# while loops

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[Pull request](https://github.com/carbon-language/carbon-lang/pull/340)

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## Table of contents

-   [Problem](#problem)
-   [Background](#background)
-   [Proposal](#proposal)
-   [Details](#details)
    -   [Executable semantics form](#executable-semantics-form)
-   [Caveats](#caveats)
    -   [C++ as baseline](#c-as-baseline)
    -   [`do`/`while`](#dowhile)
-   [Rationale based on Carbon's goals](#rationale-based-on-carbons-goals)
-   [Alternatives considered](#alternatives-considered)
    -   [Non-C++ syntax](#non-c-syntax)
    -   [Initializing variables in the `while`](#initializing-variables-in-the-while)

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## Problem

`while` is noted in the [language overview](/docs/design/README.md#while), 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.

    ```c++
    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.

    ```python
    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`.

    ```swift
    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.

    ```rust
    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`.

    ```go
    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](https://github.com/carbon-language/carbon-lang/pull/285),
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](/docs/project/goals.md#interoperability-with-and-migration-from-existing-c-code).
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](/docs/project/goals.md#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](/docs/project/goals.md#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](https://github.com/carbon-language/carbon-lang/pull/285)
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](#c-as-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:

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

In addition,
[Selections statements with initializer](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0305r1.html)
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:

```carbon
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:

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

This breaks slightly from the [C++ baseline](#c-as-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.
