Lua Modules

Runtime modules extend the Lua environment with new functionality. Modules can provide deterministic utilities, I/O operations, or async commands that yield to external systems.

The Lua runtime implementation may change in future versions.

Module Definition

Every module uses luaapi.ModuleDef:

var Module = &luaapi.ModuleDef{
    Name:        "mymodule",
    Description: "My custom module",
    Class:       []string{luaapi.ClassDeterministic},
    Types:       ModuleTypes,  // Type definitions for tooling
    Build: func() (*lua.LTable, []luaapi.YieldType) {
        mod := lua.CreateTable(0, 2)
        mod.RawSetString("hello", lua.LGoFunc(helloFunc))
        mod.RawSetString("greet", lua.LGoFunc(greetFunc))
        mod.Immutable = true
        return mod, nil
    },
}

The Build function returns:

• Module table with exported functions
• List of yield types for async operations (or nil)

Module tables are built once and cached for reuse across all Lua states.

Module Classification

The Class field determines where the module can be used:

Modules tagged only with ClassDeterministic are workflow-safe. Adding I/O or network classes restricts the module to functions and processes.

Exposing Functions

Functions have signature func(l *lua.LState) int where the return value is the number of values pushed onto the stack:

func greetFunc(l *lua.LState) int {
    name := l.CheckString(1)           // Required argument
    greeting := l.OptString(2, "Hello") // Optional with default

    l.Push(lua.LString(greeting + ", " + name + "!"))
    return 1
}

Tables

Tables passed between Go and Lua are mutable by default. Module export tables should be marked immutable:

mod := lua.CreateTable(0, 5)
mod.RawSetString("func1", lua.LGoFunc(func1))
mod.Immutable = true  // Prevent Lua from modifying exports

Data tables remain mutable for normal use:

result := l.CreateTable(0, 3)
result.RawSetString("name", lua.LString("value"))
result.RawSetString("count", lua.LNumber(42))
l.Push(result)

Type System

Modules use two separate but complementary typing mechanisms.

Type Definitions (Tooling)

The Types field provides type signatures for IDE support and documentation:

func ModuleTypes() *types.TypeManifest {
    m := types.NewManifest("mymodule")

    objectType := &types.InterfaceType{
        Name: "mymodule.Object",
        Methods: map[string]*types.FunctionType{
            "get_value": types.NewFunction(nil, []types.Type{types.String}),
            "set_value": types.NewFunction([]types.Type{types.String}, nil),
        },
    }

    m.DefineType("Object", objectType)
    m.SetExport(moduleType)
    return m
}

Available type constructs:

Function signatures support variadic parameters:

// (string, ...any) -> (string, error?)
types.FunctionType{
    Params:   []types.Type{types.String},
    Variadic: types.Any,
    Returns:  []types.Type{types.String, types.Optional(types.LuaError)},
}

See the types package in go-lua for the complete type system.

UserData Bindings (Runtime)

RegisterTypeMethods creates the actual Go-to-Lua bindings:

func init() {
    value.RegisterTypeMethods(nil, "mymodule.Object",
        map[string]lua.LGoFunc{
            "__tostring": objectToString,  // Metamethods
        },
        map[string]lua.LGoFunc{
            "get_value": objectGetValue,   // Regular methods
            "set_value": objectSetValue,
        },
    )
}

Metatables are immutable and cached globally for thread-safe reuse.

Async Operations

For operations that wait on external systems, return a yield instead of a result. The yield is dispatched to a Go handler and the process resumes when the handler completes.

Defining Yields

Declare yield types in the module's Build function:

Build: func() (*lua.LTable, []luaapi.YieldType) {
    mod := lua.CreateTable(0, 1)
    mod.RawSetString("fetch", lua.LGoFunc(fetchFunc))
    mod.Immutable = true

    yields := []luaapi.YieldType{
        {Sample: &FetchYield{}, CmdID: myapi.FetchCommand},
    }

    return mod, yields
}

Creating a Yield

Return -1 to signal a yield instead of normal return values:

func fetchFunc(l *lua.LState) int {
    url := l.CheckString(1)

    yield := AcquireFetchYield()
    yield.URL = url

    l.Push(yield)
    return -1  // Signal yield, not stack count
}

Yield Implementation

Yields bridge Lua values and dispatcher commands:

type FetchYield struct {
    *myapi.FetchCmd
}

func (y *FetchYield) String() string              { return "<fetch_yield>" }
func (y *FetchYield) Type() lua.LValueType        { return lua.LTUserData }
func (y *FetchYield) CmdID() dispatcher.CommandID { return myapi.FetchCommand }
func (y *FetchYield) ToCommand() dispatcher.Command { return y.FetchCmd }
func (y *FetchYield) Release() { releaseFetchYield(y) }

func (y *FetchYield) HandleResult(l *lua.LState, data any, err error) []lua.LValue {
    if err != nil {
        return []lua.LValue{lua.LNil, lua.NewLuaError(l, err.Error())}
    }
    resp := data.(*myapi.FetchResponse)
    return []lua.LValue{lua.LString(resp.Body), lua.LNil}
}

The dispatcher routes the command to a handler. See Command Dispatch for implementing handlers.

Error Handling

Return errors as the second value using structured errors:

func myFunc(l *lua.LState) int {
    result, err := doSomething()
    if err != nil {
        lerr := lua.NewLuaError(l, err.Error()).
            WithKind(lua.Internal).
            WithRetryable(true)
        l.Push(lua.LNil)
        l.Push(lerr)
        return 2
    }

    l.Push(lua.LString(result))
    l.Push(lua.LNil)
    return 2
}

Security

Check permissions before performing sensitive operations:

func myFunc(l *lua.LState) int {
    ctx := l.Context()

    if !security.IsAllowed(ctx, "mymodule.action", resource, nil) {
        l.Push(lua.LNil)
        l.Push(lua.NewLuaError(l, "permission denied").WithKind(lua.PermissionDenied))
        return 2
    }

    // Proceed with operation
}

Testing

Basic module tests verify structure and synchronous functions:

func TestModule(t *testing.T) {
    l := lua.NewState()
    defer l.Close()

    mod, _ := Module.Build()
    l.SetGlobal("mymodule", mod)

    err := l.DoString(`
        local m = mymodule
        assert(m.hello() == "Hello, World!")
    `)
    if err != nil {
        t.Fatal(err)
    }
}

Testing Modules with Yields

To test Lua code that uses yielding functions, create a minimal scheduler with the required dispatchers:

type testScheduler struct {
    *actor.Scheduler
    clock   *clock.Dispatcher
    mu      sync.Mutex
    pending map[string]chan *runtime.Result
}

func newTestScheduler() *testScheduler {
    ts := &testScheduler{pending: make(map[string]chan *runtime.Result)}
    reg := scheduler.NewRegistry()

    // Register dispatchers for yields your module uses
    clockSvc := clock.NewDispatcher()
    clockSvc.RegisterAll(func(id dispatcher.CommandID, h dispatcher.Handler) {
        reg.Register(id, h)
    })
    ts.clock = clockSvc

    ts.Scheduler = actor.NewScheduler(reg, actor.WithWorkers(4), actor.WithLifecycle(ts))
    return ts
}

func (ts *testScheduler) OnComplete(_ context.Context, p pid.PID, result *runtime.Result) {
    ts.mu.Lock()
    ch, ok := ts.pending[p.UniqID]
    delete(ts.pending, p.UniqID)
    ts.mu.Unlock()
    if ok {
        ch <- result
    }
}

func (ts *testScheduler) Execute(ctx context.Context, p pid.PID, proc process.Process,
    method string, input payload.Payloads) (*runtime.Result, error) {
    resultCh := make(chan *runtime.Result, 1)
    ts.mu.Lock()
    ts.pending[p.UniqID] = resultCh
    ts.mu.Unlock()

    _, err := ts.Scheduler.Submit(ctx, p, proc, method, input)
    if err != nil {
        return nil, err
    }

    select {
    case result := <-resultCh:
        return result, nil
    case <-ctx.Done():
        return nil, ctx.Err()
    }
}

Create processes from Lua scripts with the modules you're testing:

func bindMyModule(l *lua.LState) {
    tbl, _ := mymodule.Module.Build()
    l.SetGlobal(mymodule.Module.Name, tbl)
}

func newLuaProcess(script string) *engine.Process {
    proto, _ := lua.CompileString(script, "test.lua")
    return engine.NewProcess(
        engine.WithProto(proto),
        engine.WithModuleBinder(bindMyModule),
    )
}

func TestMyModuleYields(t *testing.T) {
    sched := newTestScheduler()
    sched.Start()
    defer sched.Stop()

    script := `
        local result = mymodule.fetch("http://example.com")
        return result.status
    `

    ctx, _ := ctxapi.OpenFrameContext(context.Background())
    proc := newLuaProcess(script)

    result, err := sched.Execute(ctx, pid.PID{UniqID: "test"}, proc, "", nil)
    if err != nil {
        t.Fatal(err)
    }
    // Assert on result
}

See runtime/lua/modules/time/integration_test.go for a complete example.

See Also

• Command Dispatch - Handling yield commands
• Scheduler - Process execution