This directory contains the abstract syntax tree that represents a Sass source file, regardless of which syntax it was written in (SCSS, the indented syntax, or plain CSS). The AST is constructed recursively by a parser from the leaf nodes in towards the root, which allows it to be fully immutable.
The Sass AST is broken up into three categories:
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The statement AST, which represents statement-level constructs like variable assignments, style rules, and at-rules.
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The expression AST, which represents SassScript expressions like function calls, operations, and value literals.
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Miscellaneous AST nodes that are used by both statements and expressions or don't fit cleanly into either category that live directly in this directory.
The Sass AST nodes are processed (usually from the root Stylesheet) by the
evaluator, which runs the logic they encode and builds up a CSS AST that
represents the compiled stylesheet. They can also be transformed back into Sass
source using the toString() method. Since this is only ever used for debugging
and doesn't need configuration or full-featured indentation tracking, it doesn't
use a full visitor.