RFC 1624: loop-break-value

lang (syntax | expressions | control-flow)

• Feature Name: loop_break_value
• Start Date: 2016-05-20
• RFC PR: rust-lang/rfcs#1624
• Rust Issue: rust-lang/rust#37339

Summary

(This is a result of discussion of issue #961 and related to RFCs 352 and 955.)

Let a loop { ... } expression return a value via break my_value;.

Motivation

Rust is an expression-oriented language. Currently loop constructs don't provide any useful value as expressions, they are run only for their side-effects. But there clearly is a "natural-looking", practical case, described in this thread and [this] RFC, where the loop expressions could have meaningful values. I feel that not allowing that case runs against the expression-oriented conciseness of Rust. comment by golddranks

Some examples which can be much more concisely written with this RFC:

// without loop-break-value:
let x = {
    let temp_bar;
    loop {
        ...
        if ... {
            temp_bar = bar;
            break;
        }
    }
    foo(temp_bar)
};

// with loop-break-value:
let x = foo(loop {
        ...
        if ... { break bar; }
    });

// without loop-break-value:
let computation = {
    let result;
    loop {
        if let Some(r) = self.do_something() {
            result = r;
            break;
        }
    }
    result.do_computation()
};
self.use(computation);

// with loop-break-value:
let computation = loop {
        if let Some(r) = self.do_something() {
            break r;
        }
    }.do_computation();
self.use(computation);

Detailed design

This proposal does two things: let break take a value, and let loop have a result type other than ().

Break Syntax

Four forms of break will be supported:

1. break;
2. break 'label;
3. break EXPR;
4. break 'label EXPR;

where 'label is the name of a loop and EXPR is an expression. break and break 'label become equivalent to break () and break 'label () respectively.

Result type of loop

Currently the result type of a 'loop' without 'break' is ! (never returns), which may be coerced to any type. The result type of a 'loop' with a 'break' is (). This is important since a loop may appear as the last expression of a function:

fn f() {
    loop {
        do_something();
        // never breaks
    }
}
fn g() -> () {
    loop {
        do_something();
        if Q() { break; }
    }
}
fn h() -> ! {
    loop {
        do_something();
        // this loop must diverge for the function to typecheck
    }
}

This proposal allows 'loop' expression to be of any type T, following the same typing and inference rules that are applicable to other expressions in the language. Type of EXPR in every break EXPR and break 'label EXPR must be coercible to the type of the loop the EXPR appears in.

It is an error if these types do not agree or if the compiler's type deduction rules do not yield a concrete type.

Examples of errors:

// error: loop type must be () and must be i32
let a: i32 = loop { break; };
// error: loop type must be i32 and must be &str
let b: i32 = loop { break "I am not an integer."; };
// error: loop type must be Option<_> and must be &str
let c = loop {
    if Q() {
        break "answer";
    } else {
        break None;
    }
};
fn z() -> ! {
    // function does not return
    // error: loop may break (same behaviour as before)
    loop {
        if Q() { break; }
    }
}

Example showing the equivalence of break; and break ();:

fn y() -> () {
    loop {
        if coin_flip() {
            break;
        } else {
            break ();
        }
    }
}

Coercion examples:

// ! coerces to any type
loop {}: ();
loop {}: u32;
loop {
    break (loop {}: !);
}: u32;
loop {
    // ...
    break 42;
    // ...
    break panic!();
}: u32;

// break EXPRs are not of the same type, but both coerce to `&[u8]`.
let x = [0; 32];
let y = [0; 48];
loop {
    // ...
    break &x;
    // ...
    break &y;
}: &[u8];

Result value

A loop only yields a value if broken via some form of break ...; statement, in which case it yields the value resulting from the evaulation of the statement's expression (EXPR above), or () if there is no EXPR expression.

Examples:

assert_eq!(loop { break; }, ());
assert_eq!(loop { break 5; }, 5);
let x = 'a loop {
    'b loop {
        break 'a 1;
    }
    break 'a 2;
};
assert_eq!(x, 1);

Drawbacks

The proposal changes the syntax of break statements, requiring updates to parsers and possibly syntax highlighters.

Alternatives

No alternatives to the design have been suggested. It has been suggested that the feature itself is unnecessary, and indeed much Rust code already exists without it, however the pattern solves some cases which are difficult to handle otherwise and allows more flexibility in code layout.

Unresolved questions

Extension to for, while, while let

A frequently discussed issue is extension of this concept to allow for, while and while let expressions to return values in a similar way. There is however a complication: these expressions may also terminate "naturally" (not via break), and no consensus has been reached on how the result value should be determined in this case, or even the result type.

There are three options:

1. Do not adjust for, while or while let at this time
2. Adjust these control structures to return an Option<T>, returning None in the default case
3. Specify the default return value via some extra syntax

Via Option<T>

Unfortunately, option (2) is not possible to implement cleanly without breaking a lot of existing code: many functions use one of these control structures in tail position, where the current "value" of the expression, (), is implicitly used:

// function returns `()`
fn print_my_values(v: &Vec<i32>) {
    for x in v {
        println!("Value: {}", x);
    }
    // loop exits with `()` which is implicitly "returned" from the function
}

Two variations of option (2) are possible:

• Only adjust the control structures where they contain a break EXPR; or break 'label EXPR; statement. This may work but would necessitate that break; and break (); mean different things.
• As a special case, make break (); return () instead of Some(()), while for other values break x; returns Some(x).

Via extra syntax for the default value

Several syntaxes have been proposed for how a control structure's default value is set. For example:

fn first<T: Copy>(list: Iterator<T>) -> Option<T> {
    for x in list {
        break Some(x);
    } else default {
        None
    }
}

or:

let x = for thing in things default "nope" {
    if thing.valid() { break "found it!"; }
}

There are two things to bear in mind when considering new syntax:

• It is undesirable to add a new keyword to the list of Rust's keywords
• It is strongly desirable that unbounded lookahead is not required while syntax parsing Rust code

For more discussion on this topic, see issue #961.