carbon-lang/docs/design/control_flow.md

Control flow

Table of contents

• TODO
• Overview
• Open questions
  • if blocks
  • break and continue

TODO

This is a skeletal design, added to support the overview. It should not be treated as accepted by the core team; rather, it is a placeholder until we have more time to examine this detail. Please feel welcome to rewrite and update as appropriate.

At least summarize if and else to cover basics. Especially important to surface the idea of using basic conditionals as both expressions and statements to avoid needing conditional operators.

Looping is an especially interesting topic to explore as there are lots of challenges posed by the C++ loop structure. Even C++ itself has been seeing significant interest and pressure to improve its looping facilities.

Overview

Blocks of statements are generally executed linearly. However, statements are the primary place where this flow of execution can be controlled. Carbon's control flow constructs are mostly similar to those in C, C++, and other languages.

fn Foo(Int: x) {
  if (x < 42) {
    Bar();
  } else if (x > 77) {
    Baz();
  }
}

Loops will at least be supported with a low-level primitive while statement, with break and continue statements which work the same as in C++.

Last but not least, for the basics we need to include the return statement. This statement ends the flow of execution within a function, returning it to the caller. If the function returns a value to the caller, that value is provided by an expression in the return statement. This allows us to complete the definition of our Sum function from earlier as:

fn Sum(Int: a, Int: b) -> Int {
  return a + b;
}

Open questions

if blocks

It is an open question whether a block is required or a single statement may be nested in an if statement. Similarly, it is an open question whether else if is a single keyword versus a nested if statement, and if it is a single construct whether it should be spelled elif or something else.

break and continue

If and how to support a "labeled break" or "labeled continue" is still a point of open discussion.