Labeled break and continue

[brson]

Henri needs a labeled break and continue equivalent in order to generate an HTML parser for servo. For C++ they use gotos and macros. We could possibly devise something with macros, though it may not be efficient.

[graydon]

The only difficulty is grammatical: collides when at the beginning of a block. Open brace ident colon is the prefix of a record literal. Any other good looking options?

[boggle]

Would pre-declaring of labels help to sort this out, i.e. label foo and a production that dispatches pre-declared labels as potential jump targets and non-declared ones as record fields?

[graydon]

might, but seems unfortunate. maybe some kind of sub production on the loop headers? while foo: bar() {...} should work, i think. label immediately following loop keyword. tolerable?

[boggle]

I find that hard to read when scanning for labels but with a small lookahead 'foo: while' should be distinguishable due to while being a keyword.

[bstrie]

Right now {foo: while true { break; }} is a valid record where the value of foo is ().

[bstrie]

Perhaps the most sane solution is simply to introduce a new label keyword. In that way at least it doesn't require too much cleverness, and it requires people to be explicit where they want to use labeled break and continue (which hopefully will not be too often).

[nikomatsakis]

One thing: I don't know what Henri needs precisely, but at least in Java it is possible to label an arbitrary block and break out of it:

some_block: {
    ...
    if (some_cond)
        // Jumps to (1)
        break some_block;
    ...
}
... // (1)

Whatever syntax we do may need to be applicable to blocks as well.

[brson]

I think we should try to resolve this for 0.3

[brson]

I took a peek at the parser source and I suspect that it only uses labels on for loops, mostly ones that look like for(;;).

[brson]

#[label = "foo"]
loop {
}

We don't even have to commit to a syntax.

[brson]

#[label = "foo"]
loop {
    #[label = "foo"]
    break;
}

[boggle]

Nice, I like this!

[graydon]

Attributes could work but feels odd to me given that it is control flow. What about assembly style labels? .label: { ... } ?