The exenum library offers constructors to build enumerations for datatypes, that is, functions from (arbitrarily large) integers to values. Such enumerations are typically used for unit testing. The library is efficient: the n-th element of an enumeration is returned without having computed the (n-1) previous elements. Complexity is in log(n), except for some pathological datatypes. See the homepage for details: https://github.com/lebotlan/ocaml-exenum Inspired by Feat: Functional Enumeration of Algebraic Types, by Duregard, Jansson, Wang, Chalmers University.
As an example, consider the following datatype: type term = Var of string | App of term * term | Lambda of string * term
Using exenum, one may easily generate zillions of different lambda-terms. In our specific example, term number 2000000000000 happens to be ((((x v) (fun u -> y)) ((fun u -> y) (fun y -> y))) (((x v) (fun u -> v)) (fun u -> y)))