Support "do while" loops

[mdinger]

Sometimes you want to check the condition at the end and not the beginning.

do {
   do_work();  
} while condition

These substitutes are less than ideal:

do_work();
while condition {
   do_work();
}

loop {
   do_work();
   if !condition { break }
}

A do while loop would be nice in these cases.

[dscorbett]

Not that do-while wouldn’t be nice, but you can already do this:

while {
    do_work();
    condition
} { }

[mdinger]

Huh. Didn't know that. That isn't documented. Are the extra {} on the end accidental? I would have thought something would go in them. That is weird.

Runnable example

[seanmonstar]

It's because while evaluates an expression, and an expression can be in
braces. The empty body is because the work was done evaluating the
condition.

On Thu, Oct 8, 2015, 12:58 PM mdinger notifications@github.com wrote:

Huh. Didn't know that. That isn't documented
http://doc.rust-lang.org/nightly/book/loops.html#while. Are the extra {}
on the end accidental? I would have thought something would go in them.
That is weird.

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#1313 (comment).

[steveklabnik]

It's not documented because it's more of a weird hack than it is something you should ever really write.

[mdinger]

Well, if that is to be the canonical form of it, it should probably be smarter and not require the extra brace set unless someone wants an extra expression.

while {
    do_work();
    condition
}

The extra brace is kinda inconsistent between expressions too. loop and for run it after the loop is complete. while, at the end of every loop.

EDIT: It's not clear how similar the extra {} is supposed to be to for {} else {} either.

[seanmonstar]

It's not a canonical form. It's cleverness permitted by the language
grammar. Imagine the do_work fn returned the condition, you could just
write 'while do_work() {}'

On Thu, Oct 8, 2015, 1:27 PM mdinger notifications@github.com wrote:

Well, if that is to be the canonical form of it, it should probably be
smarter and not require the extra brace set unless someone wants an extra
expression.

while {
do_work();
condition
}

The extra brace is kinda inconsistent between expressions too. loop and
for run it after the loop is complete. while, at the end of every loop.

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#1313 (comment).

If you want it to be more obvious, I usually do:

loop {
    do_work();
    if (!condition) { break }
}

as an alternative to do { do_work(); } while (condition);

[mdinger]

Me too but it just seems silly not to provide something this simple which is slightly nicer. I mean, while itself if hardly more than the following so there's no dire need for while, but it's nice to have.

loop {
    if !condition { break }
    do_work();
}

[BurntSushi]

I personally don't think that's enough of a reason to add new syntax. do...while is pretty rare in my experience, which IMO means it probably doesn't justify its own syntax. I agree there is little marginal cost to adding it, but they add up over time...

Could add something like:

loop while {
    action();
    more_code();
    condition
}

It could also support stuff like:
loop while some_condition();

Currently, I don't think that do is a reserved keyword.

[mdinger]

Or:

loop {
    action();
    more_code();
} while condition

Just so there's a canonical, proper, and non-hacking way to check the condition at the bottom of the {} without break. It doesn't need to be do while.

[petrochenkov]

Ugh, based on my C++ experience do/while loops are 1) rarely needed, 2) ugly as hell, so I'd prefer to put additional sugar somewhere else.
(For example, a nice little extension to loops I would like to see is "loop n times": loop n { ... }. Now loop is almost redundant and it would give it some additional value. The last time I've seen "loop n times" in some educational language in elementary school, but it turns out to be pretty common in numeric or test code!)

[mdinger]

How is it ugly aside from a "looks different from a regular while" sense? It's the natural thing to do when you want to exit on a specific condition. It avoids break and it uses the same number of lines of code as a while loop.

[bluss]

do {
   do_work();  
} while condition

Is not ideal either, since the scoping rules don't allow condition to use local bindings from the body of the loop. In that way, the current loop-if is superior.

[ticki]

@petrochenkov: What's wrong with

for _ in 0..n {
    do_work();
}

?