In CS theory courses, proofs by reduction are a notorious source of pain for students and instructors alike. Invariably, students use pen and paper to write down reductions that “work” in many but not all cases. When instructors observe that a student’s reduction deviates from the expected one, they have to manually compute a counterexample that exposes the mistake. In other words, reductions are subtle yet, most of the time, unimplemented programs. And for a good reason: there exists no language tailored to reductions. We introduce Karp, a language for programming and testing Karp reductions. Karp combines an array of programming languages techniques: language-oriented programming and macros, solver-aided languages, property testing, higher-order contracts and gradual typing. To validate the correctness of Karp, we prove that its core is well-defined. To validate its pragmatics, we demonstrate that it is expressive and performant enough to handle a diverse set of reduction exercises from a popular algorithms textbook. Finally, we report from a preliminary user study with Karp.