The Cratera Programming Language
================================
Cratera is a language very similar to Lua, (in fact it's a fork of Lua's syntax) and as such most of the Lua manual applies to it, with the following differences.
Trait method call syntax
------------------------
Cratera adds the following syntax on top of Lua, called a "trait method call":
mytable:[MyTrait].myfunction(myargument)
which is equivalent to:
mytable[MyTrait].myfunction(mytable, myargument)
This syntax sugar is similar to the "methods" syntax sugar ([Lua 5.3 §3.4.10](http://www.lua.org/manual/5.3/manual.html#3.4.10),
[Lua 5.2 §3.4.9](http://www.lua.org/manual/5.2/manual.html#3.4.9), [Lua 5.1 §2.5.8](http://www.lua.org/manual/5.1/manual.html#2.5.8)),
and, indeed, `mytable` is only evaluated once.
Traits
------
(TODO)
Module layout
-------------
Cratera uses a module system that integrates with Lua's, effectively making Cratera modules interchangeable with Lua modules. However, the file layout is a bit different, particularly for submodules.
For starters, Cratera module filenames end in `.cratera`, so the call:
local mymodule = require "mymodule"
will look for the file `mymodule.cratera`. But when requiring a submodule, the call:
local submodule = require "mymodule.submodule"
will look for the file `mymodule.cratera.d/submodule.cratera`. This uses the old UNIX convention of stuffing subfiles of a file `filename` into a directory `filename.d`, for example `profile.d` or `ld.so.conf.d`.
Finally, the Cratera path can be set independently of the Lua path, with the environment variable `CRATERA_PATH` or the Cratera global `craterapath`.
Lua interoperability
--------------------
### Bootstrapping
Cratera can be bootstrapped from Lua by `require "cratera.bootstrap"`, this initializes the Cratera module system and the Cratera standard library. From here, it's possible to `require` Cratera modules or `load` Cratera scripts:
local cratera = require "cratera.bootstrap"
local cratera_module = require "cratera_module"
-- N.B. cratera.loadfile for Cratera scripts, loadfile for regular Lua scripts
return cratera.loadfile("myscript.cratera")(...)
However, the provided bootstrap module may not be suitable for all Lua environments. The user is encouraged to bring their own bootstrap.
### Using Lua from Cratera
You can access Lua globals from Cratera using the `lua` table (including `lua.load` - the Lua load function). By default, the Cratera global environment also proxies over to the Lua environment, with a few overrides worth highlighting:
- In Cratera, `load` and `loadfile` refer to the Cratera loader, and by default also set the environment of the loaded function to the Cratera environment.
- Cratera does not expose `dofile` and `dostring` functions, their slots are set to `false`.
- `_VERSION` refers to the Cratera version.
### Using Cratera traits from Lua
Since Cratera is simple syntax sugar on top of Lua, it is possible (tho not recommended) to use Cratera traits from plain Lua, by simply hand-coding the desugaring oneself.
local cratera_obj, CrateraTrait = ...
cratera_obj[CrateraTrait].func(cratera_obj)
Building Cratera
================
Cratera needs to know a few things about the system before it can be used, so the `build.sh` script attempts to provide those things. Most importantly, it needs to know what Lua interpreter to use for the Cratera interpreter, as well as the default Cratera module path.
You can get an overview of these settings and more by running `./build.sh help`. You can override these settings, for example to set a different Lua interpreter, `LUA_INTERPRETER=luajit ./build.sh [command]`.
To build Cratera, run `./build.sh` or `./build.sh build`.
Cratera Compiler
================
(FIXME clean this up)
The files in `src/cratera/` include the Cratera to Lua compiler, as well as support components for the Cratera to Lua compiler, namely a pure-Lua Lua tokenizer and a table-based parser thing.
Usage
-----
To use Cratera Compiler, simply put the cratera directory somewhere in the Lua search path, and `require "cratera"`. It's compatible with Lua 5.1, Lua 5.2, and Lua 5.3.
It returns a table with the following API functions:
### `cratera.load(chunk, [chunkname, ...])`
Loads a cratera chunk.
The definition of this function is the same as the underlying `load` function, except Cratera Compiler accepts `chunk` to be a string even in Lua 5.1.
In reality, Cratera Compiler only handles the `chunk` argument. All other arguments are handed off to the native `load` function. It supports both text and binary chunks,
including when `chunk` is a function. It can also load any valid Lua chunk that would be accepted by the native `load` function, albeit with a significant performance hit.
For the definition of the native `load` function, follow the link that matches your Lua version:
- [Lua 5.1](http://www.lua.org/manual/5.1/manual.html#pdf-load)
- [Lua 5.2](http://www.lua.org/manual/5.2/manual.html#pdf-load)
- [Lua 5.3](http://www.lua.org/manual/5.3/manual.html#pdf-load)
Why not use LPeg?
-----------------
The use of a custom parsing library boils down to two reasons:
1. LPeg can't stream or produce partial outputs. This just makes it difficult to use for making a compiler.
2. LPeg can't process tables. It's still possible to use LPeg to parse table-based structures, but one must serialize them beforehand, which is... far from ideal, to say the least.
License
-------
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Cratera Compiler is released under the AGPLv3:
```text
Cratera Compiler - pure-Lua Cratera-to-Lua transpiler
Copyright (C) 2019 Soni L.
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU Affero General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU Affero General Public License for more details.
You should have received a copy of the GNU Affero General Public License
along with this program. If not, see <https://www.gnu.org/licenses/>.
```
