Use box-like algorithm in `dune fmt`

[bobot]

The rules used by dune fmt for adding a newline doesn't take into account the size of the text. So in the following example it gives not a pretty result. It would be better to use a more dynamic box-like approach for example by using the box from Format. The example would then fit on one line.

  (with-stdout-to
   x
   (echo b)
  )

[emillon]

The converse situation happens also for fields like (libraries): library names are only atoms, so the current algorithm will never break the line.

IMO there are two challenges with Format boxes:

• a small change can cause the whole file to be re-formatted differently. I guess that it is inherent to all formatting algorithms, so we'll have to live with it.
• the API is pretty difficult to understand. I think I'd be more comfortable with a custom lightweight algorithm to start.

In terms of priorities, I'd like to first have something that is able to parse and format all dune files in a project (in particular, dune itself) in order to see the problems, before implementing this.

[bobot]

• a small change can cause the whole file to be re-formatted differently. I guess that it is inherent to all formatting algorithms, so we'll have to live with it.

The format is structured enough so that it will not append, I think. The modification of one stanza will not change another one.

• the API is pretty difficult to understand. I think I'd be more comfortable with a custom lightweight algorithm to start.

You should give it a try. The introduction has exactly the example of the last conversation, with the choice of the end parenthesis:

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

or

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

In terms of priorities, I'd like to first have something that is able to parse and format all dune files

I agree, and someone else could play later with the boxes.