MoonScript 0.5.0 - Command Line Tools

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Command Line Tools

Two tools are installed with MoonScript, moon and moonc.

moonc is for compiling MoonScript code to Lua. moon is for running MoonScript code directly.


moon can be used to run MoonScript files directly from the command line, without needing a separate compile step. All MoonScript files are compiled in memory as they are executed.

$ moon my_script.moon

Any MoonScript files that are required will also be compiled on demand as they are loaded.

When an error occurs during runtime, the stack trace is rewritten to give line numbers from the original .moon file.

If you want to disable error rewriting, you can pass the -d flag. A full list of flags can be seen by passing the -h or --help flag.

Error Rewriting

Runtime errors are given special attention when running code using the moon command line tool. Because code is written in MoonScript but executed as Lua, errors that happen during runtime report Lua line numbers. This can make debugging less than ideal.

In order to solve this problem MoonScript builds up a table of line number mappings, allowing the runtime to calculate what line of MoonScript generated the line of Lua that triggered the error.

Consider the following file with a bug (note the invalid z variable):

add_numbers = (x,y) -> x + z  -- 1
print add_numbers 10,0        -- 2

The following error is generated:

moon: scrap.moon:1(3): attempt to perform arithmetic on global 'z' (a nil value)
stack traceback:
  scrap.moon:1(3): in function 'add_numbers'
  scrap.moon:2(5): in main chunk

Notice how next to the file name there are two numbers. The first number is the rewritten line number. The number in the parentheses is the original Lua line number.

The error in this example is being reported on line 1 of the moon file, which corresponds to line 3 of the generated Lua code. The entire stack trace is rewritten in addition to the error message.

Code Coverage

moon lets you run a MoonScript file while keeping track of which lines are executed with the -c flag.

For example, consider the following .moon file:

-- test.moon
first = ->
  print "hello"

second = ->
  print "world"


We can execute and get a glance of which lines ran:

$ moon -c test.moon

The following output is produced:

------| @cool.moon
     1| -- test.moon
*    2| first = ->
*    3|   print "hello"
*    5| second = ->
     6|   print "world"
*    8| first!

The star next to the line means that it was executed. Blank lines are not considered when running so by default they don’t get marked as executed.


moonc is used for transforming MoonScript files into Lua files. It takes a list of files, compiles them all, and creates the associated .lua files in the same directories.

$ moonc my_script1.moon my_script2.moon ...

You can control where the compiled files are put using the -t flag, followed by a directory.

moonc can also take a directory as an argument, and it will recursively scan for all MoonScript files and compile them.

moonc can write to standard out by passing the -p flag.

The -w flag can be used to enable watch mode. moonc will stay running, and watch for changes to the input files. If any of them change then they will be compiled automatically.

A full list of flags can be seen by passing the -h or --help flag.


moonc contains a lint tool for statically detecting potential problems with code. The linter has two tests: detects accessed global variables, detect unused declared variables.

You can execute the linter with the -l flag. When the linting flag is provided only linting takes place and no compiled code is generated.

moonc -l file1.moon file2.moon

Like when compiling, you can also pass a directory as a command line argument to recursively process all the .moon files.

Global Variable Checking

It’s considered good practice to avoid using global variables and create local variables for all the values referenced. A good case for not using global variables is that you can analyize the code ahead of time without the need to execute it to find references to undeclared variables.

MoonScript makes it difficult to declare global variables by forcing you to be explicit with the export keyword, so it’s a good candidate for doing this kind of linting.

Consider the following program with a typo: (my_number is spelled wrong as my_nmuber in the function)

-- lint_example.moon
my_number = 1234

some_function = ->
  -- a contrived example with a small chance to pass
  if math.random() < 0.01
    my_nmuber + 10


Although there is a bug in this code, it rarely happens during execution. It’s more likely to be missed during development and cause problems in the future.

Running the linter immediately identifies the problem:

$ moonc -l lint_example.moon



line 7: accessing global `my_nmuber`
>       my_nmuber + 10

Global Variable Whitelist

In most circumstances it’s impossible to avoid using some global variables. For example, to access any of the built in modules or functions you typically access them globally.

For this reason a global variable whitelist is used. It’s a list of global variables that are allowed to be used. A default whitelist is provided that contains all of Lua’s built in functions and modules.

You can create your own entires in the whitelist as well. For example, the testing framework Busted uses a collection of global functions (like describe, before_each, setup) to make writing tests easy.

It would be nice if we could allow all of those global functions to be called for .moon files located in the spec/ directory. We can do that by creating a lint_config file.

lint_config is a regular MoonScript or Lua file that provides configuration for the linter. One of those settings is whitelist_globals.

To create a configuration for Busted we might do something like this:

-- lint_config.moon
  whitelist_globals: {
    ["spec/"]: {
      "it", "describe", "setup", "teardown",
      "before_each", "after_each", "pending"

Compile the file:

$ moonc lint_config.moon

Then run the linter on your entire project:

$ moonc -l .

The whitelisted global references in spec/ will no longer raise notices.

The whitelist_globals property of the lint_config is a table where the keys are Lua patterns that match file names, and the values are an array of globals that are allowed.

Multiple patterns in whitelist_globals can match a single file, the union of the allowed globals will be used when linting that file.

Unused Variable Assigns

Sometimes when debugging, refactoring, or just developing, you might leave behind stray assignments that aren’t actually necessary for the execution of your code. It’s good practice to clean them up to avoid any potential confusion they might cause.

The unused assignment detector keeps track of any variables that are assigned, and if they aren’t accessed in within their available scope, they are reported as an error.

Given the following code:

a, b = 1, 2
print "hello", a

The linter will identify the problem:


line 1: assigned but unused `b`
> a, b = 1, 2

Sometimes you need a name to assign to even though you know it will never be accessed. The linter will treat _ as a special name that’s allowed to be written to but never accessed:

The following code would not produce any lint errors:

item = {123, "shoe", "brown", 123}
_, name, _, count = unpack item
print name, count