Invalid nested media query merge with "or" condition, when a variable is used for sub-media query

[falkenhawk]

Invalid media query is compiled when using a variable containing a media query definition with OR conditions (either with comma or "or"-keyword). Top media query should be prepended to each sub-query.

Less:

@highdpi: ~"(-webkit-min-device-pixel-ratio: 1.25), (min-device-pixel-ratio: 1.25), (min-resolution: 1.25dppx)";
.bg {
    @media (max-width: 1000px) {
        background: url(bg-small.png);

        @media @highdpi {
            background: url([email protected]);
        }
    }
}

Output:

@media (max-width: 1000px) {
  .bg {
    background: url(bg-small.png);
  }
}
@media (max-width: 1000px) and (-webkit-min-device-pixel-ratio: 1.25), (min-device-pixel-ratio: 1.25), (min-resolution: 1.25dppx) {
  .bg {
    background: url([email protected]);
  }
}

Expected output - "(max-width: 1000px) and " before each comma-separated part:

[...]
@media (max-width: 1000px) and (-webkit-min-device-pixel-ratio: 1.25), (max-width: 1000px) and (min-device-pixel-ratio: 1.25), (max-width: 1000px) and (min-resolution: 1.25dppx) {
[...]

On the other hand, it compiles fine, when not using a variable:

.bg {
    @media (max-width: 1000px) {
        background: url(bg-small.png);

        @media (-webkit-min-device-pixel-ratio: 1.25), (min-device-pixel-ratio: 1.25), (min-resolution: 1.25dppx) {
            background: url([email protected]);
        }
    }
}

[seven-phases-max]

This is more a feature request than a bug. Less is not supposed to understand what is inside those ~"" (anywhere, not just for media queries). So it's expected for Less to never know about any commas or other special symbols there.

Technically, the code like above ("list of media queries in an escaped string") is more like an old quirk/syntax-workaround rather than ever considered proper code (yet again because of ~"" itself). In today's Less, the proper pattern to achieve the same stuff would be using anonymous/detached rulesets/mixins instead of "the magic value variable", e.g.:

.highdpi(@rules) {
    @media (-webkit-min-device-pixel-ratio: 1.25), 
           (min-device-pixel-ratio: 1.25), 
           (min-resolution: 1.25dppx) {
        @rules();
    }
}

.bg {
    @media (max-width: 1000px) {
        background: url(bg-small.png);

        .highdpi({
            background: url([email protected]);
        });
    }
}

or similar methods.

Related to #1421 and similar issues/requests.

---

Note that even as a feature request (if proposed) this feature will have to use different syntax instead of ~"" anyway, i.e. the example is never supposed to work as expected above - so I'm not marking this as "feature request" (as the FR would require a little bit more insight).

Merging to #1421 maybe?

[falkenhawk]

@seven-phases-max thank you for clearing this up. I'm going to change my mixin to utilize another ".highdpi" mixin instead of the variable with escaped string.

[matthew-dean]

@seven-phases-max I think there are two separate issues / feature requests here, so not sure if we should merge to #1421 without specifying those.

1. Allow the Less parser to be more lax with expressions and lists. Allow lists to be made up of media query values or selectors, for example. So that string escaping doesn't have to be a requirement when wanting to make a media query a variable. (Which is a common need.) This is Variable selector "string" improperly parses comma #1421, and the @media example could just be something else added to that issue.
2. Merge lists of nested media query values (depends on #1, but should have a separate pass/fail from #1. Although having done that #1, #2 is a logical next step. So, I think we can track that here in Invalid nested media query merge with "or" condition, when a variable is used for sub-media query #2964.

[seven-phases-max]

Referencing #3059 here, since essentially it's almost the same problem. I.e. basically the question is: "Can we afford full featured reparsing of @media and @supports queries, etc. etc after variables are substituted with their values?" (This either requires a major refactoring of the parser itself so it can be used on an arbitrary string at arbitrary moment, or duplicating the parsing code at each specific Node and its eval/css-gen funcs).

[stale]

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