Type-safe update programming

Many software maintenance problems are caused by using text editors to change programs. A more systematic and reliable way of performing program updates is to express changes with an update language. In particular, updates should preserve the syntax- and type-correctness of the transformed object programs. We describe an update calculus that can be used to update lambdacalculus programs. We develop a type system for the update language that infers the possible type changes that can be caused by an update program. We demonstrate that type-safe update programs that fulfill certain structural constraints preserve the type-correctness of lambda terms.