# GopherLua: VM and compiler for Lua in Go.

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GopherLua is a Lua5.1(+ `goto` statement in Lua5.2) 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.

## Table of Contents

- [Design principle](#design-principle)
- [How about performance?](#how-about-performance)
- [Installation](#installation)
- [Usage](#usage)
- [Differences between Lua and GopherLua](#differences-between-lua-and-gopherlua)
- [Standalone interpreter](#standalone-interpreter)
- [How to Contribute](#how-to-contribute)
- [Libraries for GopherLua](#libraries-for-gopherlua)
- [Donation](#donation)
- [License](#license)
- [Author](#author)

## Design principle

- Be a scripting language with extensible semantics.
- User-friendly Go API
    - The stack based 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.

GopherLua has almost equivalent ( or little bit better ) performance as Python3 on micro benchmarks.

There are some benchmarks on the [wiki page](https://github.com/yuin/gopher-lua/wiki/Benchmarks) .

## Installation

```bash
$ go get github.com/yuin/gopher-lua
```

GopherLua supports >= Go1.9.

## 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.

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

Run scripts in the VM.

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

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

Refer to [Lua Reference Manual](http://www.lua.org/manual/5.1/) and [Go doc](http://godoc.org/github.com/yuin/gopher-lua) for further information.

Note that elements that are not commented in [Go doc](http://godoc.org/github.com/yuin/gopher-lua) equivalent to [Lua Reference Manual](http://www.lua.org/manual/5.1/) , except GopherLua uses objects instead of Lua stack indices.

### 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.

```go
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.")
}
```

```go
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.

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

if lv == lua.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.

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

if lua.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

The size of an `LState`'s callstack controls the maximum call depth for Lua functions within a script (Go function calls do not count).

The registry of an `LState` implements stack storage for calling functions (both Lua and Go functions) and also for temporary variables in expressions. Its storage requirements will increase with callstack usage and also with code complexity.

Both the registry and the callstack can be set to either a fixed size or to auto size.

When you have a large number of `LStates` instantiated in a process, it's worth taking the time to tune the registry and callstack options.

#### Registry

The registry can have an initial size, a maximum size and a step size configured on a per `LState` basis. This will allow the registry to grow as needed. It will not shrink again after growing.

```go
L := lua.NewState(lua.Options{
   RegistrySize: 1024 * 20,         // this is the initial size of the registry
   RegistryMaxSize: 1024 * 80,      // this is the maximum size that the registry can grow to. If set to `0` (the default) then the registry will not auto grow
   RegistryGrowStep: 32,            // this is how much to step up the registry by each time it runs out of space. The default is `32`.
})
defer L.Close()
```

A registry which is too small for a given script will ultimately result in a panic. A registry which is too big will waste memory (which can be significant if many `LStates` are instantiated).
Auto growing registries incur a small performance hit at the point they are resized but will not otherwise affect performance.

#### Callstack

The callstack can operate in two different modes, fixed or auto size.
A fixed size callstack has the highest performance and has a fixed memory overhead.
An auto sizing callstack will allocate and release callstack pages on demand which will ensure the minimum amount of memory is in use at any time. The downside is it will incur a small performance impact every time a new page of callframes is allocated.
By default an `LState` will allocate and free callstack frames in pages of 8, so the allocation overhead is not incurred on every function call. It is very likely that the performance impact of an auto resizing callstack will be negligible for most use cases.

```go
L := lua.NewState(lua.Options{
    CallStackSize: 120,                 // this is the maximum callstack size of this LState
    MinimizeStackMemory: true,          // Defaults to `false` if not specified. If set, the callstack will auto grow and shrink as needed up to a max of `CallStackSize`. If not set, the callstack will be fixed at `CallStackSize`.
})
defer L.Close()
```

#### Option defaults

The above examples show how to customize the callstack and registry size on a per `LState` basis. You can also adjust some defaults for when options are not specified by altering the values of `lua.RegistrySize`, `lua.RegistryGrowStep` and `lua.CallStackSize`.

An `LState` object that has been created by `*LState#NewThread()` inherits the callstack & registry size from the parent `LState` object.

### Miscellaneous lua.NewState options

- **Options.SkipOpenLibs bool(default false)**
    - By default, GopherLua opens all built-in libraries when new LState is created.
    - You can skip this behaviour by setting this to `true` .
    - Using the various `OpenXXX(L *LState) int` functions you can open only those libraries that you require, for an example see below.
- **Options.IncludeGoStackTrace bool(default false)**
    - By default, GopherLua does not show Go stack traces when panics occur.
    - You can get Go stack traces by setting this to `true` .

### API

Refer to [Lua Reference Manual](http://www.lua.org/manual/5.1/) and [Go doc(LState methods)](http://godoc.org/github.com/yuin/gopher-lua) for further information.

#### Calling Go from Lua

```go
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... */
}
```

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

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

```go
type LGFunction func(*LState) int
```

Working with coroutines.

```go
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
    }
}
```

#### Opening a subset of builtin modules

The following demonstrates how to open a subset of the built-in modules in Lua, say for example to avoid enabling modules with access to local files or system calls.

main.go

```go
func main() {
    L := lua.NewState(lua.Options{SkipOpenLibs: true})
    defer L.Close()
    for _, pair := range []struct {
        n string
        f lua.LGFunction
    }{
        {lua.LoadLibName, lua.OpenPackage}, // Must be first
        {lua.BaseLibName, lua.OpenBase},
        {lua.TabLibName, lua.OpenTable},
    } {
        if err := L.CallByParam(lua.P{
            Fn:      L.NewFunction(pair.f),
            NRet:    0,
            Protect: true,
        }, lua.LString(pair.n)); err != nil {
            panic(err)
        }
    }
    if err := L.DoFile("main.lua"); err != nil {
        panic(err)
    }
}
```

#### Creating a module by Go

mymodule.go

```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

```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

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

#### Calling Lua from Go

```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.

#### User-Defined types

You can extend GopherLua with new types written in Go.
`LUserData` is provided for this purpose.

```go
type Person struct {
    Name string
}

const luaPersonTypeName = "person"

// Registers my person type to given L.
func registerPersonType(L *lua.LState) {
    mt := L.NewTypeMetatable(luaPersonTypeName)
    L.SetGlobal("person", mt)
    // static attributes
    L.SetField(mt, "new", L.NewFunction(newPerson))
    // methods
    L.SetField(mt, "__index", L.SetFuncs(L.NewTable(), personMethods))
}

// Constructor
func newPerson(L *lua.LState) int {
    person := &Person{L.CheckString(1)}
    ud := L.NewUserData()
    ud.Value = person
    L.SetMetatable(ud, L.GetTypeMetatable(luaPersonTypeName))
    L.Push(ud)
    return 1
}

// Checks whether the first lua argument is a *LUserData with *Person and returns this *Person.
func checkPerson(L *lua.LState) *Person {
    ud := L.CheckUserData(1)
    if v, ok := ud.Value.(*Person); ok {
        return v
    }
    L.ArgError(1, "person expected")
    return nil
}

var personMethods = map[string]lua.LGFunction{
    "name": personGetSetName,
}

// Getter and setter for the Person#Name
func personGetSetName(L *lua.LState) int {
    p := checkPerson(L)
    if L.GetTop() == 2 {
        p.Name = L.CheckString(2)
        return 0
    }
    L.Push(lua.LString(p.Name))
    return 1
}

func main() {
    L := lua.NewState()
    defer L.Close()
    registerPersonType(L)
    if err := L.DoString(`
        p = person.new("Steeve")
        print(p:name()) -- "Steeve"
        p:name("Alice")
        print(p:name()) -- "Alice"
    `); err != nil {
        panic(err)
    }
}
```

#### Terminating a running LState

GopherLua supports the [Go Concurrency Patterns: Context](https://blog.golang.org/context) .

```go
L := lua.NewState()
defer L.Close()
ctx, cancel := context.WithTimeout(context.Background(), 1*time.Second)
defer cancel()
// set the context to our LState
L.SetContext(ctx)
err := L.DoString(`
  local clock = os.clock
  function sleep(n)  -- seconds
    local t0 = clock()
    while clock() - t0 <= n do end
  end
  sleep(3)
`)
// err.Error() contains "context deadline exceeded"
```

With coroutines

```go
L := lua.NewState()
defer L.Close()
ctx, cancel := context.WithCancel(context.Background())
L.SetContext(ctx)
defer cancel()
L.DoString(`
    function coro()
	  local i = 0
	  while true do
	    coroutine.yield(i)
		i = i+1
	  end
	  return i
    end
`)
co, cocancel := L.NewThread()
defer cocancel()
fn := L.GetGlobal("coro").(*LFunction)

_, err, values := L.Resume(co, fn) // err is nil

cancel() // cancel the parent context

_, err, values = L.Resume(co, fn) // err is NOT nil : child context was canceled
```

**Note that using a context causes performance degradation.**

```
time ./glua-with-context.exe fib.lua
9227465
0.01s user 0.11s system 1% cpu 7.505 total

time ./glua-without-context.exe fib.lua
9227465
0.01s user 0.01s system 0% cpu 5.306 total
```

#### Sharing Lua byte code between LStates

Calling `DoFile` will load a Lua script, compile it to byte code and run the byte code in a `LState`.

If you have multiple `LStates` which are all required to run the same script, you can share the byte code between them,
which will save on memory.
Sharing byte code is safe as it is read only and cannot be altered by lua scripts.

```go
// CompileLua reads the passed lua file from disk and compiles it.
func CompileLua(filePath string) (*lua.FunctionProto, error) {
    file, err := os.Open(filePath)
    defer file.Close()
    if err != nil {
        return nil, err
    }
    reader := bufio.NewReader(file)
    chunk, err := parse.Parse(reader, filePath)
    if err != nil {
        return nil, err
    }
    proto, err := lua.Compile(chunk, filePath)
    if err != nil {
        return nil, err
    }
    return proto, nil
}

// DoCompiledFile takes a FunctionProto, as returned by CompileLua, and runs it in the LState. It is equivalent
// to calling DoFile on the LState with the original source file.
func DoCompiledFile(L *lua.LState, proto *lua.FunctionProto) error {
    lfunc := L.NewFunctionFromProto(proto)
    L.Push(lfunc)
    return L.PCall(0, lua.MultRet, nil)
}

// Example shows how to share the compiled byte code from a lua script between multiple VMs.
func Example() {
    codeToShare, err := CompileLua("mylua.lua")
    if err != nil {
        panic(err)
    }
    a := lua.NewState()
    b := lua.NewState()
    c := lua.NewState()
    DoCompiledFile(a, codeToShare)
    DoCompiledFile(b, codeToShare)
    DoCompiledFile(c, codeToShare)
}
```

#### 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.

```go
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)](http://godoc.org/github.com/yuin/gopher-lua) 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:

```lua
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
```

```lua
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.

```go
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` .

```go
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.

### Unsupported functions

- `string.dump`
- `os.setlocale`
- `lua_Debug.namewhat`
- `package.loadlib`
- debug hooks

### Miscellaneous notes

- `collectgarbage` does not take any arguments and runs the garbage collector for the entire Go program.
- `file:setvbuf` does not support a line buffering.
- Daylight saving time is not supported.
- GopherLua has a function to set an environment variable : `os.setenv(name, value)`
- GopherLua support `goto` and `::label::` statement in Lua5.2.
    - `goto` is a keyword and not a valid variable name.

## Standalone interpreter

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

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

`glua` has same options as `lua` .

## How to Contribute

See [Guidelines for contributors](https://github.com/yuin/gopher-lua/tree/master/.github/CONTRIBUTING.md) .

## Libraries for GopherLua

- [gopher-luar](https://github.com/layeh/gopher-luar) : Simplifies data passing to and from gopher-lua
- [gluamapper](https://github.com/yuin/gluamapper) : Mapping a Lua table to a Go struct
- [gluare](https://github.com/yuin/gluare) : Regular expressions for gopher-lua
- [gluahttp](https://github.com/cjoudrey/gluahttp) : HTTP request module for gopher-lua
- [gopher-json](https://github.com/layeh/gopher-json) : A simple JSON encoder/decoder for gopher-lua
- [gluayaml](https://github.com/kohkimakimoto/gluayaml) : Yaml parser for gopher-lua
- [glua-lfs](https://github.com/layeh/gopher-lfs) : Partially implements the luafilesystem module for gopher-lua
- [gluaurl](https://github.com/cjoudrey/gluaurl) : A url parser/builder module for gopher-lua
- [gluahttpscrape](https://github.com/felipejfc/gluahttpscrape) : A simple HTML scraper module for gopher-lua
- [gluaxmlpath](https://github.com/ailncode/gluaxmlpath) : An xmlpath module for gopher-lua
- [gmoonscript](https://github.com/rucuriousyet/gmoonscript) : Moonscript Compiler for the Gopher Lua VM
- [loguago](https://github.com/rucuriousyet/loguago) : Zerolog wrapper for Gopher-Lua
- [gluabit32](https://github.com/PeerDB-io/gluabit32) : [Port of Lua 5.2 bit32](https://www.lua.org/manual/5.2/manual.html#6.7)
- [gluacrypto](https://github.com/tengattack/gluacrypto) : A native Go implementation of crypto library for the GopherLua VM.
- [gluasql](https://github.com/tengattack/gluasql) : A native Go implementation of SQL client for the GopherLua VM.
- [purr](https://github.com/leyafo/purr) : A http mock testing tool.
- [vadv/gopher-lua-libs](https://github.com/vadv/gopher-lua-libs) : Some usefull libraries for GopherLua VM.
- [gluasocket](https://gitlab.com/megalithic-llc/gluasocket) : A native Go implementation of LuaSocket for the GopherLua VM.
- [glua-async](https://github.com/CuberL/glua-async) : An async/await implement for gopher-lua.
- [gopherlua-debugger](https://github.com/edolphin-ydf/gopherlua-debugger) : A debugger for gopher-lua
- [gluamahonia](https://github.com/super1207/gluamahonia) : An encoding converter for gopher-lua
- [awesome-gopher-lua](https://github.com/Root-lee/awesome-gopher-lua) : Collections of awesome libraries for GopherLua.

## Donation

BTC: 1NEDSyUmo4SMTDP83JJQSWi1MvQUGGNMZB

## License

MIT

## Author

Yusuke Inuzuka
