Provide error when mixin results in [object Object] value

[uncenter]

I have a mixin called rgbify, as seen below:

#rgbify(@color) {
  @rgb: red(@color), green(@color), blue(@color);
}

It can be used as color: #rgbify(#fff)[];, and note the [] to capture the return value(s). Without the [] (just #rgbify(#fff)), Less outputs [object Object] as the value for wherever the mixin is used. I most definitely did not mean to have the value be [object Object], and I'm hoping Less could error when it encounters a value like that - otherwise i have no good way of knowing I missed it if I'm not looking carefully/specifically for that mistake.

[matthew-dean]

I marked this as a bug at first, until I read it a little more carefully and yeah, this is more like a feature request. Returning a mixin body to assign to a property is undefined behavior.

[matthew-dean]

@uncenter Sorry yes I meant any property, not specifically a custom property. I'm just noting why it's rendering "[object Object]" has to do with Less attempting to render the mixin as a value. IIRC there was an error that was added to be thrown in some instances of trying to render rule sets, but there must be an implementation bug here. So... could be a bug or feature request. Like... since it's supposed to error in the original implementation, that's kind of a bug. But also the docs don't say what should happen is all I mean, so making sure it does error as a documented feature would be good.

That's just of words to say that what you're reporting is valid (I've already created an issue ticket from it) and it's not important how it's labelled.

[matthew-dean]

Related, if you write:

#rgbify(@color) {
  @rgb: red(@color), green(@color), blue(@color);
}

.test {
  fg: #rgbify(red) l;
}

...you get:

.test {
  fg: @rgb: 255, 0, 0; l;
}

...which doesn't make sense either and should error.

[uncenter]

Right. I just wish you could have a default return value in mixins... but alas, [] will do.

[matthew-dean]

@uncenter You do have a default return value in mixins, which is the set of rules. Do you mean you wish that when rendering a set of rules does not make sense, it would return a particular value?

[uncenter]

Well I mean I wish you could return a "default" variable from a mixin, because something like this isn't a "rule" and doesn't work:

#rgbify(@color) {
  red(@color), green(@color), blue(@color)
}

:root {
  color: #rgbify(red);
}

I guess what I really want is something like Sass's functions: https://sass-lang.com/documentation/at-rules/function/.