A bidirectional transformation consists of a function get that takes a source (document or value) to a view and a function put that takes an updated view and the original source back to an updated source, governed by certain consistency conditions relating the two functions. Both the database and programming language communities have studied techniques that essentially allow a user to specify only one of get and put and have the other inferred automatically. All approaches so far to this bidirectionalization task have been syntactic in nature, either proposing a domain-specific language with limited expressiveness but built-in (and composable) backward components, or restricting get to a simple syntactic form from which some algorithm can synthesize an appropriate definition for put. Here we present a semantic approach instead. The idea is to take a general-purpose language, Haskell, and write a higher-order function that takes (polymorphic) get-functions as arguments and returns appr...