Re: [Caml-list] Deprecation of tabulation boxes

[Gabriel Scherer <gabriel.scherer@gmail.com>]

You may be interested in the discussion
  https://github.com/ocaml/ocaml/pull/229
which discussed a few ways in which the proposed tabulation interface
may be inconvenient. It is after this discussion that Pierre Weis
decided to deprecate tabulation boxes -- I believe that the reason is
that tabulation and formatting never mixed very well.

Note that if you can decide in advance a maximal size for a given
"column" of your formatted output, you can use the left or
right-justification features of formatting conversions to print
aligned text:

let data = [("x", "foo"); ("loop", "bar")]

let () =
  print_newline ();
  data |> List.iter (fun (lab, instr) -> Printf.printf "%5s: %s\n" lab instr)
(*
    x: foo
 loop: bar
*)

let () =
  print_newline ();
  data |> List.iter (fun (lab, instr) -> Printf.printf "%-5s: %s\n" lab instr)
(*
x    : foo
loop : bar
*)

let () =
  let len = List.fold_left (fun m (lab, _) -> max m (String.length
lab)) 0 data in
  print_newline ();
  data |> List.iter (fun (lab, instr) -> Printf.printf "%*s: %s\n" len
lab instr)
(*
   x: foo
loop: bar
*)

On Mon, Dec 19, 2016 at 12:59 PM, Ivan Gotovchits <ivg@ieee.org> wrote:
> Greetings,
>
> The tabulation boxes are marked as deprecated since 4.03.0. I've tried to
> google for
> any reasons that justify the removal but found only a note by Pierre Weis in
> the Matis issue tracker[1]:
>
>
>> The proposed printf-like syntax is fine, but tabulation boxes are now
>> deprecated.
>> Indeed, tabulation boxes interaction with other pretty-printing boxes have
>> never been sorted out and tabulation boxes usage is orthogonal to the rest
>> of the Format module.
>> If considered useful, tabulation boxes could be implemented out of the
>> Format module.
>
>
> First of all the tabulation boxes can't be implemented outside of the format
> module since the tab stops are actually stored in the stack of tabulation
> boxes. If this data field would be removed from the formatter we will need
> to pass an extra argument to all pretty-printers that use the tabulation
> break, or use some global variable. Neither solution can be considered
> acceptable.
>
> Speaking about the usefulness. The tabulation boxes are useful for printing
> assembly outputs. And since compiler writing is sort of an application area
> for OCaml, it shouldn't be considered as a rare case. It is also very useful
> for printing Fortran code, that can be considered an assembler for the
> numeric computing. It also just allows printing nicely formatted texts, that
> it the main purpose of the Format library. As an example, tabulation boxes
> are used in BAP and CIL frameworks.
>
> To summarize, the deprecation will eventually make few project
> non-compilable. And there is no clear substitution for the deprecated
> feature.
>
> Given that, I would like to hear the justifications for the deprecation of
> tabulation boxes and suggested workarounds.
>
> One possible workaround, that I could see, is making the `formatter` type
> extensible with existential boxes or, more generally, with existential
> attributes. In that case, we will indeed be able to implement tabulation
> boxes outside of the format module.
>
> Best wishes,
> Ivan Gotovchits
>
> [1]: https://caml.inria.fr/mantis/view.php?id=4665