's no mutation. It might be being mutated in the implementation but not from the author's perspective.

I don't think I'm confusing anything.

[1] https://clojure.org/reference/transients

▲eru 5 days ago | parent | next [-]

You could imagine a language like eg Python with a for-each loop that creates a new variable for each run through the loop body.

Basically, just pretend the loop body is a lambda that you call for each run through the loop.

It might make sense to think of the common loops as a special kind of combinator (like 'map' and 'filter', 'reduce' etc.) And just like you shouldn't write everything in terms of 'reduce', even though you perhaps could, you shouldn't write everything in terms of the common loops either.

Make up new combinators, when you need them.

For comparison, in Haskell we seldom use 'naked' recursion directly: we typically define a combinator and then use it.

That often makes sense, even if you only use the combinator once.

▲ramchip 4 days ago | parent | prev | next [-]

> There, the reference to `i` is being mutated.

That's rebinding. Mutation is when you change the state of an object. Variables are not objects. You can't have a reference (aka pointer) pointing to a variable.

▲mrkeen 4 days ago | parent | next [-]

I don't know if you're referring to a particular language's peculiarities, but it doesn't really matter. It's mutation.

tombert's point (I think) is that in a functional setting, factorial(n) is always factorial(n). In an imperative setting, first it's 1, then 2, then 6, then 24, etc.

Here's factorial calculated imperatively in C#.

  public async Task Factorial() {
      long primitiveResult = 1L;
      Wrapped<long> objectResult = new Wrapped<long>(1L);

      var observer = new Task(ObserveVariable);
      observer.Start();

      for (var i = 2; i <= 15; i++) {
        primitiveResult *= i;
        objectResult = new Wrapped<long>(objectResult.Value * i);
        await Task.Delay(100);
      }

      observer.Wait();
      return;

      void ObserveVariable() {
        while (primitiveResult != 1307674368000 || objectResult.Value != 1307674368000) {
          Thread.Sleep(100);
        }
      }
  }
There are two results (one primitive and one object) to show that it doesn't matter. Maybe there's no such thing as a Reference-to-a-Variable in whatever language you have in mind, but in the above code ObserveVariable refers to a variable (while it mutates).
▲tombert 4 days ago | parent | prev | next [-]

I mean this is getting into splitting-hairs territory, but I would argue that rebinding is a form of mutation; the variable i is changing as far as the programmer is concerned.

If I were to write

    var i = new Obj(2)

    // do stuff 1
    
    i = new Obj(3)

    // do stuff 2
Then yes, that’s technically rebinding and not directly mutating the value in memory, but from “do stuff 2’s” perspective the value has changed. It still acts more or less the same as if it had directly changed.
▲1718627440 4 days ago | parent | prev [-]

> You can't have a reference (aka pointer) pointing to a variable.

What?

    int a;
    int * p = &a;

?
▲gpderetta 4 days ago | parent | prev [-]

  for(int i : some-container) {
     do-something-with(i);
  }
Where is the mutation?
▲mrkeen 4 days ago | parent [-]

It's implied by void-returning do-something-with(...).

Otherwise you're just burning cycles and a good compiler should dead-code eliminate the whole loop.

(see https://news.ycombinator.com/item?id=44873488)

▲gpderetta 4 days ago | parent [-]

We are discussing mutation in the loop itself, but sure:

  for(int x: container) {
      yield frob(x);
  }