Re: [Caml-list] Deprecation of tabulation boxes

[Ivan Gotovchits <ivg@ieee.org>]

[-- Attachment #1: Type: text/plain, Size: 5339 bytes --]

Well, the discussion literally says, we don't understand tabulation, and it
looks like nobody is using it, so let's just throw it away :)

We're using tabulation, and they are quite useful. Yep, I agree, that the
interface for setting the marks is kind of awkward,
it would be nice if there would be a `pp_setup_tabs : _ formatter -> int
list -> unit` function, that would push into the stack a new tabular box
with
the specified tabulations.

What concerning your solution with the alignment, it is less general and
doesn't work in our case. First of all, the pretty printing functions, that
are printing
into the columns are not specified with `%s`, but with `%a` (e.g., address,
memory string, assembly string). Second, the printing functions are not
actually
defined in the same module. The tabs are initialized in the frontend, that
defines it based on the architecture (address size, maximum length of
instruction, etc),
and pretty printers are registered separately, and they just rely on a
fact, that we have three columns, the first is for address, the second is
for memory, then assembly, etc).
This design simplifies the actual instruction printers, by consolidating
the common code in the formatter setup procedure.

So, it is still not clear to me, why the tabulations are wrong. If they do
complicate the code base and raise the support cost, then it is
understandable, why you would like to remove it
from the standard library. But in this case, it would be nice, to move this
code out as a separate library, as just removing a feature, that was in the
language for years (I would say even forever,
if we will start the history from caml-light), without providing any
substitution is... not nice :)


[1]: http://caml.inria.fr/pub/docs/manual-caml-light/node15.5.html



On Mon, Dec 19, 2016 at 1:20 PM, Gabriel Scherer <gabriel.scherer@gmail.com>
wrote:

> 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
>

[-- Attachment #2: Type: text/html, Size: 6800 bytes --]