🌴 Kailua

Kailua is an experimental type checker and integrated development environment (IDE) for the Lua programming language (currently only Lua 5.1 is supported).


Installation and Usage

Kailua can be used as a standalone checker or an IDE plugin.

Standalone Checker

To install a standalone checker, install Rust first (1.15 or later required), then type the following:

cargo install -f kailua

(-f will cause the existing installation to be upgraded.)

You can run kailua check <path to the entry point> now.

You can also run kailua check <path to the directory>, if you have kailua.json or .vscode/kailua.json in that directory. The configuration format is described in the later section.

Visual Studio Code

Kailua can be used as an IDE support for Visual Studio Code. Install Kailua by typing ext install kailua from the Quick Launch (Ctrl-P). If you are not on Windows, you should also install the standalone checker as above.

You will see a warning that the configuration file is missing when you open a folder containing Lua codes. You need it for real-time checking.

You can either create .vscode/kailua.json by hand, or search "Kailua" from the Command Palette (Ctrl-Shift-P) to edit one.

The following content is required for .vscode/kailua.json, in case you are editing it by hand:

    "start_path": "<path to the entry point>",

    "preload": {
        // This indicates that we are using Lua 5.1 and all built-in libraries of it.
        "open": ["lua51"],

You need to reload the current window (Ctrl-R or Cmd-R) to apply the configuration.

Your First Kailua Code

Once you've set the entry point, you can write your first Kailua code:

--# open lua51
print('Hello, world!')

If you are using the configuration file, the first code can be made much simpler:

print('Hello, world!')

Play a bit with this code to see which errors Kailua can detect.

Supported IDE Functions

  • Real-time syntax checking (for all files).

  • Real-time type checking, starting from given start path.

  • Auto-completions for names and fields.

  • Help for function signatures.

  • Help for types of most subexpressions on hover.

  • Go to definition for local and global names.

  • Mass renaming of local and global names.

Kailua the Language

Special Comments

Kailua is a subset of valid Lua code---you don't need any transpilation or compilation. The additional annotations are described in special comments:

  • --: <type> describes the type(s) of preceding item(s).

    It is valid anywhere a new name can be possibly defined: local (either individual names or statement), function (arguments), for (individual names) and assignment (either individual names or statement).

    When used right after the name, for the ease of typing, you can put a comma or closing parenthesis before the type like this:

    function f(x, --: integer
               y, --: integer
               z) --: integer
        -- ...

    For the common case of defining multiple names you can put types after the statement. In this case types are delimited by commas.

  • --> <type> describes the type(s) of function returns.

    It is valid only after the closing parenthesis of function arguments. It is valid to put --: (for the last argument) and --> in the same line.

  • --v function(<name>: <type> ...) [--> <type>] describes a function type.

    It is valid before the function keyword (yes, also for anonymous functions). This is equivalent to --: and -->, but much more readable. All names should be identical to the corresponding declarations. Variadic arguments can be written as ...: <type> at the end of arguments. Multiple returns need parentheses.

    The general rule of the thumb is that all functions have to be typed with either --v or --:/--> unless it is obvious from the preceding context. This allows you to write a code like f(function(a, b) ... end), but only when f is known to accept such a function.

  • --v method(<name>: <type> ...) [--> <type>] describes a method type.

    It is same to function, but for declarations like function A:b(...). Kailua tries to infer the type of self, and if it's not possible you should use function A.b(self, ...) and --v function(...) instead for the clarity.

  • --# ... is a special directive for the type checker.

    --# open <built-in library name> loads the corresponding built-in names and also implicitly specifies what language variant is currently in use. The only supported name so far is lua51, for the vanilla Lua 5.1. This is what preload.open configuration options actually do, and you should probably put it to the first non-comment line in the entry point if you don't have those options.

    --# type [local | global] <name> = <type> can be used to declare a type alias. There are three flavors of typa alises: local is locally scoped (much like local statements), global is globally scoped (much like A = ...), and no modifier indicates that the type is exported from the current file and they should be locally visible after require. Only local types can be in the inner scopes. Unlike variable names, inner type names should not overwrite outer names.

    --# assume [global] <name>: <type> overrides the type for given name. The global keyword forces the global assignment, otherwise a new scope is created like local statements. It is useful for sidestepping the checker issue, but it is also highly unsafe. Use at your own risk.

    More directives are likely to come.

The equal kind of special comments can span multiple lines.

--# type Date = {
--#     hour: integer;
--#     min: integer;
--#     sec: integer;
--# }


The following basic types are recognized:

  • nil, boolean (or bool), number, string, function, userdata, thread, table for primitive Lua types.

  • integer (or int) for a check-time integral subset of number. (In the future, in the Lua 5.3 mode or later, it will be also recognized as primitive.)

  • true or false, integer and string literals are valid subtypes of boolean, integer and string, respectively.

  • The table type is divided into four useful cases.

    Importantly, first two cases are not automatically inferred from the use and should be explicitly annotated like local tab = {} --: vector<integer>.

    • vector<T> for a table with consecutive integer keys.

    • map<Key, Value> for a homogeneous associative table.

    • { key1: T1, key2: T2 } for records, whose keys are strings and fixed at the check time. You can use semicolons in place of commas.

      Explicitly declared records are "inextensible" by default, meaning that the list of fields is complete and cannot be altered. You can make it extensible by putting ... at the end of fields; this allows a lazy initialization of records like table.field = 'string'. On the other hands, a normal Lua table is implicitly typed as extensible records, only made inextensible when required.

    • { T1, T2, T3 } for tuples, whose keys are consecutive integers. Otherwise they are similar to records.

  • function(Arg, ...) or function(Arg, ...) --> Ret for functions. Ret can be multiple types, in which case you need parentheses (function(vector<T>, integer) --> (integer, string)). Arguments can be named like function(a: string, b: number).

  • T | T | ... for union types. They are mostly useful for literal types (e.g. "read" | "write" | "execute"). Kailua has very limited support for checking other kinds of union types.

  • any has no type information. --# assume is the only way to make it useful.

  • WHATEVER (note the case) is a hole that the type checker always accepts. map<integer, WHATEVER> and map<WHATEVER, string> are compatible; map<integer, WHATEVER> and map<string, string> are not. As this thwarts the basic of type checking, use at your own risk.

The Kailua types are by default not checked for nil. That is, you can assign nil to integer but you can also add two integers; the valid Kailua code can still result in a runtime error therefore. This restriction was required for making a practical type checker without changing the source programming language.

You can opt in two other nil-handling modes if you need to make it explicit. As they are (transitively) freely assignable, consider them more a machine-readable documentation.

  • T? also accepts nil but it is aware that it can contain nil. Two integer?s cannot be added. It also allows for missing fields and missing arguments (they are not allowed otherwise): the type {a: integer?, b: integer} can contain either {a = 42, b = 54} or {b = 54}, but not {a = 42}.

  • T! guarantees that it cannot contain nil.

Also, the table values are always T or T? (for the obvious reason).

Finally, types for the names and table values can optionally have a const prefix. You cannot modify the innard of const types: map<integer, const vector<string>>. You can still assign to them (otherwise this type won't be useful at all).

Avoiding the type checker

As annotating everything is not practical, Kailua supports two ways to avoid the type checking with more localized guarantees:

  • --v [NO_CHECK] function(...) disables the type checking for the following function.

    Kailua essentially believes the specified function type, which can be no longer omitted.

  • You can override what file to check by having .kailua files.

    When require() was used with a check-time string Kailua makes use of package.path and package.cpath set. For package.path, it will try F.kailua first before reading a file F. For package.cpath, it will always F.kailua as F would be probably binary. (Note that this will normally result in two extensions .lua.kailua unless you have a sole ? in the search paths.)

    .kailua files would frequently use --# assume as you should assume that the original code has given types.

Configuration Format

You can configure the exact behavior of Kailua with kailua.json. It is a JSON with comments (//) and stray comma allowed for convenience:

    // This indicates where to start. This is the only mandatory field in the file.
    // This can be a single string or an array of strings, and in the latter case
    // multiple paths are separately (but possibly parallelly) checked against.
    // Checking sessions do not affect others, but reports are merged.
    "start_path": ["entrypoint.lua", "lib/my_awesome_lib.lua"],

    // These are values for `package.path` and `package.cpath` variables, respectively.
    // They are always relative to the base directory
    // (a directory containing `.vscode` or `kailua.json` whichever being used).
    // Refer to the Lua manual for the exact format.
    // In the configuration one can use a special `{start_dir}` sequence
    // which gets replaced by the directory containing the *current* start path.
    // so if there are two start paths `foo/a.lua` and `bar/b.lua`,
    // the path of `{start_dir}/?.lua` will expand to `foo/?.lua` or `bar/?.lua`
    // for each start path. This is useful when you are working with multiple projects
    // with individual directories, only sharing a portion of common codes.
    // If they are not explicitly set, they are inferred from any assignments to
    // `package.path` and `package.cpath` variables. This can be handy for scripts,
    // but will be cumbersome for most other cases.
    // It should be also noted that any path in `package_cpath` won't be directly
    // read by Kailua; only `.kailua` files associated to them will be read.
    "package_path": "?.lua;contrib/?.lua",
    "package_cpath": "native/?",

    // The preloading options to populate the environment before checking.
    // They are executed in the following order, and in each array, in given order.
    "preload": {
        // A list of `--# open` arguments.
        "open": ["lua51"],
        // A list of `require()` arguments. Affected by `package_*` options.
        "require": ["depA", "depB.core"],