Handling scalar values with `extract` and `length` functions.

[seven-phases-max]

(Feature request)

I was about to pull length function implementation (as in #1542), but realized that it would first probably make sense to propose a minor improvement for the extract function since it's closely related.

Currently, a scalar value (e.g. any non-array/list value) handling by extract is somewhat undefined. Certain types just pass through while others rise a errors, for example:

results {
    c: extract(123, 1);   // OK, outputs c: extract(123, 1);
    d: extract(#123, 1);  // non_object_property_loadError: error evaluating function `extract`
    c: extract('123', 1); // undefined_methodError: Object 1 has no method 'genCSS'
    a: extract(name, 1);  // undefined_methodError: Object n has no method 'genCSS'
    // etc.
}

So my proposal is: both extract and length should treat a single value as an array of length 1. The rationale behind this is quite simple I think: any code working with an arbitrary number of values (as a variable(s) or a mixin argument(s)) would be able to handle both array/list and scalar input in the same uniform manner.
Here's a typical use-case example I think of:

.widget-1 {
    .transition(color);
}

.widget-2 {
    .transition(color, background-color, border-color);
}

.transition(@properties...) {
    .loop(length(@properties));
    .loop(@i) when (@i > 0) {
        .loop((@i - 1));
        transition+: extract(@properties, @i) 2s ease-out;
    }
}

CSS output:

.widget-1 {
  transition: color 2s ease-out;
}
.widget-2 {
  transition: color 2s ease-out, background-color 2s ease-out, border-color 2s ease-out;
}

That's it, no need for any conditionals, specialized mixin, etc...

Thanks.

[lukeapage]

merged