[yawaramin]

In the technique I showed, I am using a difflist to enumerate a set of HTTP POST form fields and their expected types. This difflist type is defined in such a way that one of its type parameters gets inferred as a function type which takes the decoded values in the correct order and returns a record containing the decoded form. Eg from Field.[int "id"; string "name"] we get a type parameter int -> string -> 'a, where 'a is the type of the final decoded form.

This is the kind of real-world usage where difflists or heterogeneous lists shine. The same technique is used by this library to define type-safe GraphQL resolvers: https://github.com/andreas/ocaml-graphql-server