Question

I am not really understand about the function (parse_list) at

None -> List.rev is and None -> []

let try_parse parse x = try Some (parse x) with Error _ -> None;;

let parse_list parse =
  let rec aux is = function
    | [] -> List.rev is, []
    | (hd :: tl) as xs ->
    match try_parse parse hd with
      | Some i -> aux (i::is) tl
      | None -> List.rev is, xs
  in aux [];;

and

let parse_list parse =
  let rec aux is = function
    | [] -> List.rev is, []
    | (hd :: tl) as xs ->
    match try_parse parse hd with
      | Some i -> aux (i::is) tl
      | None -> [], xs
  in aux [];;

Are they different? could you please give me an example if they are different? Thank you very much

Answer 1

Yes, they are different.

In the first one, when the parse function will fail, the function parse_list will return a partial list of “parsed” expression (List.rev is).

In the second one, when the parse function will fail, you’ll get an empty list from parse_list ([]).

Look this example with a parse function which will keep only integers lesser than 3:

let test_parse x = if x < 3 then x else raise Error "error";;

With the first implementation you’ll get:

# parse_list test_parse [1; 2; 3; 4; 5];;
  - : int list * int list = ([1; 2], [3; 4; 5])

wit the second one, you’ll get:

# parse_list test_parse [1; 2; 3; 4; 5];;
  - : int list * int list = ([], [3; 4; 5])