Generates the implementation of a deserializer for a given record type (or list of label declarations).

The outline of the generated code is:

* create a field_visitor that maps strings and ints to an ad-hoc field polyvar (field name "hello" matches `hello)

* declare field value holders (one 'a option ref per field)

* create a recursive function for consuimng fields one by one, using the visitor to validate them, and directing the deserialization to the right field deserializer

* extract the field value holders and validate that all fields are present

* construct the final result

So for a record like:

ocaml[ type person = { name: string; age: int

The generated code would look like:

ocaml[ let deserialize_person = De.(deserializer @@ fun ctx -> record ctx "person" 2 @@ fun ctx -> let field_visitor = Visitor.make ~visit_string:(fun _ctx str -> match str with | "name" -> Ok `name | "age" -> Ok `age | _ -> Error `invalid_field_type) ~visit_int:(fun _ctx int -> match int with | 0 -> Ok `name | 1 -> Ok `age | _ -> Error `invalid_field_type) () in let name = ref None in let age = ref None in let rec read_fields () = let* tag = next_field ctx field_visitor in match tag with | Some `name -> let* v = field ctx "name" string in name := Some v; read_fields () | Some `age -> let* v = field ctx "age" int in age := Some v; read_fields () | None -> Ok () in let* () = read_fields () in let* name = Option.to_result ~none:(`Msg "missing field 'name'") name in let* age = Option.to_result ~none:(`Msg "missing field 'age'") age in Ok {name;age )