]> Ker Lutyn wrote: > I need a dynamically extensible sum type. I can think of three approaches: > > (1) Use polymorphic variants: `Foo of a * b, `Bar of c * d * e, etc I have taken this approach to statically type SQL queries. I have various application modules defining sum types of polymorphic variants such as the following: module Application1 = struct type table = [ | `Table1 | `Table2 | ... | `TableN ] type field = [ | `Field_name1 of foo | `Field_name2 of bar | ... ] let string_of_table = function | `Table1 -> "name_of_table_1" | `Table2 -> "name_of_table_2" ... let string_of_field = function | `Field_name1 (_) -> "field_name_1" ... end Several modules share this common approach. I finally tie everything up in a Db_interface, whose main element is the SQL query generator function. This function takes abstract representation of queries, such as `SELECT(`From_table, field_list, where_conditions, order_by_field_list, group_by_field_list) The names of tables and fields are converted from their variant-constructor representation to their string representation by a function which is defined as the parallelization of all functions of the kind ApplicationN.string_of_table/field. (* In module Db_interface *) let string_of_field = function | #Application1.field as f -> Application1.string_of_field f | #Application2.field as f -> Application2.string_of_field f ... I like this approach, but it requires some source preprocessing to be able to selectively link only some application code. I still haven't gotten to figure out how to get camlp4 to filter out pattern-matching lines depending on what application modules are actually being compiled and linked. > (3) Use thunks: (fun () -> foo a b), (fun () -> bar c d e), etc I would greatly indebted to you if you could explain to me what thunks are. ------------------- To unsubscribe, mail caml-list-request@inria.fr Archives: http://caml.inria.fr Bug reports: http://caml.inria.fr/bin/caml-bugs FAQ: http://caml.inria.fr/FAQ/ Beginner's list: http://groups.yahoo.com/group/ocaml_beginners