Convenience functions for extracting multiple arguments

[hcarty]

Pulling from rgrinberg/opium#32 (comment) it would be nice to have a function for extracting multiple query parameters with a single call, similar to:

(* only match if all query params are present *)
val get_query_params : Uri.t -> string list -> (string list list) option
(* allow for partial matches of parameters list *)
val get_query_params' : Uri.t -> string list -> (string list option) list

usable as:

match get_query_params uri ["ua"; "xx"; "yyy"] with
| Some [ua; xx; yyy] -> ...
| _ -> ...

or, if you're expecting and want to only accept a single argument for a parameter:

match get_query_params uri ["ua"; "xx"; "yyy"] with
| Some [[ua]; [xx]; yyy] -> ... (* Only match if ua and xx are single-valued, yyy gets everything *)
| _ -> ...

[dsheets]

This is an API to consider for the fabled 2.0 release.

[hcarty]

An mostly untested implementation if someone wants something before 2.0:

let get_query_params uri keys = 
  let values = List.map (fun k -> Uri.get_query_param' uri k) keys in
  let result = 
    List.fold_left ( 
      fun accu mayv ->
        match accu, mayv with
        | None, _ -> None
        | _, None -> None
        | Some l, Some v -> Some (v :: l)
    ) (Some []) values
  in
  match result with
  | None -> None
  | Some l -> Some (List.rev l)

let get_query_params' uri keys = 
  List.map (fun k -> Uri.get_query_param' uri k) keys