GEXL is a general expression library providing core primitives for building and processing expression trees. I had abandoned it awhile ago because I quickly ran into a serious, well-known issue: The Expression Problem . Basically, the expression problem boils down to the fact that typical programming solutions are extensible in only a single direction: either the data types or the operations defined on them, but not both, can be extended without modifying existing code. It takes some sophisticated type machinery to safely solve the expression problem, and various languages, including OCaml , Haskell , Beta/gBeta , and Scala , have acceptable solutions. Languages supporting multimethods support less type safe extension , so I exclude them here (although Nice might do it properly). So GEXL, which aims to provide both data and their operations as data and visitors respectively, runs smack into the expression problem. Fortunately, Mads Torgersen's paper, The Expression Problem Revisi...