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The Embedded Sass Protocol

This repository defines a bidirectional protocol for communicating between a Sass implementation and a host environment. It allows the host environment to invoke the Sass compiler on source files, and to define custom functions and importers in the host language.

Disclaimer: this is not an official Google product.


This protocol operates between two endpoints over a bidirectional stream. One of these endpoints, the compiler, is responsible for compiling Sass stylesheets to CSS. The other, the host, is responsible for telling the compiler what to compile and for providing implementations of custom importers and functions.

Messages are sent between the host and the compiler in the form of protocol buffers, using a custom RPC system defined below. The messages and services that comprise this protocol are defined in the .proto file included in this repository. Most messages are requests which require the other endpoint to produce a response, but some are events which require no response.

In principle this protocol can work over any bidirectional stream capable of carrying protocol buffers. However, it's expected that most hosts will invoke the compiler as a subprocess and communicate using binary protocol buffers over its standard input and output streams.

For streams (like standard input and output) that don't have built-in message boundaries, every message must begin with an unsigned varint indicating the length in bytes of the remaining message. This matches the best practice described in the protocol buffer documentation. Because JavaScript can't easily represent integers larger than 2^53 - 1, messages may be no more than 2^53 - 1 bytes long. Because it's so unlikely that this will come up in practice, implementations are not required to verify it.

Note that a number of protocol buffer libraries have built-in utilities for reading and writing varint-delimited streams.


All RPCs are wrapped in an outer message that indicates the RPC's type using a oneof field. There are two wrapper messages:

  • InboundMessage is sent from the host to the compiler.
  • OutboundMessage is sent from the compiler to the host.

The host must only send InboundMessages to the compiler, and the compiler must only send OutboundMessages to the host.

Each wrapper message contains exactly one RPC. This protocol defines four types of RPC:

  • Requests always include a mandatory uint32 id field so that the other endpoint can respond. All request message types end in Request.
  • Responses include a mandatory uint32 id field whose value must be the same as their associated request's id. All response message types begin with the corresponding request name and end with Response.
  • Events may not be responded to and include no id field. All event message types end with Event.
  • The ProtocolError message, which is sent when one endpoint detects that the other is doing something invalid. See Error Handling below.

The protocol also defines some messages whose names don't end with Request, Response, or Event. These are used as structures shared between different RPCs.

Implementations must guarantee that they use a unique id for every request, although the same id may be used for an inbound request and an outbound request. They may not use the id 4294967295 (the maximum number representable by a uint32) because it's reserved for error handling.

All message-typed fields are documented as either "optional" or "mandatory". If a field is mandatory, the endpoint that sends that message must guarantee that it's set to a meaningful value, and the endpoint that receives it must reject the message if it's not set.

Some scalar-typed fields may also be documented as "mandatory", which means that the endpoint that sends the message must guarantee that its value is meaningful. However, since the default value may be meaningful in some cases, the endpoint that receives the message is not required to reject it based on mandatory scalar-typed fields.

Error Handling

When the compiler detects that the host is violating this protocol, it must send a ProtocolError message to the host. If the error was detected when processing a request, the ProtocolError must have its id field set to the request's id. Otherwise, even if the error was detected while processing a response with an id, the id field must be set to 4294967295 (the maximum number representable by a uint32).

When the host detects that the compiler is violating this protocol, it does not need to send a ProtocolError message to the compiler. Instead, it should expose an error to the host's consumers and close the connection with the compiler.

An error occurs whenever any requirements set out by this protocol (including the documentation in embedded_sass.proto) are violated. This includes, but is not limited to:

  • Sending data that can't be parsed as an InboundMessage (for the compiler) or an OutboundMessage (for the host).

  • Sending a request with an ID that's in use by another in-flight request.

  • Sending a response with an ID that doesn't correspond to an in-flight request's ID.

  • Sending a response with an ID that corresponds to the ID of an in-flight request ID of the incorrect type.

  • Sending a message with a null value for a mandatory field.

The ProtocolError message must not be used to report Sass language errors.

Host Language API

Although not strictly part of the protocol, the host language will presumably provide an API for reading and manipulating SassScript values so that custom functions can be written in the host language. In order to ensure that custom functions will behave consistently with built-in Sass functions, the host language should provide APIs that meet the following guidelines.

The Dart Value API is a good example of an object-oriented API that follows these guidelines.


All SassScript values are immutable, and the API should preserve that fact. No API calls should be able to modify any SassScript values, including collections like lists and maps. Instead, API calls should be provided to return new values with adjusted contents or to copy values into mutable host-language objects.

If API calls are provided that return a new versions of an object with adjusted contents, metadata for the returned object (such as the type of list separator or a number's units) should match that of the original object.


SassScript values use index 1 to refer to the first element and -1 to refer to the final element. The index 0 is invalid. Furthermore, indexes in Sass strings refer to Unicode code points, not bytes or UTF-16 code units. The API should provide a means to convert between Sass's indexing scheme and the host language's indexing scheme, and should encourage authors to treat any indexes they're passed as Sass-style indexes rather than host-language-style indexes.


The API should provide an easy means to assert that values are the expected type and to produce a useful error if they aren't. They should not provide a means to assert that a value is a list, though, since all Sass values should be treated as lists (see below).


API users should be encouraged to return quoted strings unless there's a particular reason not to.

Two strings are equal if they have the same text, regardless of whether either is quoted or not.


The API should provide additional assertions for numbers:

  • that the number doesn't have any units;
  • that the number's units are compatible with given expected units;
  • that the number is an integer, which for the purposes of Sass numbers means that its numeric value is within 1e-11 of an integer;
  • that the number is in a given range, where being within 1e-11 of the top or bottom of that range is considered being equal to the top or bottom.

The API should also provide means of converting a number to the equivalent number with different-but-compatible units, and for returning it as the host language's integer type if it is an integer.

Two numbers are equal if they have compatible units, and if their numerical value (with normalized units) are within 1e-11 of one another. A hash code with the same equality semantics can be generated for a number x by rounding x * 1e11 to the nearest integer and taking the hash code of the result.


The protocol includes three distinct color value types, RgbColor, HslColor, and HwbColor. In Sass code and custom functions, colors may be represented or manipulated in either RGB, HSL, or HWB form, so having multiple types allows whichever form is currently in use to be sent between endpoints without having to eagerly normalize it.

However, users of the host language API should be able to transparently treat any color object as though it were either RGB, HSL, or HWB form. The API should provide access to the red, green, and blue, hue, saturation, lightness, whiteness, and blackness channels of every color object. It should use this RGB-to-HSL algorithm, this HSL-to-RGB algorithm, this RGB-to-HWB algorithm, and this HWB-to-RGB algorithm to convert between representations as necessary.

The API should also provide means of changing one or more channels of a color while leaving other channels as-is.

Two colors are equal if their RGB forms have the same red, green, blue channels and alpha channels within 1e-11 of one another.


In Sass, every value counts as a list. Maps count as unbracketed comma-separated lists of two-element unbracketed space-separated key-value lists, and all other non-list values count as lists that contain that value. The API should make it easy to treat every value as a list, and should discourage treating values passed as Value.Lists specially.

API users should be encouraged to return unbracketed comma-separated lists unless there's a particular reason not to.

Two lists are equal if they have the same elements, separator, and if they're both bracketed or both unbracketed. An empty list is equal to an empty map.

Value.ArgumentLists should be exposed the same way as any other list, except that it should also provide access to the keyword map. For object-oriented host languages, an argument list's class should be a subtype of normal list's. It should be considered equal to a list with the same elements, regardless of its keywords.


Although maps are transferred as lists of pairs, they should be exposed to the host language as maps that can be indexed by key, using the notions of equality described for each type.

Two maps are equal if they have equal keys that map to equal values, regardless of the order of the keys in the map. An empty map is equal to an empty list.


The True and False messages are each singletons representing the Sass values true and false, respectively. In Sass, all values other than false and null can be used to represent truth, so the API should provide an easy way to tell if a value is "truthy" (one of those values) or "falsey" (false or null). It should encourage users to check this rather than directly testing for true or false.

Two booleans are equal if they're both true or both false.


The Null message is a singleton representing the Sass null value. It should not be represented as the host language's native null value, so that it can expose Sass-specific APIs like the assertions described above.

null is only equal to null.


Calculations are represented similarly to their representation in the Sass specification, as a tree of binary operations and other calculations that terminates in numbers or strings. This tree structure may be exposed to the host language, or the host may choose to keep the structure of calculations opaque.

Two calculations are equal if their names are equal and each of their arguments are equal. Two Calculation.CalculationOperations are equal if they have the same operator and their left and right values are equal, respectively.

Note that this protocol chooses not to require host implementations to simplify calculations as they're constructed, for the sake of simplicity of implementation (although hosts may do so). This means that a host can construct calculations like calc(1 + 1) which, in Sass, would simplify to 2. The host is not required to take simplification into account when determining equality.


The protocol allows first-class functions defined in the compiler to be passed to the host (as Value.CompilerFunctions) and vice-versa (as Value.HostFunctions). It allows the compiler to invoke functions defined in the host. The host API should hide the distinction between the two function types as much as possible, but it may refuse to allow host-defined functions to be invoked on the host, since doing so correctly would require parsing those functions' signatures.

Two first-class functions are equal if they have the same ID and they're either both CompilerFunctions or both HostFunctions.


This protocol is versioned according to semver 2.0.0. The current version is indicated by the VERSION file at the root of the repository. If this file has a -dev prerelease string, that indicates that the currently checked in version is in development, is not considered a release version, and must not be used by released versions of compilers or hosts. All release versions will also have GitHub tags for their version numbers.

A "breaking change" is defined as per the protocol buffer rules for updating a message type. Compatibility is considered from the perspective of the host. For example, if a new InboundMessage type is added, that's considered a "backwards compatible" change because older hosts can simply opt not to use it, even though from the perspective of the compiler a new message type would be a breaking change.

Hosts are generally expected to be responsible for installing appropriate compiler versions as part of their installation process, which should limit the potential for incompatible versions between the two. For this reason, version numbers are intended to be primarily an advisory for humans as to the degree of change over time.

In some cases, the version number will be marked as "pending". This indicates that the next version of the protocol is still under active development, and may be waiting for additional pull requests before it's finalized. Hosts and compilers should never cut releases that target pending protocol versions.


A protocol for communicating between a Sass implementation and a host language



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