sass/README.md

The Next-Generation Sass Module System

This repository houses a proposal for the @use directive and associated module system, which is intended to be the headlining feature for Sass 4. This is a living proposal: it's intended to evolve over time, and is hosted on GitHub to encourage community collaboration and contributions. Any suggestions or issues can be brought up and discussed on the issue tracker.

Although this document describes some imperative processes when describing the semantics of the module system, these aren't meant to prescribe a specific implementation. Individual implementations are free to implement this feature however they want as long as the end result is the same. However, there are specific design decisions that were made with implementation efficiency in mind—these will be called out explicitly in block-quoted "implementation note"s.

Note: at the time of writing, the initial draft of the proposal is not yet complete.

Table of Contents

• Background
• Goals
  • High-Level
  • Low-Level
  • Non-Goals
• Definitions
  • Member
  • CSS Tree
  • Configuration
  • Module
  • Module Graph
  • Source File
  • Entrypoint
• Syntax
• Semantics
  • Compilation Process
  • Loading Modules
  • Resolving Members
  • Resolving Extends

Background

The new @use directive is intended to supercede Sass's @import directive as the standard way of sharing styles across Sass files. @import is the simplest form of re-use: it does little more than directly include the target file in the source file. This has caused numerous problems in practice: including the same file more than once slows down compilation and produces redundant output, users must manually namespace everything in their libraries, and there's no encapsulation to allow them to keep moving pieces hidden.

The new module system is intended to address these shortcomings (among others) and bring Sass's modularity into line with the best practices as demonstrated by other modern languages. As such, the semantics of @use are is heavily based on other languages' module systems, with Python and Dart being particularly strong influences.

Goals

High-Level

These are the philosophical design goals for the module system at large. While they don't uniquely specify a system, they do represent the underlying motivations behind many of the lower-level design decisions.

• Locality. The module system should make it possible to understand a Sass file by looking only at that file. An important aspect of this is that names in the file should be resolved based on the contents of the file rather than the global state of the compilation. This also applies to authoring: an author should be able to be confident that a name is safe to use as long as it doesn't conflict with any name visible in the file.

• Encapsulation. The module system should allow authors, particularly library authors, to choose what API they expose. They should be able to define entities for internal use without making those entities available for external users to access or modify. This also includes the ability to "forward" public APIs from another file.

• Configuration. Sass is unusual among languages in that it encourages the use of files whose entire purpose is to produce side effects—specifically, to emit CSS. There's also a broader class of libraries that may not emit CSS directly, but do define configuration variables that are used in computations, sometimes at the top level. The module system should allow the user to flexibly use modules with side-effects, and shouldn't force global configuration.

Low-Level

These are goals that are based less on philosophy than on practicality. For the most part, they're derived from user feedback that we've collected about @import over the years.

• Using CSS files. People often have CSS files that they want to bring into their Sass compilation. Historically, @import has been unable to do this due to its overlap with the plain-CSS @import directive and the requirement that SCSS remain a CSS superset. With a new directive name, this becomes possible.

• Import once. Because @import is a literal textual inclusion, multiple @imports of the same Sass file within the scope of a compilation will compile and run that file multiple times. At best this hurts compilation time, and it can also contribute to bloated CSS output when the styles themselves are duplicated. The new module system should only compile a file once, at least for the default configuration.

Non-Goals

These are potential goals that we have explicitly decided to avoid pursuing for various reasons. Some of them may be on the table for future work, but none are expected to land in Sass 4.

• Dynamic imports. Allowing the path to a module to be defined dynamically, whether by including variables or including it in a conditional block, moves away from being declarative. In addition to making stylesheets harder to read, this makes any sort of static analysis more difficult—and actually impossible in the general case. It also limits the possibility of future implementation optimizations.

• Importing multiple files at once. In addition to the long-standing reason that this hasn't been supported—that it opens authors up to sneaky and difficult-to-debug ordering bugs—this violates the principle of locality by obfuscating which files are imported and thus where names come from.

• Extend-only imports. The idea of importing a file so that the CSS it generates isn't emitted unless it's @extended is cool, but it's also a lot of extra work. This is the most likely feature to end up in a future release, but it's not central enough to the module system to include in Sass 4.

Definitions

Member

A member is anything that's defined, either by the user or the implementation, that is identified by a Sass identifier. This currently includes variables, mixins, functions, and placeholder selectors. Each member type has its own namespace, so for example the variable $name doesn't conflict with the placeholder selector %name.

All members have definitions associated with them, whose specific structure depends on the type of the given member. Variables, mixins, and functions have intuitive definitions, but placeholder selectors' definitions just indicate which module they come from.

There's some question of whether placeholders ought to be considered members, and consequently namespaced like other members. On one hand, they're frequently used in parallel with mixins as the API exposed by a library, which suggests that they should be namespaced like the mixins they parallel. On the other hand, this usage is somewhat discouraged since it doesn't treat them like selectors, and not namespacing them would potentially free up characters like . or : to be used as namespace separators.

CSS Tree

A CSS tree is an abstract CSS syntax tree. It has multiple top-level CSS declarations like @-rules or rulesets. The ordering of the roots is significant.

A CSS tree cannot contain any Sass-specific constructs, with the notable exception of placeholder selectors. These are allowed so that modules' CSS may be @extended.

An empty CSS tree contains no top-level declarations.

Configuration

A configuration is a set of variables with associated SassScript values. It's used when executing a source file to customize its execution. It may be empty—that is, it may contain no variables.

Two configurations are considered identical if they contain the same variables, and if each pair of variables with the same name has values that are == to one another.

Module

A module is an abstract collection of members as well as a CSS tree, although that tree may be empty. Each module may have only one member of a given type and name (for example, a module may not have two variables named $name). To satisfy this requirement, placeholder selectors are de-duplicated.

Each module is uniquely identified by the combination of a URI and a configuration. A given module can be produced by executing the source file identified by the module's URI with the module's configuration.

Module Graph

Modules also track their @use directives, which point to other modules. In this sense, modules can be construed as a directed acyclic graph where the vertices are modules and the edges are @use directives. We call this the module graph.

The module graph is not allowed to contain cycles because they make it impossible to guarantee that all dependencies of a module are fully executed before that module is loaded. Although a module's members can be determined without executing it, Sass allows code to be executed while loading a module, which means those members may be executed.

Source File

A source file is an entity uniquely identified by a URI. It can be executed with a configuration to produce a module. The names of this module's members are static, and can be determined without executing the file. This means that all modules for a given source file have the same member names regardless of the configurations used for those modules.

There are five types of source file:

• Sass files, SCSS files, and CSS files are identified by file paths.

• Core libraries are identified by special URIs in an as-yet-undecided format.

• Implementations may define implementation-specific or pluggable means of defining source files, which can use any URI.

Each one has different execution semantics that are beyond the scope of this document. Note that some of these are not or may not actually be files on the file system.

Entrypoint

The entrypoint of a compilation is the source file that was initially passed to the implementation. Similarly, the entrypoint module is the module loaded from that source file with an empty configuration. The entrypoint module is the root of the module graph.

Syntax

The new directive will be called @use. The grammar for this directive is as follows:

UseDirective ::= '@use' QuotedString (AsClause | NoPrefix)? ShowClause?
                     HideClause?
AsClause     ::= 'as' Identifier
NoPrefix     ::= 'no-prefix'
ShowClause   ::= 'show' Identifier (',' Identifier)*
HideClause   ::= 'hide' Identifier (',' Identifier)*

Note: this only encompasses the syntax whose semantics are currently described in this document. As the document becomes more complete, the grammar will be expanded accordingly.

The ordering of clauses is enforced in part to ensure consistency across stylesheets, and in part to match the style of plain CSS, where supports() must precede media queries in an @import rule.

@use directives must be at the top level of the document, and must come before any directives other than @charset. Because each @use directive affects the namespace of the entire source file that contains it, whereas most other Sass constructs are purely imperative, keeping it at the top of the file helps reduce confusion.

Semantics

Compilation Process

First, let's look at the large-scale process that occurs when compiling a Sass entrypoint to CSS.

• Load the entrypoint module as though it were referenced by a @use directive containing its URI and no further clauses. Note that this transitively loads any referenced modules, producing a module graph.

• Resolve extends.

• Concatenate the CSS of each module in the module graph, in reverse topological order. This is the CSS output.

Loading Modules

When encountering a @use directive, the first step is to load the associated module. To do so:

• Look up the source file with the given URI. The process for doing this is out of scope of this document.

• If no such file can be found, throw a fatal error.

• If the source file has already been executed with an empty configuration, use the module that execution produced. This fulfills the "import once" low-level goal.

• Otherwise, execute that file with an empty configuration, and use the resulting module.

Implementation note:

Although this specification only requires that modules be cached and reused when compiling a single entrypoint, modules are intentionally context-independent enough to store and re-use across multiple entrypoints, as long as no source files change. For example, if the user requests that all Sass files beneath stylesheets/sass be compiled, modules may be shared between those separate compilations.

Each module loaded this may have an associated prefix, which is a Sass identifier that's used to identify the module's members within the current file. No two @use directives in a given file may share a prefix, although any number may have no prefix. The prefix for a given @use directive's module is determined as follows:

• If the directive has an as clause, use that clause's identifier.

• If the directive has a no-prefix clause, then it has no prefix.

• If the module's URI doesn't match the regular expression (.*/)?([^/]+)(\.[^/]*)?, the @use directive is malformed.

• Call the text captured by the second group of the regular expression the module name.

• If the module name isn't a Sass identifier, the @use directive is malformed.

• If the module name followed by a hyphen is a initial substring of previous @use directive's prefix, or if another @use directive's prefix followed by a hyphen is an initial substring of the module name, the @use directive is malformed.

• Use the module name.

This proposal follows Python and diverges from Dart in that @use imports modules with a prefix by default. This is for two reasons. First, it seems to be the case that language ecosystems with similar module systems either prefix all imports by convention, or prefix almost none. Because Sass is not object-oriented and doesn't have the built-in namespacing that classes provide many other languages, its APIs tend to be much broader at the top level and thus at higher risk for name conflict. Prefixing by default tilts the balance towards always prefixing, which mitigates this risk.

Second, a default prefix scheme drastically reduces the potential for inconsistency in prefix choice. If the prefix is left entirely up to the user, different people may choose to prefix strings.scss as strings, string, str, or strs. This taxes the reusability of code and knowledge, and mitigating it is a benefit.

Resolving Members

The main function of the module system is to control how member names are resolved across files—that is, to find the definition corresponding to a given name. Given a set of modules loaded via @use and a member type and name to resolve:

• If the name begins with a module's prefix followed by a hyphen:

  • Strip the prefix and hyphen to get the unprefixed name.

  • If the module doesn't have a member of the given type with the unprefixed name, resolution fails.

  • If the module's @use directive has a show clause that doesn't include the unprefixed name, resolution fails.

  • If the module's @use directive has a hide clause that does include the unprefixed name, resolution fails.

  • Otherwise, use that member's definition.

• If a member of the given type with the given name has already been defined in the current source file, use its definition.

• If such a member is defined later on in the file, resolution fails. This ensures that any change in name resolution caused by reordering a file causes an immediate error rather than an unexpected behavioral change.

• Take the set of unprefixed modules that have such a member. Call this the candidate module set.

• Remove all modules whose @use directives have a show clause that doesn't include the member name from the candidate module set.

• Remove all modules whose @use directives have a hide clause that does include the member name from the candidate module set.

• If there's exactly one module left in the candidate module set, use that module's definition.

• Otherwise, if there are multiple modules left, resolution fails. This ensures that if a new version of a package produces a conflicting name, it causes an immediate error.

• Otherwise, if the candidate module set is empty, resolution fails.

The hyphenated syntax (namespace-name) was chosen in preference to other syntaxes (for example namespace.name, namespace::name, or namespace|name) because it's likely to be compatible with existing code that uses manual namespaces, and because it doesn't overlap with plain CSS syntax. This is especially relevant for namespaced placeholder selectors, because most other reasonable characters are already meaningful in selector contexts.

The downside to hyphens are that they look like normal identifiers, which makes it less locally clear what's a namespace and what's a normal member name. It also allows module prefixes to shadow other members, and introduces the possibility of conflicting prefixes between modules.

Resolving Extends

The module system also scopes the resolution of the @extend directive. This helps satisfy locality, making selector extension more predictable than it is using @imports.

Extension is scoped to CSS in modules transitively used by the module in which the @extend appears. This transitivity is necessary because CSS is not considered a member of a module, and can't be controlled as explicitly as members can. Extending all transitively-used modules means that the @extend affects exactly that CSS that is guaranteed to exist by the @use directives.

Specifically, once all modules have been loaded, do a global pass to resolve the extends.

• For each module that contains CSS (call it the extended module):

  • Set this module's virtual selectors to the set of selectors in all of its CSS rules. These virtual selectors remember the original selector they were generated from.

  • Take the subgraph of the module graph that can transitively reach the extended module.

  • For every module in this subgraph (call it the extending module) in reverse topological order:

    • For each of the extended module's selectors:

      • Take the corresponding virtual selector from each module used by the extending module, if it has such a selector set.

      • Create a new selector that matches the union of all elements matched by these virtual selectors, and set it as a virtual selector of this subgraph.

  • Replace the selectors in the extended module's rules with the corresponding virtual selectors of the entrypoint module.

Implementation note:

This process as written is O(n²) for the number of modules in the compilation, and in a pathological case this is accurate. However, it's intended to be tightly constrained by the number and scope of the extensions the user chooses to write. Even a relatively naïve implementation should be able to keep it to O(n×m) for extending modules and extended modules. It's possible there's even an O(n) implementation that integrates extension with compilation, but that's beyond the scope of this document.

There is intentionally no way for a module to affect the extensions of another module that doesn't transitively use it. This promotes locality, and matches the behavior of mixins and functions in that monkey-patching is disallowed.