out

File "t.ml", line 14, characters 6-16:
Error: This pattern matches values of type param t
       but a pattern was expected which matches values of type literal t
       Type param is not compatible with type literal 

t.ml

type literal
type param

type _ t =
  | QueryString : string -> literal t
  | IntParam : string -> param t
  | StringParam : string -> param t

type 'a query = 'a t list

let to_string (q:'a query) : string =
  let t2s (r:'a t) : string = match r with
    | QueryString s -> s
    | IntParam s | StringParam s -> ":" ^ s in
  let strs = List.map t2s q in
  String.concat "" strs