What steps will reproduce the problem? 1. Define a simple 'add' function, like the one in this project's primary user guide 2. Use 'T' to give that 'add' function the type definition "add :: Int -> Int -> Int" 3. Call 'add' in any illegal way, e.g., 'add(2, "foo")' or 'add(2, 2.3)'

What is the expected output? What do you see instead? Expected output is a detailed error message describing how the contract was violated.

What I see instead: 'Uncaught TypeError: ~add' and a stacktrace

What version of the product are you using? On what operating system? Ristretto Version: 0.1.1 Operating System: Mac OSX Mountain Lion (10.8.2)

Please provide any additional information below. This library appears to be the best-kept secret of light-weight type checking/contracts for JavaScript. While the error messages are precise, they are not particularly detailed.

For example, for add(2, "foo") I would expect, at a minimum, to see a different error message than I would for add(2, 2.4), especially given the specifics around the IntegerContract. I see that there is a generic fail function defined on Contract and this simply throws a type error with a message of the stringified Label. Perhaps storing some extra information in the Label as checks are made, and then adapting Label.toString to use that data, would be an easy way to get started with better error messages.

As a proof of concept, I've made the following edit to my version of Ristretto to get more detailed messages about contract violations for the IntegerContract. If this approach would be at all desirable, I'm happy to make a proper diff (I've got other edits at the moment for RequireJS compatibility, which I'd need to take out first).

* /Users/semperos/Downloads/ristretto-js/ristretto.js 2012-02-02 21:21:42.000000000 -0500 --- /Users/semperos/dev/.../assets/javascripts/vendor/ristretto.js 2012-12-10 12:53:30.000000000 -0500

* 20,31 ** Authors: Samuel Li and Shane Stephens */

(function(){

function Label(name) { this.name = name; this.polarity = true; ! this.toString = function() { return (this.polarity ? "" : "~") + this.name; } this.complement = function() { var label = new Label(name); label.polarity = !this.polarity; --- 20,41 ---- Authors: Samuel Li and Shane Stephens */

  (function(){

      function Label(name) {
          this.name = name;
          this.polarity = true;

! this.toString = function() { ! var polarity = (this.polarity ? "" : "~"); ! var message = (this.reason ? ("\nReason: " + this.reason) : ""); ! var str = polarity + this.name; ! if (message.length > 0) { ! return str + message; ! } else { ! return str; ! } ! } this.complement = function() { var label = new Label(name); label.polarity = !this.polarity;

* 54,60 ** --- 64,74 ---- this.restrict = function(x) { if (is('Number', x) && Math.round(x) == x) { return x; + } else if (! is('Number', x)) { + this.label.reason = 'The value ' + x + ' is not a Number'; + this.fail(); } else { + this.label.reason = 'The value ' + x + ' is not an Integer'; this.fail(); } }

Diff finished. Mon Dec 10 12:56:04 2012

Taking it a step further, since you already define your own getClass function, you could use that to also return expected/actual:

this.label.reason = 'Value: ' + x; this.label.reason = 'Expected: Number'; this.label.reason += 'Actual: ' + getClass(x);

Which would print out:

Value: foo Expected: Number Actual: String

For something like add(2, "foo").

This would at least work in situations where the failure is in the is check, since it uses getClass directly.