MoonScript 0.8.0 - Compiler API

MoonScript Compiler API

Autocompiling with the moonscript Module

After installing MoonScript, you can include the moonscript module to make any Lua script MoonScript aware.

require "moonscript"

After moonscript is required, Lua’s package loader is updated to search for .moon files on any subsequent calls to require. The search path for .moon files is based on the current package.path value in Lua when moonscript is required. Any search paths in package.path ending in .lua are copied, rewritten to end in .moon, and then inserted in package.moonpath.

The moonloader is the function that is responsible for searching package.moonpath for a file available to be included. It is inserted in the second position of the package.loaders table. This means that a matching .moon file will be loaded over a matching .lua file that has the same base name.

For more information on Lua’s package.loaders see Lua Reference Manual — package.loaders

The moonloader, when finding a valid path to a .moon file, will parse and compile the file in memory. The code is then turned into a function using the built in load function, which is run as the module.

If you are executing MoonScript code with the included moon command line tool then it is not required to include this module before including any other MoonScript modules.

moonscript.base Module

moonscript = require "moonscript.base"

This module contains an assortment of functions for loading and compiling MoonScript code from within Lua.

The module provides load, loadfile, loadstring functions, which are analogous to the similarly named Lua functions. The major difference is that they load MoonScript code instead of Lua code.

moonscript = require "moonscript.base"

fn = moonscript.loadstring 'print "hi!"'
fn!

All of these functions can take an optional last argument, a table of options:

• implicitly_return_root — Setting this to false makes it so the file does not implicitly return its last statement.
• correlate — Setting this to true generates Lua code where the line numbers match the line numbers of the MoonScript source as closely as possible, the same as the --correlate flag of moonc.

moonscript = require "moonscript.base"

fn = moonscript.loadstring "10"
print fn! -- prints "10"

fn = moonscript.loadstring "10", implicitly_return_root: false
print fn! -- prints nothing

One more useful function is provided: to_lua. This function takes a string of MoonScript code and returns the compiled Lua result along with the line mapping table. If there are any errors then nil and the error message are returned.

import to_lua from require "moonscript.base"

lua_code, line_table = to_lua [[
x = 124
print "hello world #{x}"
]]

Similar to the load* functions from above, to_lua can take an optional final argument of a table of options.

The second return value of to_lua is useful if you want to perform line number reversal. It’s a table where the key is a Lua line number and the value is a character offset from the original MoonScript source.

Programmatically Compiling

If you need finer grained control over the compilation process you can use the raw parse and compile modules.

Parsing converts a string of MoonScript into an abstract syntax tree. Compiling converts an abstract syntax tree into a Lua code string.

Knowledge of this API may be useful for creating tools to aid the generation of Lua code from MoonScript code. For example, you could build a macro system by analyzing and manipulating the abstract syntax tree. Be warned though, the format of the abstract syntax tree is undocumented and may change in the future.

Here is a quick example of how you would compile a MoonScript string to a Lua String (This is effectively the same as the to_lua function described above):

parse = require "moonscript.parse"
compile = require "moonscript.compile"

moon_code = [[(-> print "hello world")!]]

tree, err = parse.string moon_code
unless tree
  error "Parse error: " .. err

lua_code, err, pos = compile.tree tree
unless lua_code
  error compile.format_error err, pos, moon_code

-- our code is ready
print lua_code

On a parse failure parse.string returns nil and a multi-line error string of the form <message> at line <n>, column <m>: followed by the offending source line and a ^ position marker.