GopherLua: VM and compiler for Lua in Go.

GopherLua is a Lua5.1 VM and compiler written in Go. GopherLua has a same goal with Lua: Be a scripting language with extensible semantics . It provides Go APIs that allow you to easily embed a scripting language to your Go host programs.

Contents

• Design principle
• How about performance?
• Installation
• Usage
• Differences between Lua and GopherLua
• Standalone interpreter
• How to Contribute
• Libraries for GopherLua
• License
• Author

Design principle

• Be a scripting language with extensible semantics.

• User-friendly Go API

  • The stack besed API like the one used in the original Lua implementation will cause a performance improvements in GopherLua (It will reduce memory allocations and concrete type <-> interface conversions). GopherLua API is not the stack based API. GopherLua give preference to the user-friendliness over the performance.

How about performance?

GopherLua is not fast but not too slow, I think.

There are some benchmarks on the wiki page .

Installation

go get github.com/yuin/gopher-lua

Usage

GopherLua APIs perform in much the same way as Lua, but the stack is used only for passing arguments and receiving returned values.

GopherLua supports channel operations. See "Goroutines" section.

Import a package.

import (
    "github.com/yuin/gopher-lua"
)

Run scripts in the VM.

L := lua.NewState()
defer L.Close()
if err := L.DoString(`print("hello")`); err != nil {
    panic(err)
}

L := lua.NewState()
defer L.Close()
if err := L.DoFile("hello.lua"); err != nil {
    panic(err)
}

Refer to Lua Reference Manual and Go doc for further information.

Data model

All data in a GopherLua program is an LValue . LValue is an interface type that has following methods.

• String() string
• Type() LValueType

Objects implement an LValue interface are

Type name	Go type	Type() value	Constants
LNilType	(constants)	LTNil	LNil
LBool	(constants)	LTBool	LTrue, LFalse
LNumber	float64	LTNumber	-
LString	string	LTString	-
LFunction	struct pointer	LTFunction	-
LUserData	struct pointer	LTUserData	-
LState	struct pointer	LTThread	-
LTable	struct pointer	LTTable	-
LChannel	chan LValue	LTChannel	-

You can test an object type in Go way(type assertion) or using a Type() value.

lv := L.Get(-1) // get the value at the top of the stack
if str, ok := lv.(lua.LString); ok {
    // lv is LString
    fmt.Println(string(str))
}
if lv.Type() != lua.LTString {
    panic("string required.")
}

lv := L.Get(-1) // get the value at the top of the stack
if tbl, ok := lv.(*lua.LTable); ok {
    // lv is LTable
    fmt.Println(L.ObjLen(tbl))
}

Note that LBool , LNumber , LString is not a pointer.

To test LNilType and LBool, You must use pre-defined constants.

lv := L.Get(-1) // get the value at the top of the stack

if lv == LTrue { // correct
}

if bl, ok == lv.(lua.LBool); ok && bool(bl) { // wrong
}

In Lua, both nil and false make a condition false. LVIsFalse and LVAsBool implement this specification.

lv := L.Get(-1) // get the value at the top of the stack
if LVIsFalse(lv) { // lv is nil or false
}

if LVAsBool(lv) { // lv is neither nil nor false
}

Objects that based on go structs(LFunction. LUserData, LTable) have some public methods and fields. You can use these methods and fields for performance and debugging, but there are some limitations.

• Metatable does not work.
• No error handlings.

Callstack & Registry size

Size of the callstack & registry is fixed for mainly performance. You can change the size of the callstack & registry.

lua.RegistrySize = 1024 * 20
lua.CallStackSize = 1024
L = lua.NewState()
defer L.Close()

API

Refer to Lua Reference Manual and Go doc(LState methods) for further information.

Calling Go from Lua

func Double(L *lua.LState) int {
    lv := L.ToInt(1)             /* get argument */
    L.Push(lua.LNumber(lv * 2)) /* push result */
    return 1                     /* number of results */
}

func main() {
    L := lua.NewState()
    defer L.Close()
    L.SetGlobal("double", L.NewFunction(Double)) /* Original lua_setglobal uses stack... */
}

print(double(20)) -- > "40"

Any function registered with GopherLua is a lua.LGFunction, defined in value.go

type LGFunction func(*LState) int

Working with coroutines.

co := L.NewThread() /* create a new thread */
fn := L.GetGlobal("coro").(*lua.LFunction) /* get function from lua */
for {
    st, err, values := L.Resume(co, fn)
    if st == lua.ResumeError {
        fmt.Println("yield break(error)")
        fmt.Println(err.Error())
        break
    }

    for i, lv := range values {
        fmt.Printf("%v : %v\n", i, lv)
    }

    if st == lua.ResumeOK {
        fmt.Println("yield break(ok)")
        break
    }
}

Creating a module by Go

mymodule.go

package mymodule

import (
    "github.com/yuin/gopher-lua"
)

func Loader(L *lua.LState) int {
    // register functions to the table
    mod := L.SetFuncs(L.NewTable(), exports)
    // register other stuff
    L.SetField(mod, "name", lua.LString("value"))

    // returns the module
    L.Push(mod)
    return 1
}

var exports = map[string]lua.LGFunction{
    "myfunc": myfunc,
}

func myfunc(L *lua.LState) int {
    return 0
}

mymain.go

package main

import (
    "./mymodule"
    "github.com/yuin/gopher-lua"
)

func main() {
    L := lua.NewState()
    defer L.Close()
    L.PreloadModule("mymodule", mymodule.Loader)
    if err := L.DoFile("main.lua"); err != nil {
        panic(err)
    }
}

main.lua

local m = require("mymodule")
m.myfunc()
print(m.name)

Calling Lua from Go

L := lua.NewState()
defer L.Close()
if err := L.DoFile("double.lua"); err != nil {
    panic(err)
}
if err := L.CallByParam(lua.P{
    Fn: L.GetGlobal("double"),
    NRet: 1,
    Protect: true,
    }, lua.LNumber(10)); err != nil {
    panic(err)
}
ret := L.Get(-1) // returned value
L.Pop(1)  // remove received value

If Protect is false, GopherLua will panic instead of returning an error value.

Goroutines

The LState is not goroutine-safe. It is recommended to use one LState per goroutine and communicate between goroutines by using channels.

Channels are represented by channel objects in GopherLua. And a channel table provides functions for performing channel operations.

Some objects can not be sent over channels due to having non-goroutine-safe objects inside itself.

• a thread(state)
• a function
• an userdata
• a table with a metatable

You must not send these objects from Go APIs to channels.

func receiver(ch, quit chan lua.LValue) {
    L := lua.NewState()
    defer L.Close()
    L.SetGlobal("ch", lua.LChannel(ch))
    L.SetGlobal("quit", lua.LChannel(quit))
    if err := L.DoString(`
    local exit = false
    while not exit do
      channel.select(
        {"|<-", ch, function(ok, v)
          if not ok then
            print("channel closed")
            exit = true
          else
            print("received:", v)
          end
        end},
        {"|<-", quit, function(ok, v)
            print("quit")
            exit = true
        end}
      )
    end
  `); err != nil {
        panic(err)
    }
}

func sender(ch, quit chan lua.LValue) {
    L := lua.NewState()
    defer L.Close()
    L.SetGlobal("ch", lua.LChannel(ch))
    L.SetGlobal("quit", lua.LChannel(quit))
    if err := L.DoString(`
    ch:send("1")
    ch:send("2")
  `); err != nil {
        panic(err)
    }
    ch <- lua.LString("3")
    quit <- lua.LTrue
}

func main() {
    ch := make(chan lua.LValue)
    quit := make(chan lua.LValue)
    go receiver(ch, quit)
    go sender(ch, quit)
    time.Sleep(3 * time.Second)
}

Go API

ToChannel, CheckChannel, OptChannel are available.

Refer to Go doc(LState methods) for further information.

Lua API

• channel.make([buf:int]) -> ch:channel

  • Create new channel that has a buffer size of buf. By default, buf is 0.

• channel.select(case:table [, case:table, case:table ...]) -> {index:int, recv:any, ok}

  • Same as the select statement in Go. It returns the index of the chosen case and, if that case was a receive operation, the value received and a boolean indicating whether the channel has been closed.

  • case is a table that outlined below.

    • receiving: {"|<-", ch:channel [, handler:func(ok, data:any)]}
    • sending: {"<-|", ch:channel, data:any [, handler:func(data:any)]}
    • default: {"default" [, handler:func()]}

channel.select examples:

local idx, recv, ok = channel.select(
  {"|<-", ch1},
  {"|<-", ch2}
)
if not ok then
    print("closed")
elseif idx == 1 then -- received from ch1
    print(recv)
elseif idx == 2 then -- received from ch2
    print(recv)
end

channel.select(
  {"|<-", ch1, function(ok, data)
    print(ok, data)
  end},
  {"<-|", ch2, "value", function(data)
    print(data)
  end},
  {"default", function()
    print("default action")
  end}
)

• channel:send(data:any)

  • Send data over the channel.

• channel:receive() -> ok:bool, data:any

  • Receive some data over the channel.

• channel:close()

  • Close the channel.

The LState pool pattern

To create per-thread LState instances, You can use the sync.Pool like mechanism.

type lStatePool struct {
    m     sync.Mutex
    saved []*lua.LState
}

func (pl *lStatePool) Get() *lua.LState {
    pl.m.Lock()
    defer pl.m.Unlock()
    n := len(pl.saved)
    if n == 0 {
        return pl.New()
    }
    x := pl.saved[n-1]
    pl.saved = pl.saved[0 : n-1]
    return x
}

func (pl *lStatePool) New() *lua.LState {
    L := lua.NewState()
    // setting the L up here.
    // load scripts, set global variables, share channels, etc...
    return L
}

func (pl *lStatePool) Put(L *lua.LState) {
    pl.m.Lock()
    defer pl.m.Unlock()
    pl.saved = append(pl.saved, L)
}

func (pl *lStatePool) Shutdown() {
    for _, L := range pl.saved {
        L.Close()
    }
}

// Global LState pool
var luaPool = &lStatePool{
    saved: make([]*lua.LState, 0, 4),
}

Now, you can get per-thread LState objects from the luaPool .

func MyWorker() {
   L := luaPool.Get()
   defer luaPool.Put(L)
   /* your code here */
}

func main() {
    defer luaPool.Shutdown()
    go MyWorker()
    go MyWorker()
    /* etc... */
}

Differences between Lua and GopherLua

Goroutines

• GopherLua supports channel operations.

  • GopherLua has a type named channel.
  • The channel table provides functions for performing channel operations.

Pattern match

• GopherLua uses the regexp package to implement the pattern match.

  • The Pattern match only works for utf8 strings.
  • The regexp package does not support back-references.
  • The regexp package does not support position-captures.

GopherLua has an option to use the Go regexp syntax as a pattern match format.

lua.LuaRegex = false
L := lua.NewState()
defer L.Close()

print(string.gsub("abc $!?", [[a(\w+)]], "${1}")) --> bc $!?

Unsupported functions

• string.dump
• os.setlocale
• collectgarbage
• lua_Debug.namewhat
• package.loadlib
• debug hooks

Miscellaneous notes

• file:setvbuf does not support a line bufferring.
• Daylight saving time is not supported.
• GopherLua has a function to set an environment variable : os.setenv(name, value)

Standalone interpreter

Lua has an interpreter called lua . GopherLua has an interpreter called glua .

go get github.com/yuin/gopher-lua/cmd/glua

glua has same options as lua .

How to Contribute

Any kind of contributions are wellcome.

Pull requests

Our workflow is based on the github-flow .

1. Create a new issue.

2. Fork the project.

3. Clone your fork and add the upstream.

   git remote add upstream https://github.com/yuin/gopher-lua.git
   

4. Pull new changes from the upstream.

   git checkout master
   git fetch upstream
   git merge upstream/master
   

5. Create a feature branch

   git checkout -b <branch-name>
   

6. Commit your changes and reference the issue number in your comment.

   git commit -m "Issue #<issue-ref> : <your message>"
   

7. Push the feature branch to your remote repository.

   git push origin <branch-name>
   

8. Open new pull request.

Libraries for GopherLua

• gopher-luar : Custom type reflection for gopher-lua
• gluamapper : Mapping a Lua table to a Go struct

License

MIT

Author

Yusuke Inuzuka