type default_tokenizer = string -> to_tokenize list;;
type lifted_tokenizer = to_tokenize list -> to_tokenize list;;

let rec lift (tokenizer:default_tokenizer): lifted_tokenizer = function
  | [] -> []
  | a::b -> match a with
            | `To_Tokenize(s) -> (tokenizer s) @ (lift tokenizer b)
            | `Info_Token(i) -> `Info_Token(i) :: (lift tokenizer b)
;;

Line 7
Error: This pattern matches values of type [? `To_Tokenize of 'a ]
       but a pattern was expected which matches values of type
         to_tokenize =
           [ `Info_Token of info_token | `To_Tokenize of string ] list