dak

Static type checker using Hindley–Milner type signatures

Install

$ npm install --global dak

Usage

$ dak --help

Usage
$ dak [options] file.js [file.js] [dir]

Options
  --throw  Throws an error

Examples

hello.js file:

// @dak
 
// @type greet :: String -> String
function greet(person) {
    return "Hello, " + person;
}
 
var user = true;
 
greet(user);
 
// @type times10 :: Number -> Number
function times10(x) {
  return x * 10;
}
 
times10('Hello, world!');
 
// @type reducer :: {count: Number} -> {type: String} -> {count: Number}
function reducer(state, action) {
  switch (action.type) {
    case 'INCREMENT':
      return {count: state.count + 1};
    case 'DECREMENT':
      return {count: state.count - 1};
    default:
      return state;
  }
}
 
reducer({count: 0}, {type: 1});

Console output:

$ dak hello.js

-- TYPE MISMATCH -----------------------------------------------------------
2|	// @type  greet :: String -> String
3|	function greet(person) {

> 9|	greet(user);

The type annotation for greet says it is a:

    String

But the argument pass to the function call (shown above) is a:

    Boolean

-- TYPE MISMATCH -----------------------------------------------------------
11|	// @type  times10 :: Number -> Number
12|	function times10(x) {

> 16|	times10('Hello, world!');

The type annotation for times10 says it is a:

    Number

But the argument pass to the function call (shown above) is a:

    String

-- TYPE MISMATCH -----------------------------------------------------------
18|	// @type  reducer :: {count: Number} -> {type: String} -> {count: Number}
19|	function reducer(state, action) {

> 30|	reducer({count: 0}, {type: 1});

The type annotation for reducer says it is a:

    {type: String}

But the argument pass to the function call (shown above) is a:

    Number for the key "type"

Found 3 errors in hello.js

License

MIT