Generating dependent pairs

[benjub]

I have had several times to generate dependent pairs in my property checks: for instance, pairs formed of an array and a valid index.

To do that, I used the function

let dep_pair a f = a >>= fun x -> tup2 (pure x) (f x)

(where QGheck2.Gen is open), rather than first generating all pairs and then discarding the unwanted ones using assume.

The above example can then be obtained with:

let array_index =
  dep_pair
    (array_size (int_range 1 100) nat_small)
    (fun t -> int_bound (pred (Array.length t)))

My use cases included more complex pairs of structures bound by relations, but I think the above example shows the basic idea.

A function like dep_pair may be worth adding to QCheck2.Gen.