Just being statically typed helps, there's nothing particularly special about Haskell that makes QuickCheck possible.

Have you read the source to Haskell's QuickCheck?

It can automatically derive automatic randomized generation of your data-types FOR you. No other generative testing library (such as for Clojure or Erlang) can do this.

Further, the random data generation can be customized using the functional combinator API.

Haskell QuickCheck's source code is a compelling example of what can be done with static types and typeclasses in Haskell.

> It can automatically derive automatic randomized generation of your data-types FOR you.

There's nothing about Haskell (or some extension to Haskell, I presume) that makes this possible that wouldn't make this possible in other languages like C#, Java, or, with a bit more verbosity, C++. If I'm wrong, please say what it is, I'd love to know.

You're wrong and you should look into how typeclasses, derivation, SYB, and GHC generics work.

That they aren't doing it is reason enough. Clojure and Erlang's QuickCheck libraries would derive Arbitrary if they could. :)

I already know how they work. Still waiting for an explanation of what they can do that you couldn't do in Java or C# or (with some verbosity) C++.

Edit: For example, to answer this question, you could define a type and explain how Haskell lets you automatically generate arbitrary instances of that type, where Java or C#, being as limited as you claim them to be, would be incapable of doing so. Edit: Also, Clojure and Erlang are irrelevant. My comment you originally replied to said that static typing was what makes QuickCheck possible.

Let's use circles and rectangels to motivate some analysis. In Haskell you might define:

  data Point = Point { x :: Float, y :: Float }
    deriving(Generic)
  data Shape =
    Circle { center :: Point, radius :: Float }
    | Rectangle { topLeft :: Point, bottomRight :: Point }
      deriving(Generic)

In C# you could similarly define

  class Point {
    public float _x, _y;
    public Point(float x, float y) {
      _x = x;
      _y = y;
    }
  }

  interface Shape {}
  class Circle {
    public Point _center;
    public float _radius;
    public Circle(Point center, float radius) {
      _center = center
      _radius = radius
    }
  }
  // etc, for Rectangle ...

Let's start from the C# end of things. Let's say we want to do some tests on random Shapes and that we want to get an instance of a random shape by calling Generate.random<Shape>(). There are a couple of different ways to do this. We could explicitly define a RandomShapeFactory and a RandomPointFactory and then wire those up into the Generate class. Maybe we could use reflection to dynamically generate instances of Points and Shapes from their definitions at runtime. It would probably end up looking like some combination of the two.

Haskell doesn't let you magically generate random instances of Shape. The cool thing it gives is the "deriving(Generic)" part. Effectively, this causes the compiler to generate a description of Point and the Shape-Circle-Rectangle set of types. In many ways, it's like C#'s reflection. The QuickCheck library uses this reflection-like definition to generate random instances.

The core functionality isn't very different, but Haskell's approach has a number of advantages. Most importantly, it's type-checked at compile-time. In the C# version, I could call Generate.random<Cat> and the code would blow up at run-time, whereas we would just get a compiler error in Haskell. The other major advantage in this case is the reduction of boiler-plate. There's no need to register factory instances inside of some singleton object at startup, the type system can infer which Generic components are needed where.

C# and Haskell are both turning complete, so you can do anything in either language. Haskell makes certain types of programming more enjoyable by reducing boiler-plate and preventing annoying runtime errors.

EDIT - code formatting

> There's no need to register factory instances inside of some singleton object at startup, the type system can infer which Generic components are needed where.

You would only need to do that if you wanted to define custom behavior, instead of automatically derived behavior, and registering a few delegates is the same amount of work as manually implementing an instance of Arbitrary and CoArbitrary.

Yep. The only really-real difference is compile time checking.

I'm interested in what you think of ScalaCheck[1]. It looks like at least someone is doing it.

[1] http://scalacheck.org/

Based on the documentation:

https://github.com/rickynils/scalacheck/wiki/User-Guide#gene...

It cannot derive random generation behavior (Arbitrary typeclass in Haskell) for arbitrary data types. You have to write the generators yourself using the Generator class.

Which goes back to my point about generics, typeclass instance derivation, and the boilerplate using Haskell kills dead.

Hmm, I think you could do something similar to QuickCheck using C# generics. The biggest problem you'd run into is probably the same as Haskell does (the expression problem). You can get some ideas from http://www.daimi.au.dk/~madst/ecoop04/index.html.

Oh and you could probably always fall back to reflection and attributes, but that's just kind of cheating isn't it? ;)

>Hmm, I think you could do something similar to QuickCheck using C# generics

You can't do this. Quick check can generate type classes instances, for example, for tuples, functions, etc. This is impossible in language like C#.

There are certainly cases which aren't expressible(things which require kinding, for example) but your examples aren't a problem. Both tuples and functions can be represented using System F, and thus can be completely transposed into C# generics.

I believe you don't understand C# generics and you should look closer at the fundamental type calculus.

No it's not. The feature C# calls reflection lets you inspect types and generate them automatically.

If we use reflection, we might know about error only in runtime. What haskell does is guaranteed to work at compile time. That's why it's so cool.

QuickCheck doesn't do anything interesting at compile time.

"What haskell does is guaranteed at compile time to work."

Immutability is good but far from enough.