Type inference and checker for JavaScript (experimental)

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This package is not currently in any snapshots. If you're interested in using it, we recommend adding it to Stackage Nightly. Doing so will make builds more reliable, and allow to host generated Haddocks.

GPL-2 licensed by Noam Lewis


Static type inference for JavaScript. (In early development.)

See the intro blog post for a short discussion comparing infernu to other type checkers.

(Formerly known as Inferno / Safe JS / SJS)


  • Full type inference: no type annotations necessary.
  • Parametric polymorphism (aka "generics"), based on Hindley-Milner type inference.
  • Row-type polymorphism, otherwise known as "static duck typing".
  • Simple type classes (which allow for example correct support of JS + and [] operators).
  • Recursive types for true representation of object-oriented methods.
  • Correct handling of JS's this dynamic scoping rules.

For more information see Infernu's Type System.


Quick and Dirty

git clone
cd infernu/
cabal install

Usage: see infernu --help

Quick example usage:

echo 'function getLength(x) { return x.length; }' > getLength.js

infernu getLength.js


    //       getLength : a.({length: b, ..c} -> b)
    function getLength(x) { return x.length; }

A bit more detailed instructions

  1. Install Haskell's cabal package manager. See for some installation options. On ubuntu, I recommend using Herbert V. Riedel's ppa.
  2. Clone this repository.

Then run:

cabal update
cd infernu
cabal install

The infernu executable will be installed to your ~/.cabal/bin. You may want to add it to your PATH.

If you have trouble in the last command due to package incompatibilities, use a cabal sandbox:

cd infernu
cabal sandbox init
cabal install

The infernu executable will be placed in infernu/.cabal-sandbox/bin




var num = 2;
var arrNums = [num, num];

Infernu infers:

//  num : Number
//  arrNums : [Number]

That is, an array of numbers.


var obj = { something: 'hi', value: num };

Inferred type:

//  obj : {something: String,
           value: Number}

That is, an object with two properties: 'something', of type string, and 'value' of type number.

Functions and this

In JS, this is one truly awful part. this is a dynamically scoped variable that takes on values depending on how the current function was invoked. Infernu knows about this (pun intended) and infers types for functions indicating what this must be.

For example:

function useThisData() {
    return + 3;

Infernu infers:

//       useThisData : {data: Number, ..a}.(() -> Number)

In words: a function which expects this to be an object with at least one property, "data" of type Number. It takes no arguments (hence the empty ()). It returns a Number.

If we call a function that needs this incorrectly, Infernu will be angry:

Error: Could not unify:
    {data: Number, ..a}

Because we called useThisData without a preceding object property access (e.g. obj.useThisData), it will get undefined for this. Infernu is telling us that our expected type for this is not unifiable with the type undefined.


Given the following function:

function makeData(x) {
    return {data: x};

Infernu infers the following type:

a.(b -> {data: b})

In words: A function that takes anything for its this, and an argument of any type b. It returns an object containing a single field, data of the same type b as the argument.

Row-type polymorphism (static duck typing)

Given the following function:

function getData(obj) {

Infernu infers:

h.({data: i, ..j} -> i)

In words: a function taking any type h for this, and a parameter that contains at least one property, named "data" that has some type i (could be any type). The function returns the same type i as the data property.

Type Classes

See here for more about Infernu's type classes.

The basic example is for the + operator:

function add(x,y) { return x + y; }

The type for add is inferred to be:

//       add : Plus b => a.((b, b) -> b)

Meaning: given any type a that is an instance of the Plus type class, the function takes two as and returns an a.

The two instances of Plus currently defined are the types Number and String.


  • [ ] consider adding sum types with guards as pattern matchers. required because some functions, like array index access, can return 'undefined' (e.g. if index is out of range)
  • [ ] allow empty var decls (use first assignment as starting point for types) - how to prevent uninitialized variable issues?
  • [ ] allow defining constructor-object properties using the notation obj.prototype.something = ...
  • [ ] find a reasonable solution for optional parameters - perhaps using an implicit "Maybe"-like type or implicit type unions, and require guards?
  • [ ] when concluding that two recursive types are equivalent, use that information to simplify the resulting types (perhaps using the simpler of the two everywhere)
  • [ ] BUG: top-level type of naked object {a:3} isn't shown unless it is wrapped in a paren ({a:3}).
  • [ ] support arguments (a tuple?) and function bind
  • [ ] Should we treat functions as objects with properties? the only properties they have are: length (very weird! we might as well leave it out), and call/bind/apply (which need special handling)


  • [ ] type annotations
  • [ ] add support for CommonJS modules
  • [ ] deal better with inferred polymorphic object properties - requires full rank-n unification
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