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Start writing implementation documentation
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Natalie Weizenbaum
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@ -9,6 +9,7 @@ Want to contribute? Great! First, read this page.
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* [Synchronizing](#synchronizing)
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* [File Headers](#file-headers)
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* [Release Process](#release-process)
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* [Package Structure](#package-structure)
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## Before You Contribute
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@ -233,3 +234,13 @@ few things to do before pushing that tag:
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You *don't* need to create tags for packages in `pkg`; that will be handled
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automatically by GitHub actions.
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## Package Structure
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The structure of the Sass package is documented in README.md files in most
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directories under `lib/`. This documentation is intended to help contributors
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quickly build a basic understanding of the structure of the compiler and how its
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various pieces fit together. [`lib/src/README.md`] is a good starting point to get
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an overview of the compiler as a whole.
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[`lib/src/README.md`]: lib/src/README.md
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230
lib/src/README.md
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230
lib/src/README.md
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## The Sass Compiler
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* [Life of a Compilation](#life-of-a-compilation)
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* [Late Parsing](#late-parsing)
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* [Early Serialization](#early-serialization)
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* [JS Support](#js-support)
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* [APIs](#apis)
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* [Importers](#importers)
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* [Custom Functions](#custom-functions)
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* [Loggers](#loggers)
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* [Built-In Functions](#built-in-functions)
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* [`@extend`](#extend)
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This is the root directory of Dart Sass's private implementation libraries. This
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contains essentially all the business logic defining how Sass is actually
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compiled, as well as the APIs that users use to interact with Sass. There are
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two exceptions:
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* [`../../bin/sass.dart`] is the entrypoint for the Dart Sass CLI (on all
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platforms). While most of the logic it runs exists in this directory, it does
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contain some logic to drive the basic compilation logic and handle errors. All
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the most complex parts of the CLI, such as option parsing and the `--watch`
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command, are handled in the [`executable`] directory. Even Embedded Sass runs
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through this entrypoint, although it gets immediately gets handed off to [the
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embedded compiler].
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[`../../bin/sass.dart`]: ../../bin/sass.dart
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[`executable`]: executable
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[the embedded compiler]: embedded/README.md
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* [`../sass.dart`] is the entrypoint for the public Dart API. This is what's
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loaded when a Dart package imports Sass. It just contains the basic
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compilation functions, and exports the rest of the public APIs from this
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directory.
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[`../sass.dart`]: ../sass.dart
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Everything else is contained here, and each file and most subdirectories have
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their own documentation. But before you dive into those, let's take a look at
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the general lifecycle of a Sass compilation.
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### Life of a Compilation
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Whether it's invoked through the Dart API, the JS API, the CLI, or the embedded
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host, the basic process of a Sass compilation is the same. Sass is implemented
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as an AST-walking [interpreter] that operates in roughly three passes:
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[interpreter]: https://en.wikipedia.org/wiki/Interpreter_(computing)
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1. **Parsing**. The first step of a Sass compilation is always to parse the
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source file, whether it's SCSS, the indented syntax, or CSS. The parsing
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logic lives in the [`parse`] directory, while the abstract syntax tree that
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represents the parsed file lives in [`ast/sass`].
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[`parse`]: parse/README.md
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[`ast/sass`]: ast/sass/README.md
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2. **Evaluation**. Once a Sass file is parsed, it's evaluated by
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[`visitor/async_evaluate.dart`]. (Why is there both an async and a sync
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version of this file? See [Synchronizing] for details!) The evaluator handles
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all the Sass-specific logic: it resolves variables, includes mixins, executes
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control flow, and so on. As it goes, it builds up a new AST that represents
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the plain CSS that is the compilation result, which is defined in
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[`ast/css`].
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[`visitor/async_evaluate.dart`]: visitor/async_evaluate.dart
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[Synchronizing]: ../../CONTRIBUTING.md#synchronizing
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[`ast/css`]: ast/css/README.md
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Sass evaluation is almost entirely linear: it begins at the first statement
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of the file, evaluates it (which may involve evaluating its nested children),
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adds its result to the CSS AST, and then moves on to the second statement. On
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it goes until it reaches the end of the file, at which point it's done. The
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only exception is module resolution: every Sass module has its own compiled
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CSS AST, and once the entrypoint file is done compiling the evaluator will go
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back through these modules, resolve `@extend`s across them as necessary, and
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stitch them together into the final stylesheet.
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SassScript, the expression-level syntax, is handled by the same evaluator.
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The main difference between SassScript and statement-level evaluation is that
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the same SassScript values are used during evaluation _and_ as part of the
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CSS AST. This means that it's possible to end up with a Sass-specific value,
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such as a map or a first-class function, as the value of a CSS declaration.
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If that happens, the Serialization phase will signal an error when it
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encounters the invalid value.
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3. **Serialization**. Once we have the CSS AST that represents the compiled
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stylesheet, we need to convert it into actual CSS text. This is done by
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[`visitor/serialize.dart`], which walks the AST and builds up a big buffer of
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the resulting CSS. It uses [a special string buffer] that tracks source and
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destination locations in order to generate [source maps] as well.
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[`visitor/serialize.dart`]: visitor/serialize.dart
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[a special string buffer]: util/source_map_buffer.dart
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[source maps]: https://web.dev/source-maps/
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There's actually one slight complication here: the first and second pass aren't
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as separate as they appear. When one Sass stylesheet loads another with `@use`,
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`@forward`, or `@import`, that rule is handled by the evaluator and _only at
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that point_ is the loaded file parsed. So in practice, compilation actually
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switches between parsing and evaluation, although each individual stylesheet
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naturally has to be parsed before it can be evaluated.
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#### Late Parsing
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Some syntax within a stylesheet is only parsed _during_ evaluation. This allows
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authors to use `#{}` interpolation to inject Sass variables and other dynamic
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values into various locations, such as selectors, while still allowing Sass to
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parse them to support features like nesting and `@extend`. The following
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syntaxes are parsed during evaluation:
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* [Selectors](parse/selector.dart)
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* [`@keyframes` frames](parse/keyframe_selector.dart)
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* [Media queries](parse/media_query.dart) (for historical reasons, these are
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parsed before evaluation and then _reparsed_ after they've been fully
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evaluated)
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#### Early Serialization
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There are also some cases where the evaluator can serialize values before the
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main serialization pass. For example, if you inject a variable into a selector
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using `#{}`, that variable's value has to be converted to a string during
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evaluation so that the evaluator can then parse and handle the newly-generated
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selector. The evaluator does this by invoking the serializer _just_ for that
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specific value. As a rule of thumb, this happens anywhere interpolation is used
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in the original stylesheet, although there are a few other circumstances as
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well.
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### JS Support
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One of the main benefits of Dart as an implementation language is that it allows
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us to distribute Dart Sass both as an extremely efficient stand-alone executable
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_and_ an easy-to-install pure-JavaScript package, using the dart2js compilation
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tool. However, properly supporting JS isn't seamless. There are two major places
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where we need to think about JS support:
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1. When interfacing with the filesystem. None of Dart's IO APIs are natively
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supported on JS, so for anything that needs to work on both the Dart VM _and_
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Node.js we define a shim in the [`io`] directory that will be implemented in
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terms of `dart:io` if we're running on the Dart VM or the `fs` or `process`
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modules if we're running on Node. (We don't support IO at all on the browser
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except to print messages to the console.)
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[`io`]: io/README.md
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2. When exposing an API. Dart's JS interop is geared towards _consuming_ JS
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libraries from Dart, not producing a JS library written in Dart, so we have
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to jump through some hoops to make it work. This is all handled in the [`js`]
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directory.
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[`js`]: js/README.md
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### APIs
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One of Sass's core features is its APIs, which not only compile stylesheets but
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also allow users to provide plugins that can be invoked from within Sass. In
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both the JS API, the Dart API, and the embedded compiler, Sass provides three
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types of plugins: importers, custom functions, and loggers.
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#### Importers
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Importers control how Sass loads stylesheets through `@use`, `@forward`, and
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`@import`. Internally, _all_ stylesheet loads are modeled as importers. When a
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user passes a load path to an API or compiles a stylesheet through the CLI, we
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just use the built-in [`FilesystemImporter`] which implements the same interface
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that we make available to users.
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[`FilesystemImporter`]: importer/filesystem.dart
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In the Dart API, the importer root class is [`importer/async_importer.dart`].
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The JS API and the embedded compiler wrap the Dart importer API in
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[`importer/node_to_dart`] and [`embedded/importer`] respectively.
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[`importer/async_importer.dart`]: importer/async_importer.dart
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[`importer/node_to_dart`]: importer/node_to_dart
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[`embedded/importer`]: embedded/importer
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#### Custom Functions
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Custom functions are defined by users of the Sass API but invoked by Sass
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stylesheets. To a Sass stylesheet, they look like any other built-in function:
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users pass SassScript values to them and get SassScript values back. In fact,
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all the core Sass functions are implemented using the Dart custom function API.
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Because custom functions take and return SassScript values, that means we need
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to make _all_ values available to the various APIs. For Dart, this is
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straightforward: we need to have objects to represent those values anyway, so we
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just expose those objects publicly (with a few `@internal` annotations here and
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there to hide APIs we don't want users relying on). These value types live in
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the [`value`] directory.
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[`value`]: value/README.md
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Exposing values is a bit more complex for other platforms. For the JS API, we do
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a bit of metaprogramming in [`node/value`] so that we can return the
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same Dart values we use internally while still having them expose a JS API that
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feels native to that language. For the embedded host, we convert them to and
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from a protocol buffer representation in [`embedded/value.dart`].
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[`node/value`]: node/value/README.md
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[`embedded/value.dart`]: embedded/value.dart
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#### Loggers
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Loggers are the simplest of the plugins. They're just callbacks that are invoked
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any time Dart Sass would emit a warning (from the language or from `@warn`) or a
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debug message from `@debug`. They're defined in:
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* [`logger.dart`](logger.dart) for Dart
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* [`node/logger.dart`](node/logger.dart) for Node
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* [`embedded/logger.dart`](embedded/logger.dart) for the embedded compiler
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### Built-In Functions
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All of Sass's built-in functions are defined in the [`functions`] directory,
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including both global functions and functions defined in core modules like
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`sass:math`. As mentioned before, these are defined using the standard custom
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function API, although in a few cases they use additional private features like
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the ability to define multiple overloads of the same function name.
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[`functions`]: functions/README.md
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### `@extend`
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The logic for Sass's `@extend` rule is particularly complex, since it requires
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Sass to not only parse selectors but to understand how to combine them and when
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they can be safely optimized away. Most of the logic for this is contained
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within the [`extend`] directory.
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[`extend`]: extend/README.md
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