Clearcut is a library that lets client code use familiar list operations to construct and take apart expressions. However, the exact representation of the expressions does not have to be S-expressions. In particular, we also support concrete syntax trees (CSTs).

The familiar operations are implemented in terms of a small number of basic operations (FIRST, REST, ATOM, NULL, CONS). Each basic operation trampolines to a generic version that dispatches on a CLIENT object, and the class of that object is what determines the real operation to be used. That way, the operations proposed by the library can be used in client code to manipulate expressions, without exposing the representation of the expressions being manipulated. To change from one representation to another, client code simply has to rebind the special variable CLIENT to an object that can handle the new desired representation.