[compiler] Functions, reborn

.chronus/changes/witemple-msft-extern-fn-2025-10-21-12-47-57.md

@@ -0,0 +1,9 @@
+---
+changeKind: feature
+packages:
+  - "@typespec/compiler"
+---
+
+Added support for Functions, a new type graph entity and language feature. Functions enable library authors to provide input-output style transforms that operate on types and values. See [the Functions Documentation](https://typespec.io/docs/language-basics/functions/) for more information about the use and implementation of functions.
+
+Added an `unknown` value that can be used to denote when a property or parameter _has_ a default value, but its value cannot be expressed in TypeSpec (for example, because it depends on the server instance where it is generated, or because the service author simply does not with to specify how the default value is generated). This adds a new kind of `Value` to the language that represents a value that is not known.

.chronus/changes/witemple-msft-extern-fn-2025-10-21-12-48-14.md

@@ -0,0 +1,7 @@
+---
+changeKind: feature
+packages:
+  - "@typespec/openapi3"
+---
+
+Updated emitter logic to ignore `unknown` values in parameter and schema property defaults.

.chronus/changes/witemple-msft-extern-fn-2025-10-21-12-48-30.md

@@ -0,0 +1,20 @@
+---
+changeKind: internal
+packages:
+  - "@typespec/events"
+  - "@typespec/html-program-viewer"
+  - "@typespec/http-client"
+  - "@typespec/http"
+  - "@typespec/json-schema"
+  - "@typespec/openapi"
+  - "@typespec/protobuf"
+  - "@typespec/rest"
+  - "@typespec/spector"
+  - "@typespec/sse"
+  - "@typespec/streams"
+  - "@typespec/tspd"
+  - "@typespec/versioning"
+  - "@typespec/xml"
+---
+
+Updated `tspd` to generate extern function signatures. Regenerated all extern signatures.

cspell.yaml

@@ -141,6 +141,7 @@ words:
   - lzutf
   - MACVMIMAGE
   - MACVMIMAGEM
+  - marshal
   - mday
   - methodsubscriptionid
   - mgmt
@@ -274,6 +275,7 @@ words:
   - unioned
   - unionified
   - unionify
+  - unmarshal
   - unparented
   - unprefixed
   - unprojected

packages/compiler/generated-defs/TypeSpec.Prototypes.ts-test.ts

@@ -7,4 +7,4 @@ import type { TypeSpecPrototypesDecorators } from "./TypeSpec.Prototypes.js";
 /**
  * An error here would mean that the exported decorator is not using the same signature. Make sure to have export const $decName: DecNameDecorator = (...) => ...
  */
-const _: TypeSpecPrototypesDecorators = $decorators["TypeSpec.Prototypes"];
+const _decs: TypeSpecPrototypesDecorators = $decorators["TypeSpec.Prototypes"];

packages/compiler/generated-defs/TypeSpec.ts-test.ts

@@ -7,4 +7,4 @@ import type { TypeSpecDecorators } from "./TypeSpec.js";
 /**
  * An error here would mean that the exported decorator is not using the same signature. Make sure to have export const $decName: DecNameDecorator = (...) => ...
  */
-const _: TypeSpecDecorators = $decorators["TypeSpec"];
+const _decs: TypeSpecDecorators = $decorators["TypeSpec"];

packages/compiler/src/core/binder.ts

@@ -13,6 +13,7 @@ import {
   EnumStatementNode,
   FileLibraryMetadata,
   FunctionDeclarationStatementNode,
+  FunctionImplementations,
   FunctionParameterNode,
   InterfaceStatementNode,
   IntersectionExpressionNode,
@@ -133,7 +134,6 @@ export function createBinder(program: Program): Binder {
 
     for (const [key, member] of Object.entries(sourceFile.esmExports)) {
       let name: string;
-      let kind: "decorator" | "function";
       if (key === "$flags") {
         const context = getLocationContext(program, sourceFile);
         if (context.type === "library" || context.type === "project") {
@@ -152,12 +152,24 @@ export function createBinder(program: Program): Binder {
             );
           }
         }
+      } else if (key === "$functions") {
+        const value: FunctionImplementations = member as any;
+        for (const [namespaceName, functions] of Object.entries(value)) {
+          for (const [functionName, fn] of Object.entries(functions)) {
+            bindFunctionImplementation(
+              namespaceName === "" ? [] : namespaceName.split("."),
+              "function",
+              functionName,
+              fn,
+              sourceFile,
+            );
+          }
+        }
       } else if (typeof member === "function") {
         // lots of 'any' casts here because control flow narrowing `member` to Function
         // isn't particularly useful it turns out.
         if (isFunctionName(key)) {
           name = getFunctionName(key);
-          kind = "decorator";
           if (name === "onValidate") {
             const context = getLocationContext(program, sourceFile);
             const metadata =
@@ -170,12 +182,9 @@ export function createBinder(program: Program): Binder {
             // nothing to do here this is loaded as emitter.
             continue;
           }
-        } else {
-          name = key;
-          kind = "function";
+          const nsParts = resolveJSMemberNamespaceParts(rootNs, member);
+          bindFunctionImplementation(nsParts, "decorator", name, member as any, sourceFile);
         }
-        const nsParts = resolveJSMemberNamespaceParts(rootNs, member);
-        bindFunctionImplementation(nsParts, kind, name, member as any, sourceFile);
       }
     }
   }