Add specification for language binding API (#257)

This adds documentation for how the language bindings works, for those that wish to create their own bindings.

docs/modules/ROOT/pages/language-bindings.adoc

@@ -4,3 +4,6 @@
 * xref:kotlin-binding:index.adoc[Kotlin]
 * xref:swift:ROOT:index.adoc[Swift]
 * xref:go:ROOT:index.adoc[Go]
+* xref:bindings-specification:index.adoc[Specification]
+** xref:bindings-specification:message-passing-api.adoc[Message Passing API]
+** xref:bindings-specification:binary-encoding.adoc[Pkl Binary Encoding]

docs/modules/ROOT/partials/component-attributes.adoc

@@ -66,3 +66,76 @@ endif::[]
 :uri-config-java-example: {uri-pkl-examples-tree}/config-java
 :uri-config-kotlin-example: {uri-pkl-examples-tree}/config-kotlin
 :uri-pkldoc-example: {uri-pkl-examples-tree}/pkldoc
+
+:uri-stdlib-baseModule: {uri-pkl-stdlib-docs}/base
+:uri-stdlib-jsonnetModule: {uri-pkl-stdlib-docs}/jsonnet
+:uri-stdlib-reflectModule: {uri-pkl-stdlib-docs}/reflect
+:uri-stdlib-xmlModule: {uri-pkl-stdlib-docs}/xml
+:uri-stdlib-protobufModule: {uri-pkl-stdlib-docs}/protobuf
+:uri-stdlib-Boolean: {uri-stdlib-baseModule}/Boolean
+:uri-stdlib-xor: {uri-stdlib-baseModule}/Boolean#xor()
+:uri-stdlib-implies: {uri-stdlib-baseModule}/Boolean#implies()
+:uri-stdlib-Any: {uri-stdlib-baseModule}/Any
+:uri-stdlib-String: {uri-stdlib-baseModule}/String
+:uri-stdlib-Int: {uri-stdlib-baseModule}/Int
+:uri-stdlib-Float: {uri-stdlib-baseModule}/Float
+:uri-stdlib-Number: {uri-stdlib-baseModule}/Number
+:uri-stdlib-NaN: {uri-stdlib-baseModule}/#NaN
+:uri-stdlib-Infinity: {uri-stdlib-baseModule}/#Infinity
+:uri-stdlib-isBetween: {uri-stdlib-baseModule}/Number#isBetween
+:uri-stdlib-isFinite: {uri-stdlib-baseModule}/Number#isFinite
+:uri-stdlib-Int8: {uri-stdlib-baseModule}/#Int8
+:uri-stdlib-Int16: {uri-stdlib-baseModule}/#Int16
+:uri-stdlib-Int32: {uri-stdlib-baseModule}/#Int32
+:uri-stdlib-UInt8: {uri-stdlib-baseModule}/#UInt8
+:uri-stdlib-UInt16: {uri-stdlib-baseModule}/#UInt16
+:uri-stdlib-UInt32: {uri-stdlib-baseModule}/#UInt32
+:uri-stdlib-UInt: {uri-stdlib-baseModule}/#UInt
+:uri-stdlib-Uri: {uri-stdlib-baseModule}/#Uri
+:uri-stdlib-matches: {uri-stdlib-baseModule}/String#matches()
+:uri-stdlib-Null: {uri-stdlib-baseModule}/Null
+:uri-stdlib-ifNonNull: {uri-stdlib-baseModule}/Null#ifNonNull()
+:uri-stdlib-List: {uri-stdlib-baseModule}/List
+:uri-stdlib-Set: {uri-stdlib-baseModule}/Set
+:uri-stdlib-Map: {uri-stdlib-baseModule}/Map
+:uri-stdlib-Listing: {uri-stdlib-baseModule}/Listing
+:uri-stdlib-Listing-default: {uri-stdlib-baseModule}/Listing#default
+:uri-stdlib-Listing-isDistinct: {uri-stdlib-baseModule}/Listing#isDistinct
+:uri-stdlib-Listing-isDistinctBy: {uri-stdlib-baseModule}/Listing#isDistinctBy()
+:uri-stdlib-Mapping: {uri-stdlib-baseModule}/Mapping
+:uri-stdlib-Mapping-default: {uri-stdlib-baseModule}/Mapping#default
+:uri-stdlib-Duration: {uri-stdlib-baseModule}/Duration
+:uri-stdlib-Duration-value: {uri-stdlib-baseModule}/Duration#value
+:uri-stdlib-Duration-unit: {uri-stdlib-baseModule}/Duration#unit
+:uri-stdlib-DurationUnit: {uri-stdlib-baseModule}/#DurationUnit
+:uri-stdlib-DataSize: {uri-stdlib-baseModule}/DataSize
+:uri-stdlib-DataSize-value: {uri-stdlib-baseModule}/DataSize#value
+:uri-stdlib-DataSize-unit: {uri-stdlib-baseModule}/DataSize#unit
+:uri-stdlib-DataSizeUnit: {uri-stdlib-baseModule}/#DataSizeUnit
+:uri-stdlib-Dynamic: {uri-stdlib-baseModule}/Dynamic
+:uri-stdlib-Dynamic-toTyped: {uri-stdlib-baseModule}/Dynamic#toTyped()
+:uri-stdlib-Typed: {uri-stdlib-baseModule}/Typed
+:uri-stdlib-Regex: {uri-stdlib-baseModule}/Regex
+:uri-stdlib-Regex-method: {uri-stdlib-baseModule}/#Regex()
+:uri-stdlib-Regex-match: {uri-stdlib-baseModule}/Regex#match
+:uri-stdlib-RegexMatch: {uri-stdlib-baseModule}/RegexMatch
+:uri-stdlib-Pair: {uri-stdlib-baseModule}/Pair
+:uri-stdlib-IntSeq: {uri-stdlib-baseModule}/IntSeq
+:uri-stdlib-Class: {uri-stdlib-baseModule}/Class
+:uri-stdlib-TypeAlias: {uri-stdlib-baseModule}/TypeAlias
+:uri-stdlib-Deprecated: {uri-stdlib-baseModule}/Deprecated
+:uri-stdlib-ValueRenderer: {uri-stdlib-baseModule}/ValueRenderer
+:uri-stdlib-PcfRenderer-converters: {uri-stdlib-baseModule}/PcfRenderer#converters
+:uri-stdlib-Function: {uri-stdlib-baseModule}/Function
+:uri-stdlib-Function0: {uri-stdlib-baseModule}/Function0
+:uri-stdlib-Function1: {uri-stdlib-baseModule}/Function1
+:uri-stdlib-Function1-apply: {uri-stdlib-baseModule}/Function1#apply()
+:uri-stdlib-Function2: {uri-stdlib-baseModule}/Function2
+:uri-stdlib-Function3: {uri-stdlib-baseModule}/Function3
+:uri-stdlib-Function4: {uri-stdlib-baseModule}/Function4
+:uri-stdlib-Function5: {uri-stdlib-baseModule}/Function5
+:uri-stdlib-Resource: {uri-stdlib-baseModule}/Resource
+:uri-stdlib-outputFiles: {uri-stdlib-baseModule}/ModuleOutput#files
+
+:uri-messagepack: https://msgpack.org/index.html
+:uri-messagepack-spec: https://github.com/msgpack/msgpack/blob/master/spec.md

docs/modules/bindings-specification/pages/binary-encoding.adoc

@@ -0,0 +1,197 @@
+= Pkl Binary Encoding
+include::ROOT:partial$component-attributes.adoc[]
+include::partial$component-attributes.adoc[]
+
+Pkl values can be encoded into a binary format.
+This format is used for Pkl's non-JVM language bindings, for example, for its Go and Swift bindings.
+
+The binary format is uses link:{uri-messagepack}[MessagePack] encoding.
+
+== Primitives
+
+All Pkl primitives turn into their respective MessagePack primitive.
+
+|===
+|Pkl Type|MessagePack format
+
+|link:{uri-stdlib-Int}[Int]
+|link:{uri-messagepack-int}[int]
+
+|link:{uri-stdlib-Float}[Float]
+|link:{uri-messagepack-float}[float]
+
+|link:{uri-stdlib-String}[String]
+|link:{uri-messagepack-str}[string]
+
+|link:{uri-stdlib-Boolean}[Boolean]
+|link:{uri-messagepack-bool}[bool]
+
+|link:{uri-stdlib-Null}[Null]
+|link:{uri-messagepack-nil}[nil]
+|===
+
+NOTE: Pkl integers are encoded into the smallest int type that the number will fit into.
+For example, value `8` gets encoded as MessagePack `int8` format.
+
+== Non-primitives
+
+All non-primitive values are encoded as MessagePack arrays.
+The first slot of the array designates the value's type. The remaining slots have fixed meanings depending on the type.
+
+The array's length is the number of slots that are filled. For example, xref:{uri-stdlib-List}[List] is encoded as an MessagePack array with two elements.
+
+|===
+|Pkl type |Slot 1 2+|Slot 2 2+|Slot 3 2+|Slot 4
+
+||code |type |description |type |description |type |description
+
+|link:{uri-stdlib-Typed}[Typed], link:{uri-stdlib-Dynamic}[Dynamic]
+|`0x1`
+|link:{uri-messagepack-str}[str]
+|Fully qualified class name
+|link:{uri-messagepack-str}[str]
+|Enclosing module URI
+|link:{uri-messagepack-array}[array]
+|Array of <<object-members,object members>>
+
+|link:{uri-stdlib-Map}[Map]
+|`0x2`
+|link:{uri-messagepack-map}[map]
+|Map of `<value>` to `<value>`
+|
+|
+|
+|
+
+|link:{uri-stdlib-Mapping}[Mapping]
+|`0x3`
+|link:{uri-messagepack-map}[map]
+|Map of `<value>` to `<value>`
+|
+|
+|
+|
+
+|link:{uri-stdlib-List}[List]
+|`0x4`
+|link:{uri-messagepack-array}[array]
+|Array of `<value>`
+|
+|
+|
+|
+
+|link:{uri-stdlib-Listing}[Listing]
+|`0x5`
+|link:{uri-messagepack-array}[array]
+|Array of `<value>`
+|
+|
+|
+|
+
+|link:{uri-stdlib-Set}[Set]
+|`0x6`
+|link:{uri-messagepack-array}[array]
+|Array of `<value>`
+|
+|
+|
+|
+
+|link:{uri-stdlib-Duration}[Duration]
+|`0x7`
+|{uri-messagepack-float}[float64]
+|Duration value
+|link:{uri-messagepack-str}[str]
+|link:{uri-stdlib-DurationUnit}[Duration unit] (`"ns"`, `"ms"`, etc.)
+|
+|
+
+|link:{uri-stdlib-DataSize}[DataSize]
+|`0x8`
+|link:{uri-messagepack-float}[float64]
+|Value (float64)
+|link:{uri-messagepack-str}[str]
+|link:{uri-stdlib-DataSizeUnit}[DataSize unit] (`"b"`, `"kb"`, etc.)
+|
+|
+
+|link:{uri-stdlib-Pair}[Pair]
+|`0x9`
+|`<value>`
+|First value
+|`<value>`
+|Second value
+|
+|
+
+|link:{uri-stdlib-IntSeq}[IntSeq]
+|`0xA`
+|link:{uri-messagepack-int}[int]
+|Start
+|link:{uri-messagepack-int}[int]
+|End
+|link:{uri-messagepack-int}[int]
+|Step
+
+|link:{uri-stdlib-Regex}[Regex]
+|`0xB`
+|link:{uri-messagepack-str}[str]
+|Regex string representation
+|
+|
+|
+|
+
+|link:{uri-stdlib-Class}[Class]
+|`0xC`
+|
+|
+|
+|
+|
+|
+
+|link:{uri-stdlib-TypeAlias}[TypeAlias]
+|`0xD`
+|
+|
+|
+|
+|
+|
+|===
+
+[[object-members]]
+== Object Members
+
+Like non-primitive values, object members are encoded as MessagePack arrays, where the first slot designates the value's type.
+
+|===
+|Member type |Slot 1 2+|Slot 2 2+|Slot 3
+
+| |code |type |description |type |description
+
+|Property
+|`0x10`
+|link:{uri-messagepack-str}[str]
+|key
+|`<value>`
+|property value
+
+|Entry
+|`0x11`
+|`<value>`
+|entry key
+|`<value>`
+|entry value
+
+|Element
+|`0x12`
+|link:{uri-messagepack-int}[int]
+|index
+|`<value>`
+|element value
+|===
+

docs/modules/bindings-specification/pages/index.adoc

@@ -0,0 +1,173 @@
+= Language Binding Specification
+
+:uri-pkl-go-github: https://github.com/apple/pkl-go
+:uri-pkl-swift-github: https://github.com/apple/pkl-swift
+
+Pkl can be embedded within any host application.
+The host application has access to low level controls.
+It is able to manage the lifecycle of evaluators, as well as provide custom modules and resources to Pkl.
+
+Currently, Pkl must be embedded as a child process, by shelling out to the CLI using the xref:pkl-cli:index.adoc#command-server[`pkl server`] command. In the future, a C library will also be provided.
+
+When embedded, communication between a host application and Pkl happens via message passing.
+The message passing specification can be found in xref:message-passing-api.adoc[].
+
+For examples of language bindings in practice, review the xref:{uri-pkl-go-github}[pkl-go], or the xref:{uri-pkl-swift-github}[pkl-swift] codebases.
+
+NOTE: Pkl's Java and Kotlin libraries binds to Pkl directly, and do not use message passing.
+
+== Features of a language binding
+
+A language binding for Pkl should generally have the following components:
+
+. A client that spawns `pkl server`, and talks to it using message passing.
+. A deserializer that turns xref:binary-encoding.adoc[pkl binary encoding] into a structure in the host language.
+. A code generator that transforms Pkl schemas into schemas written in the host language.
+  The code generator is mostly written in Pkl, with a lightweight executable that acts as the glue layer.
+  For examples of code generators, consult link:{uri-pkl-go-github}/tree/main/codegen[pkl-go] and link:{uri-pkl-swift-github}/tree/main/codegen[pkl-swift].
+
+== Sample flow
+
+Here is a sample flow for evaluating a module with the following contents:
+
+.\file:///path/to/myModule.pkl
+[source,{pkl}]
+----
+module MyModule
+
+theModules = import*("customfs:/*.pkl")
+----
+
+. Client sends xref:message-passing-api.adoc#create-evaluator-request[Create Evaluator Request], including `customfs` as a custom module reader.
++
+[source,json]
+----
+[
+  0x20,
+  {
+    "requestId": 135,
+    "allowedModules": ["pkl:", "repl:", "file:", "customfs:"],
+    "clientModuleReaders": [
+      {
+        "scheme": "customfs",
+        "hasHierarchicalUris": true,
+        "isGlobbable": true,
+        "isLocal": true
+      }
+    ]
+  }
+]
+----
+. Server sends xref:message-passing-api.adoc#create-evaluator-response[Create Evaluator Response], with an evaluator id.
++
+[source,json]
+----
+[
+  0x21,
+  {
+    "requestId": 135,
+    "evaluatorId": -135901
+  }
+]
+----
+. Client sends xref:message-passing-api.adoc#evaluate-request[Evaluate Request], providing the module's URI.
++
+[source,json]
+----
+[
+  0x23,
+  {
+    "requestId": 9805131,
+    "evaluatorId": -13901,
+    "moduleUri": "file:///path/to/myModule.pkl"
+  }
+]
+----
+. During evaluation, server evaluates `import*("customfs:/*.pkl")`, and sends xref:message-passing-api.adoc#list-modules-request[List Modules Request].
++
+[source,json]
+----
+[
+  0x2c,
+  {
+    "requestId": -6478924,
+    "evaluatorId": -13901,
+    "uri": "customfs:/"
+  }
+]
+----
+. Client responds with xref:message-passing-api.adoc#list-modules-response[List Modules Response].
+In our pretend scenario, there is only one file; `foo.pkl`.
++
+[source,json]
+----
+[
+  0x2d,
+  {
+    "requestId": -6478924,
+    "evaluatorId": -13901,
+    "pathElements": [
+      {
+        "name": "foo.pkl",
+        "isDirectory": false
+      }
+    ]
+  }
+]
+----
+. Server sends xref:message-passing-api.adoc#read-module-request[Read Module Request] to read `foo.pkl`.
++
+[source,json]
+----
+[
+  0x28,
+  {
+    "requestId": 36408291,
+    "evaluatorId": -13901,
+    "uri": "customfs:/foo.pkl"
+  }
+]
+----
+. Client responds with the module's contents
++
+[source,json]
+----
+[
+  0x29,
+  {
+    "requestId": 36408291,
+    "evaluatorId": -13901,
+    "contents": "foo = 1"
+  }
+]
+----
+. Server finishes evaluation, and responds with the xref:message-passing-api.adoc#evaluate-response[Evaluate Response].
++
+[source,json]
+----
+[
+  0x24,
+  {
+    "requestId": 9805131,
+    "evaluatorId": -13901,
+    "result": <pkl binary value>
+  }
+]
+----
+. Client sends xref:message-passing-api.adoc#close-evaluator[Close Evaluator].
++
+[source,json]
+----
+[
+  0x22,
+  {
+    "evaluatorId": -13901
+  }
+]
+----
+
+== Debug logs
+
+Set the env var `PKL_DEBUG=1` to enable more verbose logging from Pkl.
+It is recommended that clients also use this environment variable to enable debug logs of their own.
+

docs/modules/bindings-specification/pages/message-passing-api.adoc

@@ -0,0 +1,531 @@
+= Message Passing API
+
+include::ROOT:partial$component-attributes.adoc[]
+include::partial$component-attributes.adoc[]
+
+All messages are encoded in link:{uri-messagepack}[MessagePack], as an array with two elements.
+
+The first element of the array is a code that designates the message's type, encoded as an int.
+The second element of the array is the message body, encoded as a map.
+
+Messages are passed between the _client_ and the _server_.
+The _client_ is the host language (for example, the Swift application when using pkl-swift).
+The _server_ is the entity that provides controls for interacting with Pkl.
+
+For example, in JSON representation:
+
+[source,json]
+----
+[
+  1,                           // <1>
+  { "someKey": "someValue" }   // <2>
+]
+----
+<1> Code indicating message type
+<2> Message body
+
+== Message types
+
+[[client-message]]
+=== Client Message
+A message passed from the client to the server.
+
+[[server-message]]
+=== Server Message
+A message passed from the server to the client.
+
+[[request-message]]
+=== Request Message
+A message sent with a `requestId` value.
+The `requestId` should be a unique number at the time of message send.
+The other side is expected to respond with a <<response-message>> with the same `requestId`.
+
+[[response-message]]
+=== Response Message
+A message that is the response to a <<request-message>>.
+It contains the same `requestId` of the request message.
+
+[[one-way-message]]
+=== One Way Message
+
+A fire-and-forget message where no response is expected.
+
+== Messages
+
+All messages have their schema described in Pkl.
+A nullable type means that the property should be omitted (as opposed to the property's value being `nil`).
+
+[[create-evaluator-request]]
+=== Create Evaluator Request
+
+Code: `0x20` +
+Type: <<client-message,Client>> <<request-message,Request>>
+
+Create an evaluator with the provided evaluator settings.
+Upon creating the evaluator, the server sends back a <<create-evaluator-response>> message.
+
+Schema:
+[source,pkl]
+----
+/// A number identifying this request
+requestId: Int
+
+/// Regex patterns to determine which modules are allowed for import.
+///
+/// API version of the CLI's `--allowed-modules` flag
+allowedModules: Listing<String>?
+
+/// Regex patterns to dettermine which resources are allowed to be read.
+///
+/// API version of the CLI's `--allowed-resources` flag
+allowedResources: Listing<String>?
+
+/// Register client-side module readers.
+clientModuleReaders: Listing<ClientModuleReader>?
+
+/// Register client-side resource readers.
+clientResourceReaders: Listing<ClientResourceReader>?
+
+/// Directories, ZIP archives, or JAR archives
+/// to search when resolving `modulepath:` URIs.
+///
+/// API version of the CLI's `--module-path` flag.
+modulePaths: Listing<String>?
+
+/// Environment variable to set.
+///
+/// API version of the CLI's `--env-var` flag.
+env: Mapping<String, String>?
+
+/// External properties to set.
+///
+/// API version of the CLI's `--properties` flag.
+properties: Mapping<String, String>?
+
+/// Duration, in seconds, after which evaluation of a source module will be timed out.
+///
+/// API version of the CLI's `--timeout` flag.
+timeoutSeconds: Int?
+
+/// Restricts access to file-based modules and resources to those located under the root directory.
+rootDir: String?
+
+/// The cache directory for storing packages.
+cacheDir: String?
+
+/// The format to generate.
+///
+/// This sets the `pkl.outputFormat` external property.
+outputFormat: String?
+
+/// The project dependency settings.
+project: Project?
+
+class ClientResourceReader {
+  /// The URI scheme this reader is responsible for reading.
+  scheme: String
+
+  /// Tells whether the path part of ths URI has a
+  /// [hier-part](https://datatracker.ietf.org/doc/html/rfc3986#section-3).
+  ///
+  /// An example of a hierarchical URI is `file:///path/to/my/file`, where
+  /// `/path/to/my/file` designates a nested path through the `/` character.
+  ///
+  /// An example of a non-hierarchical URI is `pkl.base`, where the `base` does not denote
+  /// any form of hierarchy.
+  hasHierarchicalUris: Boolean
+
+  /// Tells whether this reader supports globbing.
+  isGlobbable: Boolean
+}
+
+class ClientModuleReader {
+  /// The URI scheme this reader is responsible for reading.
+  scheme: String
+
+  /// Tells whether the path part of ths URI has a
+  /// [hier-part](https://datatracker.ietf.org/doc/html/rfc3986#section-3).
+  ///
+  /// An example of a hierarchical URI is `file:///path/to/my/file`, where
+  /// `/path/to/my/file` designates a nested path through the `/` character.
+  ///
+  /// An example of a non-hierarchical URI is `pkl.base`, where the `base` does not denote
+  /// any form of hierarchy.
+  hasHierarchicalUris: Boolean
+
+  /// Tells whether this reader supports globbing.
+  isGlobbable: Boolean
+
+  /// Tells whether the module is local to the system.
+  ///
+  /// A local resource that [hasHierarchicalUris] supports triple-dot imports.
+  isLocal: Boolean
+}
+
+class Project {
+  type: "local"
+
+  /// The canonical URI of this project's package
+  packageUri: String?
+
+  /// The URI pointing to the location of the project file.
+  projectFileUri: String
+
+  /// The dependencies of this project.
+  dependencies: Mapping<String, Project|RemoteDependency>
+}
+
+class RemoteDependency {
+  type: "remote"
+
+  /// The canonical URI of this dependency
+  packageUri: String?
+
+  /// The checksums of this remote dependency
+  checksums: Checksums?
+}
+
+class Checksums {
+  /// The sha-256 checksum of this dependency's metadata.
+  sha256: String
+}
+----
+
+Example:
+[source,json5]
+----
+[
+  0x20,
+  {
+    "requestId": 193501,
+    "allowedModules": ["pkl:", "repl:"],
+    "allowedResources": ["file:", "package:", "projectpackage:"]
+  }
+]
+----
+
+[[create-evaluator-response]]
+=== Create Evaluator Response
+
+Code: `0x21` +
+Type: <<server-message,Server>> <<response-message,Response>>
+
+The response for a <<create-evaluator-request>>.
+If the evaluator was created successfully, `evaluatorId` is set. Otherwise, `error` is set to the resulting error.
+
+[source,pkl]
+----
+/// A number identifying this request
+requestId: Int
+
+/// A number identifying the created evaluator.
+evaluatorId: Int?
+
+/// A message detailing why the evaluator was not created.
+error: String?
+----
+
+[[close-evaluator]]
+=== Close Evaluator
+
+Code: `0x22` +
+Type: <<client-message,Client>> <<one-way-message,One Way>>
+
+Tells the Pkl server to close an evaluator, releasing any resources it may be holding.
+
+[source,pkl]
+----
+/// A number identifying this evaluator.
+evaluatorId: Int
+----
+
+[[evaluate-request]]
+=== Evaluate Request
+
+Code: `0x23` +
+Type: <<client-message,Client>> <<request-message,Request>>
+
+Evaluate a module.
+
+[source,pkl]
+----
+/// A number identifying this request
+requestId: Int
+
+/// A number identifying this evaluator.
+evaluatorId: Int
+
+/// The absolute URI of the module to be evaluated.
+moduleUri: String
+
+/// The module's contents.
+///
+/// If [null], Pkl will load the module at runtime.
+moduleText: String?
+
+/// The Pkl expression to be evaluated within the module.
+///
+/// If [null], evaluates the whole module.
+expr: String?
+----
+
+[[evaluate-response]]
+=== Evaluate Response
+
+Code: `0x24` +
+Type: <<server-message,Server>> <<response-message,Response>>
+
+The server's response to <<evaluate-request>>.
+If the evaluation is successful, the response is the Pkl value encoded in xref:binary-encoding.adoc[binary encoding].
+
+[source,pkl]
+----
+/// The requestId of the Evaluate request
+requestId: Int
+
+/// A number identifying this evaluator.
+evaluatorId: Int
+
+/// The evaluation contents, if successful.
+result: Binary? // <1>
+
+/// A message detailing why evaluation failed.
+error: String?
+
+typealias Binary = Any // <1>
+----
+<1> xref:binary-encoding.adoc[Pkl Binary Encoding] in link:{uri-messagepack-bin}[bin format] (not expressable in Pkl)
+
+[[log]]
+=== Log
+
+Code: `0x25` +
+Type: <<server-message,Server>> <<one-way-message,One Way>>
+
+Tells the client to emit a log message, during the execution of a Pkl program.
+A log can occur through a xref:language-reference:index.adoc#debugging[trace()] expression, or through a warning (for example, when encountering a link:{uri-stdlib-Deprecated}[Deprecated] value.)
+
+[source,pkl]
+----
+/// A number identifying this evaluator.
+evaluatorId: Int
+
+/// A number identifying the log level.
+///
+/// - 0: trace
+/// - 1: warn
+level: Int(this == 0 || this == 1)
+
+/// The message to be logged
+message: String
+
+/// A string representing the source location within Pkl code producing this log output.
+frameUri: String
+----
+
+[[read-resource-request]]
+=== Read Resource Request
+
+Code: `0x26` +
+Type: <<server-message,Server>> <<request-message,Request>>
+
+Read a resource at the given URI.
+This message occurs when a read expression (`read`/`read?`/`read*`) is encountered within a program, and its scheme matches a client resource reader.
+
+[source,pkl]
+----
+/// A number identifying this request.
+requestId: Int
+
+/// A number identifying this evaluator.
+evaluatorId: Int
+
+/// The URI of the resource.
+uri: String
+----
+
+[[read-resource-response]]
+=== Read Resource Response
+
+Code: `0x27` +
+Type: <<client-message,Client>> <<response-message,Response>>
+
+The response to <<read-resource-request>>.
+If successful, `contents` is set.
+Otherwise, `error` is set.
+
+[source,pkl]
+----
+/// A number identifying this request.
+requestId: Int
+
+/// A number identifying this evaluator.
+evaluatorId: Int
+
+/// The contents of the resource.
+contents: Binary? // <1>
+
+/// The description of the error that occured when reading this resource.
+error: String?
+
+typealias Binary = Any // <1>
+----
+<1> MessagePack's link:https://github.com/msgpack/msgpack/blob/master/spec.md#bin-format-family[bin format family] (not expressable in Pkl)
+
+[[read-module-request]]
+=== Read Module Request
+
+Code: `0x28` +
+Type: <<server-message,Server>> <<request-message,Request>>
+
+Read a module at the given URI.
+This message occurs during the evaluation of an import statement or expression (`import`/`import*`), when a scheme matches a client module reader.
+
+[source,pkl]
+----
+/// A number identifying this request.
+requestId: Int
+
+/// A number identifying this evaluator.
+evaluatorId: Int
+
+/// The URI of the module.
+uri: String
+----
+
+[[read-module-response]]
+=== Read Module Response
+
+Code: `0x29` +
+Type: <<client-message,Client>> <<response-message,Response>>
+
+The response to <<read-module-request>>.
+If successful, `contents` is set.
+Otherwise, `error` is set.
+
+[source,pkl]
+----
+/// A number identifying this request.
+requestId: Int
+
+/// A number identifying this evaluator.
+evaluatorId: Int
+
+/// The string contents of the module.
+contents: String?
+
+/// The description of the error that occured when reading this resource.
+error: String?
+----
+
+[[list-resources-request]]
+=== List Resources Request
+
+Code: `0x2a` +
+Type: <<server-message,Server>> <<request-message,Request>>
+
+List resources at the specified base path.
+This message occurs during the evaluation of a xref:language-reference:index.adoc#globbed-reads[globbed read], when a scheme matches a client resource reader's scheme.
+
+If the resource reader does not have hierarchical URIs, `dummy` is used as the path, and the response is expected to contain all resource elements for that scheme.
+
+[source,pkl]
+----
+/// A number identifying this request.
+requestId: Int
+
+/// A number identifying this evaluator.
+evaluatorId: Int
+
+/// The base URI to list resources.
+uri: String
+----
+
+[[list-resources-response]]
+=== List Resources Response
+
+Code: `0x2b` +
+Type: <<client-message,Client>> <<response-message,Response>>
+
+The response to <<list-resources-request>>.
+If successful, `pathElements` is set.
+Otherwise, `error` is set.
+
+[source,pkl]
+----
+/// A number identifying this request.
+requestId: Int
+
+/// A number identifying this evaluator.
+evaluatorId: Int
+
+/// The elements at the provided base path.
+pathElements: Listing<PathElement>?
+
+/// The description of the error that occured when listing elements.
+error: String?
+
+class PathElement {
+  /// The name of the element at this path
+  name: String
+
+  /// Tells whether the element is a directory.
+  isDirectory: Boolean
+}
+----
+
+[[list-modules-request]]
+=== List Modules Request
+
+Code: `0x2c` +
+Type: <<server-message,Server>> <<request-message,Request>>
+
+List modules at the specified base path.
+This message occurs during the evaluation of a xref:language-reference:index.adoc#globbed-imports[globbed import], when a scheme matches a client resource reader's scheme.
+
+If the module reader does not have hierarchical URIs, `dummy` is used as the path, and the response is expected to contain all module elements for that scheme.
+
+[source,pkl]
+----
+/// A number identifying this request.
+requestId: Int
+
+/// A number identifying this evaluator.
+evaluatorId: Int
+
+/// The base URI to list modules.
+uri: String
+----
+
+[[list-modules-response]]
+=== List Modules Response
+
+Code: `0x2d` +
+Type: <<client-message,Client>> <<response-message,Response>>
+
+The response to <<list-modules-request>>.
+If successful, `pathElements` is set.
+Otherwise, `error` is set.
+
+[source,pkl]
+----
+/// A number identifying this request.
+requestId: Int
+
+/// A number identifying this evaluator.
+evaluatorId: Int
+
+/// The elements at the provided base path.
+pathElements: Listing<PathElement>?
+
+/// The description of the error that occured when listing elements.
+error: String?
+
+class PathElement {
+  /// The name of the element at this path
+  name: String
+
+  /// Tells whether the element is a directory.
+  isDirectory: Boolean
+}
+----

docs/modules/bindings-specification/partials/component-attributes.adoc

@@ -0,0 +1,9 @@
+:uri-github-binary-encoding-snippet-tests: {uri-github-tree}/pkl-server/src/test/files/SnippetTests
+:uri-messagepack-bool: {uri-messagepack-spec}#bool-format-family
+:uri-messagepack-int: {uri-messagepack-spec}#int-format-family
+:uri-messagepack-float: {uri-messagepack-spec}#float-format-family
+:uri-messagepack-str: {uri-messagepack-spec}#str-format-family
+:uri-messagepack-bin: {uri-messagepack-spec}#bin-format-family
+:uri-messagepack-array: {uri-messagepack-spec}#array-format-family
+:uri-messagepack-map: {uri-messagepack-spec}#map-format-family
+:uri-messagepack-nil: {uri-messagepack-spec}#nil-format

docs/modules/language-reference/pages/index.adoc

@@ -9,70 +9,6 @@ include::ROOT:partial$component-attributes.adoc[]
 :uri-javadoc-Pattern: https://docs.oracle.com/javase/8/docs/api/java/util/regex/Pattern.html
 :uri-github-PklLexer: {uri-github-tree}/pkl-core/src/main/antlr/PklLexer.g4
 :uri-github-PklParser: {uri-github-tree}/pkl-core/src/main/antlr/PklParser.g4
-:uri-stdlib-baseModule: {uri-pkl-stdlib-docs}/base
-:uri-stdlib-jsonnetModule: {uri-pkl-stdlib-docs}/jsonnet
-:uri-stdlib-reflectModule: {uri-pkl-stdlib-docs}/reflect
-:uri-stdlib-xmlModule: {uri-pkl-stdlib-docs}/xml
-:uri-stdlib-protobufModule: {uri-pkl-stdlib-docs}/protobuf
-:uri-stdlib-Boolean: {uri-stdlib-baseModule}/Boolean
-:uri-stdlib-xor: {uri-stdlib-baseModule}/Boolean#xor()
-:uri-stdlib-implies: {uri-stdlib-baseModule}/Boolean#implies()
-:uri-stdlib-Any: {uri-stdlib-baseModule}/Any
-:uri-stdlib-String: {uri-stdlib-baseModule}/String
-:uri-stdlib-Int: {uri-stdlib-baseModule}/Int
-:uri-stdlib-Float: {uri-stdlib-baseModule}/Float
-:uri-stdlib-Number: {uri-stdlib-baseModule}/Number
-:uri-stdlib-NaN: {uri-stdlib-baseModule}/#NaN
-:uri-stdlib-Infinity: {uri-stdlib-baseModule}/#Infinity
-:uri-stdlib-isBetween: {uri-stdlib-baseModule}/Number#isBetween
-:uri-stdlib-isFinite: {uri-stdlib-baseModule}/Number#isFinite
-:uri-stdlib-Int8: {uri-stdlib-baseModule}/#Int8
-:uri-stdlib-Int16: {uri-stdlib-baseModule}/#Int16
-:uri-stdlib-Int32: {uri-stdlib-baseModule}/#Int32
-:uri-stdlib-UInt8: {uri-stdlib-baseModule}/#UInt8
-:uri-stdlib-UInt16: {uri-stdlib-baseModule}/#UInt16
-:uri-stdlib-UInt32: {uri-stdlib-baseModule}/#UInt32
-:uri-stdlib-UInt: {uri-stdlib-baseModule}/#UInt
-:uri-stdlib-Uri: {uri-stdlib-baseModule}/#Uri
-:uri-stdlib-matches: {uri-stdlib-baseModule}/String#matches()
-:uri-stdlib-Null: {uri-stdlib-baseModule}/Null
-:uri-stdlib-ifNonNull: {uri-stdlib-baseModule}/Null#ifNonNull()
-:uri-stdlib-List: {uri-stdlib-baseModule}/List
-:uri-stdlib-Set: {uri-stdlib-baseModule}/Set
-:uri-stdlib-Map: {uri-stdlib-baseModule}/Map
-:uri-stdlib-Listing: {uri-stdlib-baseModule}/Listing
-:uri-stdlib-Listing-default: {uri-stdlib-baseModule}/Listing#default
-:uri-stdlib-Listing-isDistinct: {uri-stdlib-baseModule}/Listing#isDistinct
-:uri-stdlib-Listing-isDistinctBy: {uri-stdlib-baseModule}/Listing#isDistinctBy()
-:uri-stdlib-Mapping: {uri-stdlib-baseModule}/Mapping
-:uri-stdlib-Mapping-default: {uri-stdlib-baseModule}/Mapping#default
-:uri-stdlib-Duration: {uri-stdlib-baseModule}/Duration
-:uri-stdlib-Duration-value: {uri-stdlib-baseModule}/Duration#value
-:uri-stdlib-Duration-unit: {uri-stdlib-baseModule}/Duration#unit
-:uri-stdlib-DurationUnit: {uri-stdlib-baseModule}/#DurationUnit
-:uri-stdlib-DataSize: {uri-stdlib-baseModule}/DataSize
-:uri-stdlib-DataSize-value: {uri-stdlib-baseModule}/DataSize#value
-:uri-stdlib-DataSize-unit: {uri-stdlib-baseModule}/DataSize#unit
-:uri-stdlib-DataSizeUnit: {uri-stdlib-baseModule}/#DataSizeUnit
-:uri-stdlib-Dynamic: {uri-stdlib-baseModule}/Dynamic
-:uri-stdlib-Dynamic-toTyped: {uri-stdlib-baseModule}/Dynamic#toTyped()
-:uri-stdlib-Typed: {uri-stdlib-baseModule}/Typed
-:uri-stdlib-Regex: {uri-stdlib-baseModule}/Regex
-:uri-stdlib-Regex-method: {uri-stdlib-baseModule}/#Regex()
-:uri-stdlib-Regex-match: {uri-stdlib-baseModule}/Regex#match
-:uri-stdlib-RegexMatch: {uri-stdlib-baseModule}/RegexMatch
-:uri-stdlib-ValueRenderer: {uri-stdlib-baseModule}/ValueRenderer
-:uri-stdlib-PcfRenderer-converters: {uri-stdlib-baseModule}/PcfRenderer#converters
-:uri-stdlib-Function: {uri-stdlib-baseModule}/Function
-:uri-stdlib-Function0: {uri-stdlib-baseModule}/Function0
-:uri-stdlib-Function1: {uri-stdlib-baseModule}/Function1
-:uri-stdlib-Function1-apply: {uri-stdlib-baseModule}/Function1#apply()
-:uri-stdlib-Function2: {uri-stdlib-baseModule}/Function2
-:uri-stdlib-Function3: {uri-stdlib-baseModule}/Function3
-:uri-stdlib-Function4: {uri-stdlib-baseModule}/Function4
-:uri-stdlib-Function5: {uri-stdlib-baseModule}/Function5
-:uri-stdlib-Resource: {uri-stdlib-baseModule}/Resource
-:uri-stdlib-outputFiles: {uri-stdlib-baseModule}/ModuleOutput#files
 :uri-pkl-core-ModuleSchema: {uri-pkl-core-main-sources}/ModuleSchema.java
 :uri-pkl-core-SecurityManager: {uri-pkl-core-main-sources}/SecurityManager.java
 :uri-pkl-core-ResourceReader: {uri-pkl-core-main-sources}/resource/ResourceReader.java

docs/nav.adoc

@@ -23,6 +23,9 @@
 **** xref:kotlin-binding:pkl-config-kotlin.adoc[pkl-config-kotlin Library]
 *** xref:swift:ROOT:index.adoc[Swift]
 *** xref:go:ROOT:index.adoc[Go]
+*** xref:bindings-specification:index.adoc[Specification]
+**** xref:bindings-specification:message-passing-api.adoc[Message Passing API]
+**** xref:bindings-specification:binary-encoding.adoc[Pkl Binary Encoding]
 
 * xref:ROOT:tools.adoc[Tools]
 ** xref:pkl-cli:index.adoc[CLI]
@@ -38,3 +41,4 @@
 * xref:release-notes:index.adoc[Release Notes]
 ** xref:release-notes:0.25.adoc[0.25 Release Notes]
 ** xref:release-notes:changelog.adoc[Changelog]
+