\Ve describe the design of a transaction facilit y for a language that supports higher-order functions.tVe factor transactions into four separable features: persistence, undoability, locking, and threads.Then, relying on function composition, we show how we can put them together again.Our modular approach toward building transactions enables us to construct a model of concurrent, nested, multi threaded transactions, as well as other nontradi tional models where not all features of traditional transactions are present.Key to our approach is the use of higher-order functions to make transactions first-class.Not only do we get clean composability of transactional features, but also we avoid the need to introduce special control General