Command Line Tools
Two tools are installed with MoonScript,
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
If you want to disable error rewriting, you can pass the
-d flag. A full list of flags can be seen by passing the
Runtime errors are given special attention when running code using the
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
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.
moon lets you run a MoonScript file while keeping track of which lines
are executed with the
For example, consider the following
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" 4| * 5| second = -> 6| print "world" 7| * 8| first! 9|
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
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
-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
A full list of flags can be seen by passing the
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
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)
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
./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
setup) to make writing
It would be nice if we could allow all of those global functions to be called
.moon files located in the
spec/ directory. We can do that by creating
lint_config is a regular MoonScript or Lua file that provides configuration
for the linter. One of those settings is
To create a configuration for Busted we might do something like this:
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.
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:
The linter will identify the problem:
./lint_example.moon 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: