What? The way the Arbitrary class works is only possible thanks to typeclasses. Without typeclasses, you would have to explicitly specify which generator to us

What? The way the Arbitrary class works is only possible thanks to typeclasses. Without typeclasses, you would have to explicitly specify which generator to use. Thanks to the typeclass system, Haskell can just infer this. On the other hand, none of the Arbitrary machinery is particularly tied to algebraic data types per se. It would work as well if you had OO style constructors instead. Just take a look at the OCaml version of QuickCheck to see what I mean. OCaml obviously has algebraic data types, but it's still far more awkward to use QuickCheck there.