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golua

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A Lua compiler / runtime in Go

// repository documentation

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# GoLua

Implementation of Lua **5.5** in Go with **no third party dependencies**. The
compiler and runtime are complete (including coroutines), the standard Lua
library is mostly implemented.

**Note**: This is the `lua5.5` branch implementing Lua 5.5. For the Lua 5.4 implementation, see the [`lua5.4` branch](https://github.com/arnodel/golua/tree/lua5.4).

- [GoLua](#golua)
	- [Quick start: running golua](#quick-start-running-golua)
		- [Safe execution environment (alpha)](#safe-execution-environment-alpha)
		- [Importing and using Go packages](#importing-and-using-go-packages)
	- [Quick start: embedding golua](#quick-start-embedding-golua)
	- [Quick start: extending golua](#quick-start-extending-golua)
	- [Aim](#aim)
	- [Design constraints](#design-constraints)
	- [Components](#components)
		- [Lexer / Parser](#lexer--parser)
		- [AST → IR Compilation](#ast--ir-compilation)
		- [IR → Code Compilation](#ir--code-compilation)
		- [Runtime](#runtime)
		- [Test Suite](#test-suite)
		- [The "official" Lua Test Suite](#the-official-lua-test-suite)
		- [Standard Library](#standard-library)

## Quick start: running golua

To install, run:

```sh
$ go get github.com/arnodel/golua
```

To run interactively (in a repl):

### Safe execution environment (alpha)

A unique feature of Golua is that you can run code in a safe execution
environment where cpu and memory are restricted.  E.g.

```lua
$ golua -cpulimit=10000000
> while true do end
!!! CPU limit of 10000000 exceeded
Reset limits and continue? [yN] 
```

You can even do this within Lua itself:

```lua
$ golua
> a = "a"
> runtime.callcontext({kill={memory=1000000}}, function() while true do a = a..a end end)
killed
> #a
262144
```

For more details read more [here](quotas.md).

### Importing and using Go packages

You can dynamically _import Go packages_ very easily as long as they are already
downloaded - this include the standard library. Here is an example of running an
http server in the repl:

```lua
$ golua
> go = require('golib')
> http = go.import('net/http')
> http.HandleFunc('/hello/', function(w, r)
|   w.Write('hi there from Lua! You requested ' .. r.URL.Path)
| end)
> http.ListenAndServe(':7777')
```

In another terminal you can do:

```sh
$ curl http://localhost:7777/hello/golua
hi there from Lua! You requested /hello/golua
```

To run a lua file:

```sh
$ golua myfile.lua
```

```sh
cat myfile.lua | golua
```

E.g. if the file `myfile.lua` contains:

```lua
local function counter(start, step)
    return function()
        local val = start
        start = start + step
        return val
    end
end

local nxt = counter(5, 3)
print(nxt(), nxt(), nxt(), nxt())
```

Then:

```sh
$ golua myfile.lua
5	8	11	14
```

Errors produce useful tracebacks, e.g. if the file `err.lua` contains:

```lua
function foo(x)
    print(x)
    error("do not do this")
end

function bar(x)
    print(x)
    foo(x*x)
end

bar(2)
```

Then:

```
$ golua err.lua
2
4
!!! error: do not do this
in function foo (file err.lua:3)
in function bar (file err.lua:8)
in function <main chunk> (file err.lua:11)
```

## Quick start: embedding golua

It's very easy to embed the golua compiler / runtime in a Go program. The example below compiles a lua function, runs it and displays the result.

```golang
	// First we obtain a new Lua runtime which outputs to stdout
	r := rt.New(os.Stdout)

// This is the chunk we want to run.  It returns an adding function.
	source := []byte(`return function(x, y) return x + y end`)

// Compile the chunk. Note that compiling doesn't require a runtime.
	chunk, _ := r.CompileAndLoadLuaChunk("test", source, rt.TableValue(r.GlobalEnv()))

// Run the chunk in the runtime's main thread.  Its output is the Lua adding
	// function.
	add, _ := rt.Call1(r.MainThread(), rt.FunctionValue(chunk))

// Now, run the Lua function in the main thread.
	sum, _ := rt.Call1(r.MainThread(), add, rt.IntValue(40), rt.IntValue(2))

// --> 42
	fmt.Println(sum.AsInt())
```

## Quick start: extending golua

It's also very easy to add write Go functions that can be called from Lua code.
The example below shows how to.

This is the Go function that we are going to call from Lua. Its inputs are:

- `t`: the thread the function is running in.
- `c`: the go continuation that represents the context the function is called in.
  It contains the arguments to the function and the next continuation (the
  one which receives the values computed by this function).

It returns the next continuation on success, else an error.

```golang
func addints(t *rt.Thread, c *rt.GoCont) (rt.Cont, error) {
	var x, y rt.Int

// First check there are two arguments
	err := c.CheckNArgs(2)
	if err == nil {
		// Ok then try to convert the first argument to a lua integer (rt.Int).
		x, err = c.IntArg(0)
	}
	if err == nil {
		// Ok then try to convert the first argument to a lua integer (rt.Int).
		y, err = c.IntArg(1)
	}
	if err != nil {
		// Some error occured, we return it in our context
		return nil, err
	}
	// Arguments parsed!  First get the next continuation.
	next := c.Next()

// Then compute the result and push it to the continuation.
	t.Push1(next, x + y)

// Finally return the next continuation.
	return next, nil

// Note: the last 3 steps could have been written as:
	// return c.PushingNext(x + y), nil
}
```

The code sample below shows how this function can be added to the Lua runtime
environment and demonstrates calling it from Lua.

```golang
	// First we obtain a new Lua runtime which outputs to stdout
	r := rt.New(os.Stdout)

// Load the basic library into the runtime (we need print)
	base.Load(r)

// Then we add our addints function to the global environment of the
	// runtime.
	r.SetEnvGoFunc(r.GlobalEnv(), "addints", addints, 2, false)

// This is the chunk we want to run.  It calls the addints function.
	source := []byte(`print("hello", addints(40, 2))`)

// Compile the chunk.
	chunk, _ := rt.CompileAndLoadLuaChunk("test", source, r.GlobalEnv())

// Run the chunk in the runtime's main thread.  It should output 42!
	_, _ = rt.Call1(r.MainThread(), chunk)
```

You can also make custom libraries and use Go values in Lua (using e.g. the
`runtime.UserData` type). There is an example implementing a `regex` Lua
package that uses Go `regexp.Regexp` in [examples/userdata](examples/userdata)

## Aim

To implememt the Lua programming language in Go, easily embeddable in
Go applications. It should be able to run any pure Lua code

## Design constraints

- clean room implementation: do not look at existing implementations
- self contained: no dependencies
- small: avoid re-implementing features which are already present in
  the Go language or in the standard library (e.g. garbage collection)
- register based VM
- no call stack (continuation passing), tail call optimisation.

## Components

### Lexer / Parser

- The lexer is implemented in the package `scanner`.
- The parser is hand-written and implemented in the `parsing` package.

The Lua 5.5 `global` keyword is treated as a context-sensitive "soft" keyword by
default (matching the reference Lua behavior when `LUA_COMPAT_GLOBAL` is
defined). In this mode the scanner emits `global` as a regular identifier, and
the parser recognizes it as a keyword only at the start of a statement when
followed by a name, `function`, `*`, or `<`. This allows `global` to be used as
a variable name in existing code. The scanner can be configured to treat
`global` as a reserved keyword instead (see Runtime section).

### AST → IR Compilation

The `ast` package defines all the AST nodes.  The `astcomp` package defines a
`Compiler` type that is able to compile an AST to IR, using an instance of
`ir.CodeBuilder`.

The `ir` package defines all the IR instructions and the IR compiler.

### IR → Code Compilation

The runtime bytecode is defined in the `code` package. The `ircomp` package
defines a `ConstantCompiler` type that is able to compile IR code to runtime
bytecode, using an instance of `code.Builder`.

### Runtime

The runtime is implemented in the `runtime` package. This defines a
`Runtime` type which contains the global state of a runtime, a
`Thread` type which can run a continuation, can yield and can be
resumed, the various runtime data types (e.g. `String`, `Int`...). The
bytecode interpreter is implemented in the `RunInThread` method of the
`LuaCont` data type.

The `global` keyword can be made reserved (disallowing its use as a variable or
function name) with the `WithReservedGlobal` runtime option. This propagates to
`load()` and all other compilation paths automatically.

```go
r := rt.New(os.Stdout, rt.WithReservedGlobal())
```

On the command line, pass the `-G` flag:

```sh
$ golua -G myfile.lua
```

### Test Suite

There is a framework for running lua tests in the package `luatesting`. In the
various Go packages, if there is a `lua` directory, each `.lua` file is a test.
Expected output is specified in the file as comments of a special form, starting
with `-->`:

```lua
print(1 + 2)
--> =3
-- "=" means match literally the output line

print("ababab")
--> ~^(ab)*$
-- "~" means match with a regexp (syntax is go regexp)
```

Most of the code is covered with such Lua tests. Specific packages or functions
are covered with Go tests.

### The "official" Lua Test Suite

Lua provides test suites for each version (https://www.lua.org/tests/).

The `lua5.4` branch passes an adapted version of the 5.4.3 test suite, available
[here](https://github.com/arnodel/golua-tests/pull/3). The adaptations are shown
as a PR so the differences from the original suite can be seen easily.

The `lua5.5` branch (this branch) passes an adapted version of the Lua 5.5 test
suite, available on the
[`golua-5.5` branch](https://github.com/arnodel/golua-tests/tree/golua-5.5).
Some tests are skipped or adapted because they are C-specific or verify
implementation details of the reference C implementation rather than language
semantics. See [CONFORMANCE.md](CONFORMANCE.md) for a detailed list of known
behavioral differences and their status.

### Standard Library

The `lib` directory contains a number of package, each implementing a
lua library.

- `base`: basic library. It is complete.
- `coroutine`: the coroutine library, which is done.
- `packagelib`: the package library. It is able to load lua modules
  but not "native" modules, which would be written in Go. Obviously
  this is not part of the official Lua specification. Perhaps using
  the plugin mechanism (https://golang.org/pkg/plugin/) would be a way
  of doing it. I have no plan to support Lua C modules!
- `stringlib`: the string library. It is complete.
- `mathlib`: the math library, It is complete.
- `tablelib`: the table library. It is complete.
- `iolib`: the io library. It is complete.
- `utf8lib`: the utf8 library. It is complete.
- `debug`: partially implemented (mainly to pass the lua test suite). The
  `getupvalue`, `setupvalue`, `upvalueid`, `upvaluejoin`, `setmetatable`,
  functions are implemented fully. The `getinfo` function is partially
  implemented.  The `traceback` function is implemented but its output is
  different from the C Lua implementation.  The `sethook` and `gethook` values
  are implemented - line hooks may not be as accurate as for C Lua.
- `os` package is almost complete - `exit` doesn't support "closing" the Lua
  state (need to figure out what it means.)

golua

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