Programs such as simulators and fuzz testers often use randomness to walk through a large state space in search of interesting paths or outcomes. These explorations can be made more efficient by employing heuristics that “zero-in” on paths through the state space that are more likely to lead to interesting solutions. Given one path that exhibits a desired property, it may be beneficial to generate and explore similar paths to determine if they produce similarly interesting results. When the random decisions made during this path exploration can be manipulated in such a way that they correspond to discrete structural changes in the result, we call it parametric randomness.

Many programming languages, including Racket, provide only simple randomness primitives, making the implementation of parametric randomness somewhat difficult. To address this deficiency, we present Clotho: a Racket library for parametric randomness, designed to be both easy to use and flexible. Clotho supports multiple strategies for using parametric randomness in Racket applications without hassle.