opDispatch and opApply parts should be split, plus incorrect example of opApply

[baryluk]

https://tour.dlang.org/tour/en/gems/opdispatch-opapply

has few issues

• It bunches together two very different things - opDispatch and opApply
• It does not link to opApply documentation, only opDispatch. This is because opApply is not really operator overloading. opApply is documented in statements and foreach and foreach_reverse - https://dlang.org/spec/statement.html#foreach_over_struct_and_classes
• There is no runnable example for opApply
• In-line example is buggy

It is:

class Tree {
    Tree lhs;
    Tree rhs;
    int opApply(int delegate(Tree) dg) {
        if (lhs && lhs.opApply(dg)) return 1;
        if (dg(this)) return 1;
        if (rhs && rhs.opApply(dg)) return 1;
        return 0;
    }
}
Tree tree = new Tree;
foreach (node; tree) {
    ...
}

but doing blind return 0 is not correct, especially when doing nested loops, with return statements in the body of the loops, or labeled continue/break (on outer loop), or with goto to label that is outside of the inner most loop.

This is documented in subsections 5 and 6 of the opApply documentation.

Proper way, is to return from opApply same value that dg returned:

class Tree {
    Tree lhs;
    Tree rhs;
    int opApply(int delegate(Tree) dg) {
        if (lhs) if (auto ret = lhs.opApply(dg)) return ret;
        if (auto ret = dg(this)) return ret;
        if (rhs) if (auto ret = rhs.opApply(dg)) return ret;
        return 0;
    }
}
Tree tree = new Tree;
foreach(node; tree) {
    ...
}

Or something similar.

[baryluk]

It would be also interesting to ask @WalterBright or some other dmd guru, if maybe D should add a warning to detect return of non-zero literal values from opApply, to automatically detect such errors.