What is the purpose of "&" AFTER a nested selector in Less

Question

Less

.list a{
    .landscape&{
        height: 100%;
    }
}

Outputs

.landscape.list a {
  height: 100%;
}

Which means "all a tags whose parents have both .landscape and .list"

Less

.list a{
    &.landscape{
        height: 100%;
    }
}

Outputs

.list a.landscape {
  height: 100%;
}

Which means "all a tags which have class 'landscape' and whose parents have .list"

And that makes sense. But if I remove the "a" tag from those selectors, the '&' only changes the concatenation order of .list and .landscape.

What's the point? When should I use &.class and when should I use class.&?

Answer 1

The article "LESS CSS: Secrets of the Ampersand" details the difference well. I'll highlight the key uses:

• Attach a class to an existing selector
• Change state based on parent classes
• Filter a nested selector to only match certain elements
• Avoid repetition when selecting repeated elements
• Simplify combinatorial explosions

The latter is my favorite. I've used it to handle some crazy IE issues. Check this out:

/**
 * Add a top border to paragraphs,
 * but remove that border when a preceding paragraph already has one.
 */
p  {
    border-top: 1px solid gray;
    & + & {
        border-top: 0;
    }
}

I think if you can wrap your mind around what this usage of & does, all the other uses become obvious.