doc: please document the mean of "blk"

[bronze1man]

I have seen some code:

test "for else" {
    // For allows an else attached to it, the same as a while loop.
    var items = []?i32 { 3, 4, null, 5 };

    // For loops can also be used as expressions.
    var sum: i32 = 0;
    const result = for (items) |value| {
        if (value == null) {
            break 9;
        } else {
            sum += value.?;
        }
    } else blk: {
        assert(sum == 7);
        break :blk sum;
    };
}

What is blk mean in here? Why the break 9 do not have blk?

And the Grammar part do not have blk in it.

This is the biggest unknown for me.

[andrewrk]

I'll leave this issue open until this answer is available in the official docs, but to answer your question:

break accepts an expression just like return. return returns the expression from the current function. break returns the expression from the current loop.

break also accepts an optional label, so that you can choose the loop or block you wish to return an expression from:

const result = this_is_a_label: {
    break :this_is_a_label i32(1234);
};
assert(result == 1234);

[bronze1man]

Here is my understanding:
The break can be a goto in c language.
It jump to the label and return the expression to that label.
It also can jump across several for or if.

[isaachier]

@bronze1man you bring up a good point comparing break to goto. In fact, a number of languages do not support goto, like Zig itself, in order to avoid complex jumping in code. However, nested loops are one case where languages seem to be a bit more flexible. In Java, for example, goto is disallowed, but break can use a label argument (see https://docs.oracle.com/javase/tutorial/java/nutsandbolts/branch.html).

outer:
    for (int i = 0; i < x; i++) {
        for (int j = 0; j < y; j++) {
            if (...) {
                break outer;
            }
        }
    }

I like that Zig builds on this concept by not only extending control flow in blocks, but allows the returning of values.

[thejoshwolfe]

something that strikes me about the example code in the OP is that we maybe shouldn't allow loops to return values. You can accomplish that with block break statements, which i think is clearer anyway:

    const result = blk: for (items) |value| {
        if (value == null) {
            break :blk 9;
        } else {
            sum += value.?;
        }
    } else {
        assert(sum == 7);
        break :blk sum;
    };

[isaachier]

@thejoshwolfe why not?

[andrewrk]

@thejoshwolfe related to #1004

[raulgrell]

What about the proposed result keyword? #732

    const val = for (items) |value| {
        if (value == null) {
            result 9;
        } else {
            sum += value.?;
        }
    } else {
        assert(sum == 7);
        result sum;
    };

[andrewrk]

• https://ziglang.org/documentation/master/#blocks
• https://ziglang.org/documentation/master/#Labeled-while
• https://ziglang.org/documentation/master/#Labeled-for