[Language] for-else and while-else

[igotfr]

when assigning to a variable, break in for and while isn't allowed, this proposal allow it using a else

• when break is not reached, evaluates to value in else

• semantic similar to Zig

https://ziglang.org/documentation/master/#for

read the comment:

// For loops can also be used as expressions.
// Similar to while loops, when you break from a for loop, the else branch is not evaluated.

• semantic different of Python

Example:

fn main() {
  let result = for counter in 0..5 {
    if counter == 3 {
      break counter
    }
  } else { -7 };

  println!("The result is {result}") // 3
}

fn main() {
  let result = for counter in 0..5 {
    if counter == 90 {
      break counter
    }
  } else { -7 };

  println!("The result is {result}") // -7
}

Alternatives

https://rust-lang.github.io/rfcs/2046-label-break-value.html

Alternatives to else keyword

IDK

[PatchMixolydic]

This was previously suggested in #3163, which was closed due to concerns over the clarity of the syntax.

[igotfr]

This was previously suggested in #3163, which was closed due to concerns over the clarity of the syntax.

when break is not reached, evaluates to value in else, what's the struggle?

[SOF3]

@cindRoberta the behavior in most other languages only runs else if no iterations happened at all, no matter break or not

[igotfr]

@cindRoberta the behavior in most other languages only runs else if no iterations happened at all, no matter break or not

@SOF3 in other languages, for and while aren't expressions. It's a weird behaviour for and while ever return (), the behaviour that I proposed is the same of Zig, Zig uses else

I proposed a different behaviour than Python

https://ziglang.org/documentation/master/#for

read the comment:

// For loops can also be used as expressions.
// Similar to while loops, when you break from a for loop, the else branch is not evaluated.

I'm not creating nothing, it already exists

[SOF3]

Yes, but that's exactly the reason why it is extremely confusing if someone ends up using it as a non-expression. This is just as confusing as case without break not falling through in golang because everyone is used to that fact.

[igotfr]

Yes, but that's exactly the reason why it is extremely confusing if someone ends up using it as a non-expression. This is just as confusing as case without break not falling through in golang because everyone is used to that fact.

if, for, while and loop in Rust are ever evaluating for a value, same when not binded to a variable. Just that for and while are ever evaluating to () and if and loop not

if the struggle is the confusion with Python, use other keyword than else

[SOF3]

But what you can do with for-else can already be done with iterators right? break-else is basically the same as iter.find_map(...).unwrap_or_else(...).

[igotfr]

But what you can do with for-else can already be done with iterators right? break-else is basically the same as iter.find_map(...).unwrap_or_else(...).

@safinaskar what can to be done with for-else can already be done too with labels, for-else is just fill a gap of ever evaluate to ()

[safinaskar]

@cindRoberta , what you want is already possible thanks to recently stabilized "break from labeled blocks" ( see https://blog.rust-lang.org/2022/11/03/Rust-1.65.0.html ). Here is how we can write your example with recent stable Rust: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=4981da6fe916fc970922ebcd8e639e2c

[igotfr]

@cindRoberta , what you want is already possible thanks to recently stabilized "break from labeled blocks" ( see https://blog.rust-lang.org/2022/11/03/Rust-1.65.0.html ). Here is how we can write your example with recent stable Rust: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=4981da6fe916fc970922ebcd8e639e2c

https://rust-lang.github.io/rfcs/2046-label-break-value.html

[sollyucko]

Here's are the relevant links I've found:

Previously proposed in:

• Allow loops to return values other than () #352
• Allow loops to return values other than () #961
• Allow for and while loop to return value (without discussion of keyword names) #1767
• Reserving "nobreak" as a keyword for Rust Edition 2021 #3152
• Proposal: Else clauses for for and while loops #3163
• https://www.reddit.com/r/rust/comments/n7l028/for_else/
• https://internals.rust-lang.org/t/allow-loops-to-return-values-other-than/567
• https://internals.rust-lang.org/t/pre-rfc-break-with-value-in-for-while-loops/11208/
• https://github.com/ftxqxd/rfcs/blob/loop-else/text/0000-loops-returning-values.md
• https://github.com/fstirlitz/rust-rfcs/blob/exhaustive_ballot/text/0000-loop-exhaustion.md
• https://github.com/rust-lang/rfcs/blob/master/text/1624-loop-break-value.md#extension-to-for-while-while-let

Alternative features and proposals:

• https://programming-idioms.org/idiom/223/for-else-loop/3852/rust
• https://rust-lang.github.io/rfcs/2046-label-break-value.html, https://github.com/rust-lang/rfcs/blob/798b894319c4b79e168dd160678166456be633c6/text/2046-label-break-value.md, RFC: label-break-value #2046, Tracking issue for RFC 2046, label-break-value rust#48594
• https://doc.rust-lang.org/reference/expressions/loop-expr.html#break-and-loop-values
• https://internals.rust-lang.org/t/pre-rfc-generator-integration-with-for-loops/6625
• https://internals.rust-lang.org/t/break-with-value-alternatives/11240
• https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.try_find, Tracking Issue for Iterator::try_find rust#63178, Add Iterator::try_find rust#63177

Similar features or proposals in other languages with the semantics you're proposing:

• Python: https://docs.python.org/3/tutorial/controlflow.html#break-and-continue-statements-and-else-clauses-on-loops, https://docs.python.org/3.4/reference/compound_stmts.html#for, https://docs.python.org/3.4/reference/compound_stmts.html#the-while-statement, https://s16h.medium.com/the-forgotten-optional-else-in-python-loops-90d9c465c830 (but see also https://blog.glyphobet.net/blurb/2187/, which includes https://twitpic.com/4a52sh)
• Zig: https://ziglang.org/documentation/master/, search for test_while_else.zig and test "for else"
• Go: proposal: Go 2: For Else branch golang/go#41348, proposal: spec: improve for-loop ergonomics golang/go#24282
• Julia: for-else and while-else JuliaLang/julia#1289 (comment), break with value + loop else clauses JuliaLang/julia#22891, break-with-value and for/else implementation JuliaLang/julia#23260

Features or proposals with similar syntax but semantics to run the else if the loop never iterates:

• PHP: https://wiki.php.net/rfc/loop_else, https://bugs.php.net/bug.php?id=26411, https://bugs.php.net/bug.php?id=46240, https://bugs.php.net/bug.php?id=61222
• Jinja (Python template engine): https://jinja.palletsprojects.com/en/3.0.x/templates/#for
• Julia: for-else and while-else JuliaLang/julia#1289
• Twig (PHP template engine): https://twig.symfony.com/doc/2.x/tags/for.html#the-else-clause/

[erkinalp]

For-else (with Python semantics) implemented as a macro: https://github.com/erkinalp/pythonic-for

[igotfr]

For-else (with Python semantics) implemented as a macro: https://github.com/erkinalp/pythonic-for

I have updated this proposal, please, read it again

[sollyucko]

Isn't this proposal the same as Python's semantics, except that the for loop is an expression rather than a statement, and so can have a value? If it's different beyond that, what is the difference?

https://docs.python.org/3/tutorial/controlflow.html#break-and-continue-statements-and-else-clauses-on-loops:

In a for or while loop the break statement may be paired with an else clause. If the loop finishes without executing the break, the else clause executes.

In a for loop, the else clause is executed after the loop finishes its final iteration, that is, if no break occurred.

[...]

In either kind of loop, the else clause is not executed if the loop was terminated by a break. Of course, other ways of ending the loop early, such as a return or a raised exception, will also skip execution of the else clause.

^{(formatting copied from source)}

Here's an approximate port of your example to Python, which gives the same result: Try it online!

for stop_condition in [3, 90]:
    print("stop_condition =", stop_condition)
    for counter in range(5):
        if counter == stop_condition:
            result = counter
            print("breaking at", counter)
            break
    else:
        result = -7
        print("in else clause")
    print("The result is", result)
    print()

Output:

stop_condition = 3
breaking at 3
The result is 3

stop_condition = 90
in else clause
The result is -7

[kennytm]

with label-break-value this is supported as:

    let result = 'result: {
        for counter in 0..5 {
            if counter == 3 {
                break 'result counter;
            }
        }
        -7
    };

Pro: Need to explain how the else clause works
Con: One more indentation level