sass/spec/built-in-modules.md

Built-In Modules

Sass provides a number of built-in modules that may be loaded from URLs with the scheme sass. These modules have no extensions, CSS trees, dependencies, or source files. Their canonical URLs are the same as the URLs used to load them.

Built-In Functions and Mixins

Each function and mixin defined in a built-in modules is specified with a signature of the form

<ident-token> ArgumentDeclaration

followed by a procedure. It's available as a member (either function or mixin) in the module whose name is the value of the <ident-token>. When it's executed with args:

• With an empty scope with no parent as the current scope:

  • Evaluate args with the signature's ArgumentDeclaration.

  • Run the procedure, and return its result if this is a function.

Built-in mixins don't accept content blocks unless explicitly specified otherwise.

By convention, in these procedures $var is used as a shorthand for "the value of the variable var in the current scope".

In other words, $var is the value passed to the argument $var.