OCaml Format and structural boxes: Why does my output not match the example?

Question

There is an example on the Using the Format module page on the OCaml web site that gives an example of the differences between structural and packing hov boxes. I am trying to replicate this example. (I am using OCaml 3.12.1.)

With the following input:

Format.printf "@[<hov 2>(---@\n@[<hov 2>(---@\n@[<hov 2>(---@,)@]@,)@]@,)@]@\n"

I get the expected output:

(---
  (---
    (---)))

What I can not figure out is how to get the output the page ascribes to "structural boxes":

(---
  (---
    (---
    )
  )
)

I was uncertain as to which Format.printf identifier maps to these boxes, so I tried several variations:

Format.printf "@[<hv 2>(---@\n@[<hv 2>(---@\n@[<hv 2>(---@,)@]@,)@]@,)@]@\n"
Format.printf "@[<2>(---@\n@[<2>(---@\n@[<2>(---@,)@]@,)@]@,)@]@\n"
Format.printf "@[<b 2>(---@\n@[<b 2>(---@\n@[<b 2>(---@,)@]@,)@]@,)@]@\n"

But all of the above give the same output as with the <hov 2> initial example. Would anyone have any ideas how I can get output similar to the second example from web page?

Answer 1

I would have thought that your second example (with hv boxes) should work. But it seems that @\n does not have the behavior we expect.

Solution 1: force the line break before the right parenthesis, i.e. replace @, by @\n.

let () =
  Format.printf "@.";
  Format.printf "@[<hov 2>(---@\n@[<hov 2>(---@\n@[<hov 2>(---@\n)@]@\n)@]@\n)@]@\n";
  Format.printf "@."

Result:

(---
  (---
    (---
      )
    )
  )

Problem: there will always be a line break, it will never print:

(---(---(---)))

even if it has the room for it. If that is an issue for you, see solution 2.

Solution 2: Use longer lines :) If the break is caused by a long line then an hv box does break the line before the closing parenthesis.

let () =
  Format.printf "@.";
  Format.printf "@[<hv 2>(------------------------------------------------------------------------@,@[<hv 2>(---@,@[<hv 2>(---@,)@]@,)@]@,)@]";
  Format.printf "@."

Result:

(------------------------------------------------------------------------
  (---(---))
  )

However, the closing parenthesis is not aligned with the opening one.

Solution 3: If you want the right parenthesis to be aligned with the left one, you need two boxes.

let () =
  Format.printf "@.";
  Format.printf
    "@[<hv>@[<hv 2>(------------------------------------------------------------------------\
     @[<hv>@[<hv 2>(------------------------------------------------------------------------\
     @[<hv>@[<hv 2>(------------------------------------------------------------------------\
     @]@,)@]@]@,)@]@]@,)@]";
  Format.printf "@."

Result:

(------------------------------------------------------------------------
  (------------------------------------------------------------------------
    (------------------------------------------------------------------------
    )
  )
)