haskell Lazy SmallCheck is a library for exhaustive, demand-driven testing of Haskell programs. It is based on the idea that if a property holds for a partially-defined input then it must also hold for all fully-defined refinements of the that input. Compared to ``eager'' input generation as in SmallCheck, Lazy SmallCheck may require significantly fewer test-cases to verify a property for all inputs up to a given depth.