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0.0.1 • Public • Published


Contracts for higher-order intersection and union.

Getting started

Ensure node and npm are installed and run the following command at the root of the directory:

npm install

To build the project run:

npm run build

this will compile the TypesScript files into dist/. The main file will be ./dist/index.js.

To run the tests:

npm run test


The examples directory contains a series of examples along with commentary. The aim of these files is to give an introduction to using the library, and also discuss some of the details of blame assignment for intersection and union.


Importing the library

Point your client at the dist/index.js file. In many cases it is helpful to create aliase for the type constructors and pre-defined base types.

const contract = require("path to dist/index.js");
const Base = contract.Base;
const Type = contract.Type;

Constructing Types

  • Base Types are constructed from predicates (functions that take a value and return a boolean). For example:
const zeroType = Type.makeBaseType("zero", x => x === 0);

There is one siginifcant caveat to making base types. Blame assignment will be incorrect for base types that apply their argument, for example:

const zeroFunction = Type.makeBaseType("zeroFunction", f => f(0) === 0);

This is due to complexities in intersection and union contracts. Keil and Thiemann [1] present a solution to this problem but it is non-trivial to implement in practice.

  • Function Types are constructed from a sequence of argument types and a return type. For example:
const fnType =[Base.number, zeroType], Base.string);

defines the contract type for the function that accepts two arguments, a number and 0, and returns a string. There is not support for optional or variadic functions, although optional arguments can be implemented using a union type with null and undefined.

Function types do not assert that the value is a function, only that domain are codomain are respected when the value is applied. To create a traditional function contract we can use and.

  • Branching Types are constructed from two types. There are three branching types: and, intersection, and union. For example:
// A type that checks the value is a function, and it respects the type [number,0] -> string.
const aFunctionType = Type.and(Base.function, fnType);
// Creates an intersection of function types, acting like an overloaded function.
const overloaded = Type.intersection(aFunctionType,[Base.string, Base.string], Base.boolean));
// Creates a 'optional' number, something that can be a number or undefined.
const maybeNumber = Type.union(Base.number, Base.undefined);

Applying contracts

The main way to apply a contract is using assert. For example:

const three = contract.assert(3, maybeNumber);

which will apply the maybeNumber contract to 3, returning the result. The assert function can be given a string to identify annotate the blame label:

const maybeThree = contract.assert(undefined, "my blame label", maybeNumber);

Function can be wrapped by applying a function contract:

function foo(x,y) {
    if(typeof x === "string") {
        return x.length > y.length
    return x + y + x;
// Wrap the function
foo = contract.assert(foo, "contract for foo", overloaded);
// Apply the wrapped function
foo("a", "b");
foo(4, 0);

Blame Errors

When a contract is violated a blame error is throw, indicating if the blame was positive (inside the contract), or negative (due to the context of the contract). For example:

foo(4, 1);
Negative blame @ label Symbol(contract for foo)
Reason: Value not of expected type zeroType, received number.
Type: {
  "branch": "intersection",
  "left": {
    "branch": "and",
    "left": "function",
    "right": {
      "args": [
      "ret": "string"
  "right": {
    "args": [
    "ret": "boolean"

[1] Matthias Keil and Peter Thiemann, Blame assignment for higher-order contracts with intersection and union




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