p0340.md

while loops

Pull request

Table of contents

• Problem
• Background
• Proposal
• Details
  • Executable semantics form
• Caveats
  • C++ as baseline
  • do/while
• Rationale based on Carbon's goals
• Alternatives considered
  • Non-C++ syntax
  • Initializing variables in the while

Problem

while is noted in the language overview, but is provisional. Control flow is important, and while is basic; the form is similar in many languages, even if details may change.

Background

• C++: A couple example languages following C++'s syntax closely are Java and TypeScript.

  while (x) {
    DoSomething();
  }
  
  do {
    DoSomethingElse();
  } while (y);

• Python: Python does not provide do/while. However, the else syntax for having code execute if the condition is never true may be of interest.

  while x:
    DoSomething()
  
  while True:
    DoSomethingElse()
    if not y:
      break
  
  while z:
    print("z is true")
  else:
    print("z was never true")

• Swift: Swift uses repeat instead of do.

  while x {
    DoSomething()
  }
  
  repeat {
    DoSomethingElse()
  } while y

• Rust: Rust provides only a basic while loop, relying on the condition-less loop to achieve do/while-like behavior.

  while x {
    DoSomething();
  }
  
  loop {
    DoSomethingElse();
    if (!y) { break; }
  }

• Go: Go has no while loops, only for loops. However, a for can be written similar to a while.

  for x {
    DoSomething()
  }
  
  for {
    DoSomethingElse();
    if !y { break; }
  }

Proposal

Carbon should adopt while loop syntax consistent with C and C++. In particular, it should adopt these three kinds of statements:

• while: declares that we're doing a loop, containing the condition.
• continue: continues with the next loop iteration, starting with the loop condition.
• break: breaks out of the loop, without testing the loop condition.

Details

Loop syntax looks like:

• while ( boolean expression ) { statements }

While will evaluate the loop condition before each pass of the loop, only continuing if the loop condition is true. When the loop condition evaluates to false, the loop completes.

Similar to the if/else proposal, the braces are optional and must be paired ({ ... }) if present. When there are no braces, only one statement is allowed.

continue will continue with the next loop iteration directly, skipping any other statements in the loop body. The next loop iteration behaves as normal, starting with the condition being tested.

break exits the loop immediately, without testing the condition.

All of this is consistent with C and C++ behavior.

Executable semantics form

%token WHILE
%token CONTINUE
%token BREAK

statement:
  WHILE '(' expression ')' statement
| CONTINUE ';'
| BREAK ';'
| /* preexisting statements elided */
;

Note that continue and break should only be valid in a loop context.

Caveats

C++ as baseline

This baseline syntax is based on C++, following the migration sub-goal Familiarity for experienced C++ developers with a gentle learning curve. To the extent that this proposal anchors on a particular approach, it aims to anchor on C++'s existing syntax, consistent with that sub-goal.

Alternatives will generally reflect breaking consistency with C++ syntax. While most proposals may consider alternatives more, this proposal suggests a threshold of only accepting alternatives that skew from C++ syntax if they are clearly better; the priority in this proposal is to avoid debate and produce a trivial proposal. Where an alternative would trigger debate, it should be examined by an advocate in a separate proposal.

do/while

do/while is omitted from this proposal because of disagreement about whether it should be included in Carbon. It's better to have do/while considered separately as a result, in order to separate review of the non-contentious while.

Rationale based on Carbon's goals

Relevant goals are:

• 3. Code that is easy to read, understand, and write:

  • while loops are easy to read and very helpful.

• 7. Interoperability with and migration from existing C++ code:

  • Keeping syntax close to C++ will make it easier for developers to transition.

Alternatives considered

Both alternatives from the if/else proposal apply to while as well: we could remove parentheses, require braces, or both. The conclusions mirror here in order to avoid a divergence in syntax.

Additional alternatives follow.

Non-C++ syntax

Various non-C++ features that came up and are not suggested by this proposal because they aren't in C++ are:

• else on while, as in Python.
• A loop statement with while(true) behavior, as in Rust.
• Labeled break and continue statements, as in Java or TypeScript.

These may be added later, but they are not part of the C++ baseline, and have not received much consideration beyond that adopting the proposed syntax would not significantly impair adoption of such features.

Initializing variables in the while

This proposal does not offer a way to initialize variables in the while.

For comparison, C++ does allow declaring a variable in the condition, such as:

while (optional<T> next = get_next()) { ... }

In addition, Selections statements with initializer could be inferred to suggest a corresponding while (init; cond) syntax.

Neither of these is suggested in this proposal because we are likely to consider a different route of allowing declaration of a variable in expressions. For example, the following would be legal not because while would use a condition semantic that allows variable declarations as a form, but because it uses expression semantics and var would be part of expression semantics:

while (var optional<T> next = get_next()) { ... }

In particular, this would also allow more flexible usage addressing more complex use-cases that C++ does not, such as:

while ((var status_code c = bar()) != SUCCESS) { ... }`

This breaks slightly from the C++ baseline by offering a subset of C++ functionality. However, we can choose to add related functionality later if expression semantics end up not including var. Temporarily omitting condition functionality avoids having to reconcile it later if we pursue the expression route, and it is not crucial to while loop functionality.