Looping Constructs

[eliaslfox]

[Moved from #840]

I had some initial thoughts about adding looping constructs to Coalton. This isn't a concrete plan for implementation.

Possible looping constructs could include:

• loop/forever
• while
• until (the inverse of while)
• c style for loops
• rust style iterator based for loops

I think whatever looping constructs get added, they should support break and continue.

A few other looping related feature that are worth considering:

Breaking from loop with a value like in Rust.

let mut counter = 0;

let result = loop {
    counter += 1;

    if counter == 10 {
        break counter * 2;
    }
};

Breaking and continue with named blocks like in JavaScript and many other languages.

let i, j;

// The first for statement is labeled "loop1"
loop1: for (i = 0; i < 3; i++) {
  // The second for statement is labeled "loop2"
  loop2: for (j = 0; j < 3; j++) {
    if (i === 1 && j === 1) {
      break loop1;
    }
    console.log(`i = ${i}, j = ${j}`);
  }
}

// Logs:
// i = 0, j = 0
// i = 0, j = 1
// i = 0, j = 2
// i = 1, j = 0

I also want to design something with a plan for how it can be optimized. Ideally the following two examples would compile identically.

;; Coalton
(for (x (iter:into-iter (make-list 1 2 3 4 5)))
  (f x))

;; Lisp
(dolist (x (list 1 2 3 4 5))
  (f x))

[simendsjo]

F# has Computational Expressions where you can define how looping works for
instance. Programmable let, for, etc is quite powerful. See https://learn.microsoft.com/en-us/dotnet/fsharp/language-reference/computation-expressions#creating-a-new-type-of-computation-expression