Support generating into existing MoonBit package (not module rewrite)

README.md

@@ -102,6 +102,7 @@ After updating your OpenAPI spec, regenerate the typed scaffolding without touch
 mapi generate --spec openapi.json --project ./my-api
 ```
 
+`generate` treats `--project` as an existing MoonBit package root: it reads the package name from `moon.mod.json`, regenerates `src/generated/*`, and leaves your existing module manifest untouched.
 Only files with the `// Code generated by mapi. DO NOT EDIT.` header are modified. Your handlers are safe.
 
 ---

main/generator.mbt

@@ -5,9 +5,15 @@ let generated_header = "// Code generated by mapi. DO NOT EDIT.\n"
 let user_header = "// User-owned file. Safe from regeneration.\n"
 
 ///|
-fn generate_project(ir : ApiIR, module_name : String) -> Map[String, String] {
+fn generate_project(
+  ir : ApiIR,
+  module_name : String,
+  include_module_manifest : Bool,
+) -> Map[String, String] {
   let files = {}
-  files["moon.mod.json"] = generate_moon_mod(module_name)
+  if include_module_manifest {
+    files["moon.mod.json"] = generate_moon_mod(module_name)
+  }
   files["src/generated/moon.pkg"] = generate_generated_pkg(module_name)
   files["src/generated/schemas.mbt"] = generate_schemas_file(ir)
   files["src/generated/operations.mbt"] = generate_operations_file(ir)

main/main.mbt

@@ -43,10 +43,17 @@ fn run_command(args : Array[String], mode : CommandMode) -> Unit {
           { title: "", version: "", schemas: [], operations: [] }
         }
       }
-      let module_name = infer_module_name(config.output_dir)
-      let files = generate_project(ir, module_name)
-      write_project(files, config.output_dir, mode == Generate) catch {
-        err => fail_with_message("Failed to write project: ", debug_string(err))
+      let include_module_manifest = mode == Init
+      match resolve_module_name(config.output_dir, include_module_manifest) {
+        Ok(module_name) => {
+          let files = generate_project(ir, module_name, include_module_manifest)
+          write_project(files, config.output_dir, mode == Generate) catch {
+            err =>
+              fail_with_message("Failed to write project: ", debug_string(err))
+          }
+        }
+        Err(message) =>
+          fail_with_message("Failed to resolve module name: ", message)
       }
       write_state(ir, config.spec_path, config.output_dir) catch {
         err => fail_with_message("Failed to write state: ", debug_string(err))
@@ -67,6 +74,11 @@ struct CliConfig {
   output_dir : String
 }
 
+///|
+struct MoonModConfig {
+  name : String
+} derive(FromJson)
+
 ///|
 fn parse_cli_options(
   args : Array[String],
@@ -155,6 +167,62 @@ fn infer_module_name(output_dir : String) -> String {
   }
 }
 
+///|
+fn resolve_module_name(
+  output_dir : String,
+  include_module_manifest : Bool,
+) -> Result[String, String] {
+  if include_module_manifest {
+    Ok(infer_module_name(output_dir))
+  } else {
+    read_module_name_from_moon_mod(output_dir)
+  }
+}
+
+///|
+fn read_module_name_from_moon_mod(
+  project_dir : String,
+) -> Result[String, String] {
+  let mod_path = project_dir + "/moon.mod.json"
+  if !@fs.path_exists(mod_path) {
+    return Err("moon.mod.json not found in " + project_dir)
+  }
+  let raw_result = Ok(@fs.read_file_to_string(mod_path)) catch {
+    err => Err("cannot read moon.mod.json: " + debug_string(err))
+  }
+  let raw = match raw_result {
+    Ok(value) => value
+    Err(message) => return Err(message)
+  }
+  parse_module_name_from_moon_mod(raw)
+}
+
+///|
+fn parse_module_name_from_moon_mod(raw : String) -> Result[String, String] {
+  let parsed = @json.parse(raw) catch {
+    _ => return Err("moon.mod.json is not valid JSON")
+  }
+  let mod_config : MoonModConfig = @json.from_json(parsed) catch {
+    _ => return Err("missing or invalid \"name\" field in moon.mod.json")
+  }
+  let module_name = mod_config.name.trim().to_string()
+  if module_name.is_empty() {
+    Err("empty \"name\" value in moon.mod.json")
+  } else if contains_whitespace(module_name) {
+    Err("invalid \"name\" value in moon.mod.json: contains whitespace")
+  } else {
+    Ok(module_name)
+  }
+}
+
+///|
+fn contains_whitespace(value : String) -> Bool {
+  value.find(" ") is Some(_) ||
+  value.find("\t") is Some(_) ||
+  value.find("\n") is Some(_) ||
+  value.find("\r") is Some(_)
+}
+
 ///|
 fn success_message(mode : CommandMode, output_dir : String) -> String {
   match mode {

main/moon.pkg

@@ -1,5 +1,6 @@
 import {
   "moonbitlang/core/debug" @debug,
+  "moonbitlang/core/json" @json,
   "moonbitlang/x/fs" @fs,
   "moonbitlang/x/sys" @sys,
   "cogna-dev/x/openapi" @openapi,