feat: use builtin.Empty instead of Unit typealias in Kotlin (#904)

fixes #878

examples/kotlin/ftl-module-api/src/main/kotlin/ftl/api/Api.kt

@@ -5,6 +5,7 @@ import com.google.gson.GsonBuilder
 import com.google.gson.JsonDeserializer
 import com.google.gson.JsonPrimitive
 import com.google.gson.JsonSerializer
+import ftl.builtin.Empty
 import ftl.builtin.HttpRequest
 import ftl.builtin.HttpResponse
 import xyz.block.ftl.*
@@ -18,8 +19,6 @@ data class Todo(
   val completed: Boolean = false,
 )
 
-typealias GetStatusRequest = Unit
-
 data class GetStatusResponse(
   val status: String,
 )
@@ -59,7 +58,7 @@ class Api {
 
   @Verb
   @HttpIngress(Method.GET, "/api/status")
-  fun status(context: Context, req: HttpRequest<GetStatusRequest>): HttpResponse<GetStatusResponse> {
+  fun status(context: Context, req: HttpRequest<Empty>): HttpResponse<GetStatusResponse> {
     return HttpResponse<GetStatusResponse>(status = 200, headers = mapOf(), body = GetStatusResponse("OK"))
   }
 

integration/integration_test.go

@@ -96,12 +96,12 @@ func TestHttpIngress(t *testing.T) {
 				httpCall(rd, http.MethodPut, "/users/123", jsonData(t, obj{"postId": "346"}), func(t testing.TB, resp *httpResponse) {
 					assert.Equal(t, 200, resp.status)
 					assert.Equal(t, []string{"Header from FTL"}, resp.headers["Put"])
-					assert.Equal(t, nil, resp.body)
+					assert.Equal(t, map[string]any{}, resp.body)
 				}),
 				httpCall(rd, http.MethodDelete, "/users/123", jsonData(t, obj{}), func(t testing.TB, resp *httpResponse) {
 					assert.Equal(t, 200, resp.status)
 					assert.Equal(t, []string{"Header from FTL"}, resp.headers["Delete"])
-					assert.Equal(t, nil, resp.body)
+					assert.Equal(t, map[string]any{}, resp.body)
 				}),
 
 				httpCall(rd, http.MethodGet, "/html", jsonData(t, obj{}), func(t testing.TB, resp *httpResponse) {

integration/testdata/go/httpingress/echo.go

@@ -66,10 +66,10 @@ type PutResponse struct{}
 
 //ftl:verb
 //ftl:ingress http PUT /users/{userId}
-func Put(ctx context.Context, req builtin.HttpRequest[PutRequest]) (builtin.HttpResponse[ftl.Unit], error) {
-	return builtin.HttpResponse[ftl.Unit]{
+func Put(ctx context.Context, req builtin.HttpRequest[PutRequest]) (builtin.HttpResponse[builtin.Empty], error) {
+	return builtin.HttpResponse[builtin.Empty]{
 		Headers: map[string][]string{"Put": {"Header from FTL"}},
-		Body:    ftl.Unit{},
+		Body:    builtin.Empty{},
 	}, nil
 }
 
@@ -81,11 +81,11 @@ type DeleteResponse struct{}
 
 //ftl:verb
 //ftl:ingress http DELETE /users/{userId}
-func Delete(ctx context.Context, req builtin.HttpRequest[DeleteRequest]) (builtin.HttpResponse[ftl.Unit], error) {
-	return builtin.HttpResponse[ftl.Unit]{
+func Delete(ctx context.Context, req builtin.HttpRequest[DeleteRequest]) (builtin.HttpResponse[builtin.Empty], error) {
+	return builtin.HttpResponse[builtin.Empty]{
 		Status:  200,
 		Headers: map[string][]string{"Delete": {"Header from FTL"}},
-		Body:    ftl.Unit{},
+		Body:    builtin.Empty{},
 	}, nil
 }
 

integration/testdata/kotlin/database/Echo.kt

@@ -1,20 +1,20 @@
 package ftl.echo
 
+import ftl.builtin.Empty
 import xyz.block.ftl.Context
 import xyz.block.ftl.Verb
 import xyz.block.ftl.Database
 
 data class InsertRequest(val data: String)
-typealias InsertResponse = Unit
 
 val db = Database("testdb")
 
 class Echo {
 
   @Verb
-  fun insert(context: Context, req: InsertRequest): InsertResponse {
+  fun insert(context: Context, req: InsertRequest): Empty {
     persistRequest(req)
-    return InsertResponse
+    return Empty()
   }
 
   fun persistRequest(req: InsertRequest) {

integration/testdata/kotlin/httpingress/Echo.kt

@@ -1,5 +1,6 @@
 package ftl.echo
 
+import ftl.builtin.Empty
 import ftl.builtin.HttpRequest
 import ftl.builtin.HttpResponse
 import kotlin.String
@@ -25,8 +26,8 @@ data class GetResponse(
 )
 
 data class PostRequest(
-  @Alias("user_id") val userID: Int,
-  val postID: Int,
+  @Alias("user_id") val userId: Int,
+  val postId: Int,
 )
 
 data class PostResponse(
@@ -38,14 +39,9 @@ data class PutRequest(
   @Alias("postId") val postID: String,
 )
 
-typealias PutResponse = Unit
-
 data class DeleteRequest(
   @Alias("userId") val userID: String,
 )
-typealias DeleteResponse = Unit
-
-typealias HtmlRequest = Unit
 
 class Echo {
   @Verb
@@ -74,27 +70,27 @@ class Echo {
 
   @Verb
   @HttpIngress(Method.PUT, "/users/{userId}")
-  fun put(context: Context, req: HttpRequest<PutRequest>): HttpResponse<PutResponse> {
-    return HttpResponse<PutResponse>(
+  fun put(context: Context, req: HttpRequest<PutRequest>): HttpResponse<Empty> {
+    return HttpResponse<Empty>(
       status = 200,
       headers = mapOf("Put" to arrayListOf("Header from FTL")),
-      body = PutResponse
+      body = Empty()
     )
   }
 
   @Verb
   @HttpIngress(Method.DELETE, "/users/{userId}")
-  fun delete(context: Context, req: HttpRequest<DeleteRequest>): HttpResponse<DeleteResponse> {
-    return HttpResponse<DeleteResponse>(
+  fun delete(context: Context, req: HttpRequest<DeleteRequest>): HttpResponse<Empty> {
+    return HttpResponse<Empty>(
       status = 200,
       headers = mapOf("Delete" to arrayListOf("Header from FTL")),
-      body = DeleteResponse
+      body = Empty()
     )
   }
 
   @Verb
   @HttpIngress(Method.GET, "/html")
-  fun html(context: Context, req: HttpRequest<HtmlRequest>): HttpResponse<String> {
+  fun html(context: Context, req: HttpRequest<Empty>): HttpResponse<String> {
     return HttpResponse<String>(
       status = 200,
       headers = mapOf("Content-Type" to arrayListOf("text/html; charset=utf-8")),

kotlin-runtime/ftl-generator/src/main/kotlin/xyz/block/ftl/generator/ModuleGenerator.kt

@@ -46,9 +46,9 @@ class ModuleGenerator() {
 
     val types = module.decls.mapNotNull { it.data_ }
     types.forEach {
-      if (it.fields.isEmpty()) {
-        file.addTypeAlias(
-          TypeAliasSpec.builder(it.name, Unit::class)
+      if (namespace == "ftl.builtin" && it.name == "Empty") {
+        file.addType(
+          TypeSpec.classBuilder(it.name)
             .addKdoc(it.comments.joinToString("\n"))
             .build()
         )

kotlin-runtime/ftl-generator/src/test/kotlin/xyz/block/ftl/generator/ModuleGeneratorTest.kt

@@ -45,7 +45,6 @@ public class TestModule()
           )
         )
       ),
-      Decl(data_ = Data(comments = listOf("Request comments"), name = "TestRequest")),
       Decl(
         data_ = Data(
           comments = listOf("Response comments"), name = "TestResponse",
@@ -110,7 +109,6 @@ import kotlin.ByteArray
 import kotlin.Float
 import kotlin.Long
 import kotlin.String
-import kotlin.Unit
 import kotlin.collections.ArrayList
 import kotlin.collections.Map
 import xyz.block.ftl.Alias
@@ -120,11 +118,6 @@ public data class ParamTestData<T>(
   public val t: T,
 )
 
-/**
- * Request comments
- */
-public typealias TestRequest = Unit
-
 /**
  * Response comments
  */
@@ -158,22 +151,20 @@ public class TestModule()
   @Test
   fun `should generate all Verbs`() {
     val decls = listOf(
-      Decl(data_ = Data(comments = listOf("Request comments"), name = "TestRequest")),
-      Decl(data_ = Data(comments = listOf("Response comments"), name = "TestResponse")),
       Decl(
         verb = Verb(
           name = "TestVerb",
           comments = listOf("TestVerb comments"),
-          request = Type(dataRef = DataRef(name = "TestRequest")),
-          response = Type(dataRef = DataRef(name = "TestResponse"))
+          request = Type(dataRef = DataRef(name = "Empty", module = "builtin")),
+          response = Type(dataRef = DataRef(name = "Empty", module = "builtin"))
         )
       ),
       Decl(
         verb = Verb(
           name = "TestIngressVerb",
           comments = listOf("TestIngressVerb comments"),
-          request = Type(dataRef = DataRef(name = "TestRequest")),
-          response = Type(dataRef = DataRef(name = "TestResponse")),
+          request = Type(dataRef = DataRef(name = "Empty", module = "builtin")),
+          response = Type(dataRef = DataRef(name = "Empty", module = "builtin")),
           metadata = listOf(
             Metadata(
               ingress = MetadataIngress(
@@ -192,30 +183,20 @@ public class TestModule()
 // Module comments
 package ftl.test
 
-import kotlin.Unit
+import ftl.builtin.Empty
 import xyz.block.ftl.Context
 import xyz.block.ftl.HttpIngress
 import xyz.block.ftl.Ignore
 import xyz.block.ftl.Method.GET
 import xyz.block.ftl.Verb
 
-/**
- * Request comments
- */
-public typealias TestRequest = Unit
-
-/**
- * Response comments
- */
-public typealias TestResponse = Unit
-
 @Ignore
 public class TestModule() {
   /**
    * TestVerb comments
    */
   @Verb
-  public fun TestVerb(context: Context, req: TestRequest): TestResponse = throw
+  public fun TestVerb(context: Context, req: Empty): Empty = throw
       NotImplementedError("Verb stubs should not be called directly, instead use context.call(TestModule::TestVerb, ...)")
 
   /**
@@ -226,9 +207,109 @@ public class TestModule() {
     GET,
     "/test",
   )
-  public fun TestIngressVerb(context: Context, req: TestRequest): TestResponse = throw
+  public fun TestIngressVerb(context: Context, req: Empty): Empty = throw
       NotImplementedError("Verb stubs should not be called directly, instead use context.call(TestModule::TestIngressVerb, ...)")
 }
+"""
+    assertEquals(expected, file.toString())
+  }
+
+  @Test
+  fun `builtins are handled correctly`() {
+    val decls = listOf(
+      Decl(
+        data_ = Data(
+          comments = listOf("HTTP request structure used for HTTP ingress verbs."),
+          name = "HttpRequest",
+          fields = listOf(
+            Field(name = "method", type = Type(string = String())),
+            Field(name = "path", type = Type(string = String())),
+            Field(
+              name = "pathParameters",
+              type = Type(map = Map(key = Type(string = String()), value_ = Type(string = String())))
+            ),
+            Field(
+              name = "query",
+              type = Type(
+                map = Map(
+                  key = Type(string = String()),
+                  value_ = Type(array = Array(element = Type(string = String())))
+                )
+              )
+            ),
+            Field(
+              name = "headers",
+              type = Type(
+                map = Map(
+                  key = Type(string = String()),
+                  value_ = Type(array = Array(element = Type(string = String())))
+                )
+              )
+            ),
+            Field(name = "body", type = Type(dataRef = DataRef(name = "Body")))
+          ),
+          typeParameters = listOf(TypeParameter(name = "Body"))
+        )
+      ),
+      Decl(
+        data_ = Data(
+          comments = listOf("HTTP response structure used for HTTP ingress verbs."),
+          name = "HttpResponse",
+          fields = listOf(
+            Field(name = "status", type = Type(int = Int())),
+            Field(
+              name = "headers",
+              type = Type(
+                map = Map(
+                  key = Type(string = String()),
+                  value_ = Type(array = Array(element = Type(string = String())))
+                )
+              )
+            ),
+            Field(name = "body", type = Type(dataRef = DataRef(name = "Body")))
+          ),
+          typeParameters = listOf(TypeParameter(name = "Body"))
+        )
+      ),
+      Decl(data_ = Data(name = "Empty")),
+    )
+    val module = Module(name = "builtin", comments = listOf("Built-in types for FTL"), decls = decls)
+    val file = generator.generateModule(module)
+    val expected = """// Code generated by FTL-Generator, do not edit.
+// Built-in types for FTL
+package ftl.builtin
+
+import kotlin.Long
+import kotlin.String
+import kotlin.collections.ArrayList
+import kotlin.collections.Map
+import xyz.block.ftl.Ignore
+
+/**
+ * HTTP request structure used for HTTP ingress verbs.
+ */
+public data class HttpRequest<Body>(
+  public val method: String,
+  public val path: String,
+  public val pathParameters: Map<String, String>,
+  public val query: Map<String, ArrayList<String>>,
+  public val headers: Map<String, ArrayList<String>>,
+  public val body: Body,
+)
+
+/**
+ * HTTP response structure used for HTTP ingress verbs.
+ */
+public data class HttpResponse<Body>(
+  public val status: Long,
+  public val headers: Map<String, ArrayList<String>>,
+  public val body: Body,
+)
+
+public class Empty
+
+@Ignore
+public class BuiltinModule()
 """
     assertEquals(expected, file.toString())
   }