diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md index 934a0576..b01d899c 100644 --- a/CONTRIBUTING.md +++ b/CONTRIBUTING.md @@ -9,6 +9,7 @@ Want to contribute? Great! First, read this page. * [Synchronizing](#synchronizing) * [File Headers](#file-headers) * [Release Process](#release-process) +* [Package Structure](#package-structure) ## Before You Contribute @@ -233,3 +234,13 @@ few things to do before pushing that tag: You *don't* need to create tags for packages in `pkg`; that will be handled automatically by GitHub actions. + +## Package Structure + +The structure of the Sass package is documented in README.md files in most +directories under `lib/`. This documentation is intended to help contributors +quickly build a basic understanding of the structure of the compiler and how its +various pieces fit together. [`lib/src/README.md`] is a good starting point to get +an overview of the compiler as a whole. + +[`lib/src/README.md`]: lib/src/README.md diff --git a/lib/src/README.md b/lib/src/README.md new file mode 100644 index 00000000..e213bc6d --- /dev/null +++ b/lib/src/README.md @@ -0,0 +1,230 @@ +## The Sass Compiler + +* [Life of a Compilation](#life-of-a-compilation) + * [Late Parsing](#late-parsing) + * [Early Serialization](#early-serialization) +* [JS Support](#js-support) +* [APIs](#apis) + * [Importers](#importers) + * [Custom Functions](#custom-functions) + * [Loggers](#loggers) +* [Built-In Functions](#built-in-functions) +* [`@extend`](#extend) + +This is the root directory of Dart Sass's private implementation libraries. This +contains essentially all the business logic defining how Sass is actually +compiled, as well as the APIs that users use to interact with Sass. There are +two exceptions: + +* [`../../bin/sass.dart`] is the entrypoint for the Dart Sass CLI (on all + platforms). While most of the logic it runs exists in this directory, it does + contain some logic to drive the basic compilation logic and handle errors. All + the most complex parts of the CLI, such as option parsing and the `--watch` + command, are handled in the [`executable`] directory. Even Embedded Sass runs + through this entrypoint, although it gets immediately gets handed off to [the + embedded compiler]. + + [`../../bin/sass.dart`]: ../../bin/sass.dart + [`executable`]: executable + [the embedded compiler]: embedded/README.md + +* [`../sass.dart`] is the entrypoint for the public Dart API. This is what's + loaded when a Dart package imports Sass. It just contains the basic + compilation functions, and exports the rest of the public APIs from this + directory. + + [`../sass.dart`]: ../sass.dart + +Everything else is contained here, and each file and most subdirectories have +their own documentation. But before you dive into those, let's take a look at +the general lifecycle of a Sass compilation. + +### Life of a Compilation + +Whether it's invoked through the Dart API, the JS API, the CLI, or the embedded +host, the basic process of a Sass compilation is the same. Sass is implemented +as an AST-walking [interpreter] that operates in roughly three passes: + +[interpreter]: https://en.wikipedia.org/wiki/Interpreter_(computing) + +1. **Parsing**. The first step of a Sass compilation is always to parse the + source file, whether it's SCSS, the indented syntax, or CSS. The parsing + logic lives in the [`parse`] directory, while the abstract syntax tree that + represents the parsed file lives in [`ast/sass`]. + + [`parse`]: parse/README.md + [`ast/sass`]: ast/sass/README.md + +2. **Evaluation**. Once a Sass file is parsed, it's evaluated by + [`visitor/async_evaluate.dart`]. (Why is there both an async and a sync + version of this file? See [Synchronizing] for details!) The evaluator handles + all the Sass-specific logic: it resolves variables, includes mixins, executes + control flow, and so on. As it goes, it builds up a new AST that represents + the plain CSS that is the compilation result, which is defined in + [`ast/css`]. + + [`visitor/async_evaluate.dart`]: visitor/async_evaluate.dart + [Synchronizing]: ../../CONTRIBUTING.md#synchronizing + [`ast/css`]: ast/css/README.md + + Sass evaluation is almost entirely linear: it begins at the first statement + of the file, evaluates it (which may involve evaluating its nested children), + adds its result to the CSS AST, and then moves on to the second statement. On + it goes until it reaches the end of the file, at which point it's done. The + only exception is module resolution: every Sass module has its own compiled + CSS AST, and once the entrypoint file is done compiling the evaluator will go + back through these modules, resolve `@extend`s across them as necessary, and + stitch them together into the final stylesheet. + + SassScript, the expression-level syntax, is handled by the same evaluator. + The main difference between SassScript and statement-level evaluation is that + the same SassScript values are used during evaluation _and_ as part of the + CSS AST. This means that it's possible to end up with a Sass-specific value, + such as a map or a first-class function, as the value of a CSS declaration. + If that happens, the Serialization phase will signal an error when it + encounters the invalid value. + +3. **Serialization**. Once we have the CSS AST that represents the compiled + stylesheet, we need to convert it into actual CSS text. This is done by + [`visitor/serialize.dart`], which walks the AST and builds up a big buffer of + the resulting CSS. It uses [a special string buffer] that tracks source and + destination locations in order to generate [source maps] as well. + + [`visitor/serialize.dart`]: visitor/serialize.dart + [a special string buffer]: util/source_map_buffer.dart + [source maps]: https://web.dev/source-maps/ + +There's actually one slight complication here: the first and second pass aren't +as separate as they appear. When one Sass stylesheet loads another with `@use`, +`@forward`, or `@import`, that rule is handled by the evaluator and _only at +that point_ is the loaded file parsed. So in practice, compilation actually +switches between parsing and evaluation, although each individual stylesheet +naturally has to be parsed before it can be evaluated. + +#### Late Parsing + +Some syntax within a stylesheet is only parsed _during_ evaluation. This allows +authors to use `#{}` interpolation to inject Sass variables and other dynamic +values into various locations, such as selectors, while still allowing Sass to +parse them to support features like nesting and `@extend`. The following +syntaxes are parsed during evaluation: + +* [Selectors](parse/selector.dart) +* [`@keyframes` frames](parse/keyframe_selector.dart) +* [Media queries](parse/media_query.dart) (for historical reasons, these are + parsed before evaluation and then _reparsed_ after they've been fully + evaluated) + +#### Early Serialization + +There are also some cases where the evaluator can serialize values before the +main serialization pass. For example, if you inject a variable into a selector +using `#{}`, that variable's value has to be converted to a string during +evaluation so that the evaluator can then parse and handle the newly-generated +selector. The evaluator does this by invoking the serializer _just_ for that +specific value. As a rule of thumb, this happens anywhere interpolation is used +in the original stylesheet, although there are a few other circumstances as +well. + +### JS Support + +One of the main benefits of Dart as an implementation language is that it allows +us to distribute Dart Sass both as an extremely efficient stand-alone executable +_and_ an easy-to-install pure-JavaScript package, using the dart2js compilation +tool. However, properly supporting JS isn't seamless. There are two major places +where we need to think about JS support: + +1. When interfacing with the filesystem. None of Dart's IO APIs are natively + supported on JS, so for anything that needs to work on both the Dart VM _and_ + Node.js we define a shim in the [`io`] directory that will be implemented in + terms of `dart:io` if we're running on the Dart VM or the `fs` or `process` + modules if we're running on Node. (We don't support IO at all on the browser + except to print messages to the console.) + + [`io`]: io/README.md + +2. When exposing an API. Dart's JS interop is geared towards _consuming_ JS + libraries from Dart, not producing a JS library written in Dart, so we have + to jump through some hoops to make it work. This is all handled in the [`js`] + directory. + + [`js`]: js/README.md + +### APIs + +One of Sass's core features is its APIs, which not only compile stylesheets but +also allow users to provide plugins that can be invoked from within Sass. In +both the JS API, the Dart API, and the embedded compiler, Sass provides three +types of plugins: importers, custom functions, and loggers. + +#### Importers + +Importers control how Sass loads stylesheets through `@use`, `@forward`, and +`@import`. Internally, _all_ stylesheet loads are modeled as importers. When a +user passes a load path to an API or compiles a stylesheet through the CLI, we +just use the built-in [`FilesystemImporter`] which implements the same interface +that we make available to users. + +[`FilesystemImporter`]: importer/filesystem.dart + +In the Dart API, the importer root class is [`importer/async_importer.dart`]. +The JS API and the embedded compiler wrap the Dart importer API in +[`importer/node_to_dart`] and [`embedded/importer`] respectively. + +[`importer/async_importer.dart`]: importer/async_importer.dart +[`importer/node_to_dart`]: importer/node_to_dart +[`embedded/importer`]: embedded/importer + +#### Custom Functions + +Custom functions are defined by users of the Sass API but invoked by Sass +stylesheets. To a Sass stylesheet, they look like any other built-in function: +users pass SassScript values to them and get SassScript values back. In fact, +all the core Sass functions are implemented using the Dart custom function API. + +Because custom functions take and return SassScript values, that means we need +to make _all_ values available to the various APIs. For Dart, this is +straightforward: we need to have objects to represent those values anyway, so we +just expose those objects publicly (with a few `@internal` annotations here and +there to hide APIs we don't want users relying on). These value types live in +the [`value`] directory. + +[`value`]: value/README.md + +Exposing values is a bit more complex for other platforms. For the JS API, we do +a bit of metaprogramming in [`node/value`] so that we can return the +same Dart values we use internally while still having them expose a JS API that +feels native to that language. For the embedded host, we convert them to and +from a protocol buffer representation in [`embedded/value.dart`]. + +[`node/value`]: node/value/README.md +[`embedded/value.dart`]: embedded/value.dart + +#### Loggers + +Loggers are the simplest of the plugins. They're just callbacks that are invoked +any time Dart Sass would emit a warning (from the language or from `@warn`) or a +debug message from `@debug`. They're defined in: + +* [`logger.dart`](logger.dart) for Dart +* [`node/logger.dart`](node/logger.dart) for Node +* [`embedded/logger.dart`](embedded/logger.dart) for the embedded compiler + +### Built-In Functions + +All of Sass's built-in functions are defined in the [`functions`] directory, +including both global functions and functions defined in core modules like +`sass:math`. As mentioned before, these are defined using the standard custom +function API, although in a few cases they use additional private features like +the ability to define multiple overloads of the same function name. + +[`functions`]: functions/README.md + +### `@extend` + +The logic for Sass's `@extend` rule is particularly complex, since it requires +Sass to not only parse selectors but to understand how to combine them and when +they can be safely optimized away. Most of the logic for this is contained +within the [`extend`] directory. + +[`extend`]: extend/README.md