Implement infix:<^^> (in a module)

[masak]

Can't believe I haven't been missing this one in 007 yet.

We follow Perl 6's semantics for this one, that is, the operator is list associative, short-circuiting, and iff exactly one of its operands is truthy, then it hands back that value. That is, for

a ^^ b ^^ c

The following could happen:

• All values are falsy. Then it returns c (the last value, which is falsy).
• Exactly one of a, b, or c is truthy. Then it returns that one.
• In all the other cases, it returns none (which is a falsy value, but distinct from false). Furthermore, it will not evaluate any argument beyond the second truthy one. (E.g. if a and b are both truthy, c won't be evaluated.)

The last point makes this (ah!) a macro, which is great news. Here's an attempt at an implementation:

macro infix:<^^>(...exprs) is equiv(infix:<||>) is assoc("list") {
    return quasi {
        my n = 0;
        my result = none;
        for exprs -> e {
            my val = {{{e}}};
            if val {
                result = val;  # the "exactly one" case
                n++;
                if n > 1 {
                    result = none;  # the "second truthy one" case
                    last;
                }
            }
            else if result == none {
                result = val;  # the "all values are falsy" case
            }
        }
        result;
    };
}

A small modification involving lvalue() can make it hand back lvalues, too. (Perl 6's does.) But I only feel we need to do that when we can make infix:<||> and infix:<&&> hand back lvalues.

[masak]

I kind of glossed over the fact that in order for this to work, we kind of need a new "list" associativity, too. But yeah, we do.

The other logical infixes are also list-associative, but you can't really tell with them, since the semantics ends up being the same as if they were left-associative. infix:<^^> is different exactly because it needs to keep the n counter (in the code above) across the entire list.

[masak]

This, by the way, is a delightful core module to have early, because it's not waiting on is parsed. Just blocking on modules and imports in general.

[masak]

...and varargs, aka slurpy parameters (#259).

[masak]

...and list associativity.