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Interpolation in At-Rule Names: Draft 1

(Issue)

This proposal defines support for interpolation in at-rule names.

Table of Contents

Summary

This section is non-normative.

Any at-rule may include interpolation in its name. This interpolation automatically causes it to be parsed as an unknown at-rule, even if the resolved name of the rule is known to Sass. The only exception is the @keyframes rule, which will still allows sub-rules with keyframes selectors such as 10%.

Design Decisions

It would be possible to treat CSS at-rules that Sass knows about, like @media, specially even when they're generated from interpolated at-rule names. However, this would add a lot of implementation complexity, since implementations would need to be able to re-parse those at-rules' values at runtime.

It's also unclear that there's any value to be gained in return for this complexity. At-rule interpolation is primarily useful for adding vendor prefixes, and the two CSS at-rules that Sass has special support for (@media and @supports) don't use vendor prefixes.

Syntax

This proposal defines a replacement for the production UnknownAtRule. The grammar for this production is:

UnknownAtRule ::= '@' InterpolatedIdentifier InterpolatedValue?
                    ('{' Statements '}')?

No whitespace is allowed after @. As with all statements, an UnknownAtRule without a block must be separated from other statements with a semicolon.

Note that this is the same as the previous syntax, except that the at-rule's name can be interpolated.

When an at-rule is evaluated, its name is evaluated to produce an unquoted string which is used as the name of the generated at-rule. Then that generated name is checked to see if it's an at-rule that has special runtime handling.

Note that only @keyframes has special runtime handling that's triggered here. Other CSS at-rules that Sass handles specially, like @media or @supports, are detected at parse-time. This means that @m#{ed}ia will be treated as an unknown at-rule rather than a media rule.