1Two Ways Software Gets Hard 9Generic Operations 2Contracts as a Design Tool 10Languages for the Domain 3Illegal States 11Evaluators You Can Extend 4Parse, Don't Validate 12Propagators 5Workflows as Transformations 13Unification and Search 6Effects at the Edges 14Where to Draw the Line 7Combinators 15Capstone 8Layering ·Full Contents