Type that depends on VALUE?

zeroexcuses · May 30, 2023, 4:31am

Can someone explain the OCaml side of the following blob of code:

type _ response =
  | ArrayBuffer : Typed_array.arrayBuffer t Opt.t response
  | Blob : #File.blob t Opt.t response
  | Document : Dom.element Dom.document t Opt.t response
  | JSON : 'a Opt.t response
  | Text : js_string t response
  | Default : string response

https://github.com/ocsigen/js_of_ocaml/blob/master/lib/js_of_ocaml/xmlHttpRequest.ml#L31

I understand the JS side fine. When doing a XMLHttpRequest - Web APIs | MDN
we can set the type of respone we want.

Here is what I do not understand:

perform_raw appears to return something of type:

type 'response generic_http_frame =
  { url : string
  ; code : int
  ; headers : string -> string option
  ; content : 'response
  ; content_xml : unit -> Dom.element Dom.document t option
  }

Here is the crazy thing. This starts to look like dependent types.

When I pass in a VALUE of Default, I get a TYPE of string response
When I pass in a VALUE of ArrayBuffer, I get a TYPE of Typed_array.arrayBuffer t Opt.t response

This looks like a TYPE that DEPENDS on a VALUE.

Does OCaml support DEPENDENT TYPES, or if not, what I misunderstanding ?

LINKS:
perform_raw:

github.com

ocsigen/js_of_ocaml/blob/master/lib/lwt/lwt_xmlHttpRequest.ml#L108


      
            ; content_xml = (fun () -> assert false)
            ; headers
            }
          
          
let has_get_args url =
            try
              ignore (String.index url '?');
              true
            with Not_found -> false
          
          
let perform_raw
              ?(headers = [])
              ?content_type
              ?(get_args = [])
              ?(check_headers = fun _ _ -> true)
              ?progress
              ?upload_progress
              ?contents
              ?override_mime_type
              ?override_method
              ?with_credentials

response type:

github.com

ocsigen/js_of_ocaml/blob/master/lib/lwt/lwt_xmlHttpRequest.ml#L37


      
            String.concat
              "&"
              (List.map
                 (function
                   | name, `String s -> Url.urlencode name ^ "=" ^ Url.urlencode (to_string s)
                   | name, `File s -> Url.urlencode name ^ "=" ^ Url.urlencode (to_string s##.name))
                 l)
          
          
(* Higher level interface: *)
          
          
type 'response generic_http_frame =
            { url : string
            ; code : int
            ; headers : string -> string option
            ; content : 'response
            ; content_xml : unit -> Dom.element Dom.document t option
            }
          (** type of the http headers *)
          
          
type http_frame = string generic_http_frame

nojb · May 30, 2023, 6:08am

OCaml supports a particular case of dependent types called generalized algebraic datatypes (GADT). Learn more at OCaml - Generalized algebraic datatypes and GADTs - Real World OCaml.

Cheers,
Nicolas