What is contract.js?

Contract.js is a JavaScript translation of [Contracts as Pairs of Projections] 1.

How does it work?

1. Define some predicates

function evenp (x) { return (x % 2) === 0 };
function oddp  (x) { return (x % 2) === 1 };

2. Define some contracts

oddin_oddout = ho(flat(oddp), flat(oddp));
oddin_oddout_evenout = ho(ho(flat(oddp), flat(oddp)), flat(evenp));

3. Define some guarded procedures

Note: the callee is called "server", the caller "client".

p1 = guard(oddin_oddout,
           function (y) { return y },
           "server", "client");
p2 = guard(oddin_oddout,
           function (y) { return y * y - y },
           "server", "client");
p3 = guard(oddin_oddout_evenout,
           function (f) { return f(1) },
           "server", "client");

4. Call the procedures

p1(2); // Error: client violates the contract
p2(3); // Error: server violates the contract
p3(function (x) { return x + 2 }); // Error: server violates the contract

Future direction of contract.js?

It would be nice to have a contract checking library à la [Racket's contract system] 3.