Discussion on Interpolation

Related to #12. Moved from that issue:

For interpolators, I could see us doing one of two things:

1. just use variable interpolation syntax for both:

.class@{#ns[@var]} { }

.class@{#ns[prop]} { }

...but that leaves a redundant @ in the first example:
2. Use a more generic interpolation syntax, such as borrowing from mustache: {{ }} (but could be another pair of something, if the language starts to look too curly-bracy. But used here just to demo concept)

.class{{#ns[@var]}} { }
.class{{#ns[prop]}} { }
// or for local
.class{{@var}} { }
.class{{$prop}} { }

Existing interpolation syntax (1) could be deprecated but still supported throughout 3.x.

For single property accessing, syntax doesn't need to change.

color: red;
border-color: $color;

One possible benefit of adopting a new interpolation syntax is that it doesn't need to just contain identifiers but could, like mustache, contain expressions:

.icon_{{ @startNum + @i + 1 }} { }

That's just a possible benefit, not a necessity.

Response from @seven-phases-max (which I agreed with):

As for the selector interpolation stuff, I would not take it so far yet. It's a big story on its own.
After all, the @{} syntax initially was invented for strings only, then when (~"@{var}") (almost accidentally) was found to also be usable in selectors, it was decided to be reduced to just @{var}.
The biggest problem with those curly braces in selectors is that they really make things harder to read because of the same symbol used for CSS {} block itself. So while @{} found its way to selector interpolation for some historical reasons, if we are about inventing a new syntax (especially if it's targeting for an arbitrary Less statements to appear there), the {{ }} is the last thing I'd vote for (ref.).

Start by reading related issue #12 and the referenced discussion in pull request #1648. The issue presents alternative interpolation syntaxes and explicitly defers selector interpolation, but names no files or tests. A maintainer decision and acceptance criteria are needed before implementation can be considered done.