sass/spec/free-interpolation.md

Free Interpolation

"Free interpolation" refers to interpolation that appears in SassScript values outside of a quoted string. Earlier versions of Sass had very convoluted rules for parsing free interpolation, but the current rule is very simple: interpolation is parsed as though it were an alphabetic character in an identifier. This means that:

• It can be used within an identifier, as in a#{b}c, and is considered part of that identifier. The contents of the interpolation is evaluated at runtime and concatenated with the rest of the identifier to produce an unquoted string.

• It can be used on its own, as in #{b}. This is a single expression that converts the contents of the interpolation to an unquoted string.

• Because it's parsed as an alphabetic character, when it comes after a hyphen that hyphen is considered part of the identifier. This means that $var -#{b} is parsed as a space-separated list containing $var and -#{b} rather than as the operation $var - #{b}.

• Because numbers are not an identifier start characters and the interpolation is parsed as an alphabetic character, 0#{$b} is parsed as a space-separated list containing 0 and #{$b}