Pp - Pretty printing

This library provides a lean alternative to the Format module of the OCaml standard library. It aims to make it easy for users to do the right thing. If you have tried Format before but find its API complicated and difficult to use, then Pp might be a good choice for you.

Pp uses the same concepts of boxes and break hints, and the final rendering is done to formatter from the Format module. However it defines its own algebra which I personaly find easier to work with and reason about. No previous knowledge is required to start using this library, however the various guides for the Format module such as this one should be applicable to Pp as well.

Examples

# #require "pp";;
# let print pp = Format.printf "%a@." Pp.to_fmt pp;;
val print : 'a Pp.t -> unit = <fun>
# print (Pp.enumerate (List.init 10 Fun.id) ~f:(Pp.textf "%d"));;
- 0
- 1
- 2
- 3
- 4
- 5
- 6
- 7
- 8
- 9
# print (Pp.box ~indent:2 (Pp.text
         "Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed \
          do eiusmod tempor incididunt ut labore et dolore magna \
          aliqua. Ut enim ad minim veniam, quis nostrud exercitation \
          ullamco laboris nisi ut aliquip ex ea commodo \
          consequat. Duis aute irure dolor in reprehenderit in \
          voluptate velit esse cillum dolore eu fugiat nulla \
          pariatur. Excepteur sint occaecat cupidatat non proident, \
          sunt in culpa qui officia deserunt mollit anim id est \
          laborum."));;
Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor
  incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis
  nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.
  Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore
  eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident,
  sunt in culpa qui officia deserunt mollit anim id est laborum.
- : unit = ()
# print
    (Pp.vbox
       ( Pp.box (Pp.text "Error: something went wrong!")
       ++ Pp.cut
       ++ Pp.box (Pp.text "Here are a few things you can do:")
       ++ Pp.cut
       ++ Pp.enumerate ~f:Fun.id
            [ Pp.text
                "read the documentation, double check the way you are using \
                 this software to make sure you are not doing something wrong, \
                 and hopefully fix the problem on your side and move on"
            ; Pp.text
                "strace furiously the program to try and understand why \
                 exactly it is trying to do what it is doing"
            ; Pp.text "report an issue upstream"
            ; Pp.text "if all else fails"
              ++ Pp.cut
              ++ Pp.enumerate ~f:Pp.text
                   [ "scream loudly at your computer"
                   ; "take a break from your keyboard"
                   ; "clear your head and try again"
                   ]
            ] ));;
Error: something went wrong!
Here are a few things you can do:
- read the documentation, double check the way you are using this software to
  make sure you are not doing something wrong, and hopefully fix the problem on
  your side and move on
- strace furiously the program to try and understand why exactly it is trying
  to do what it is doing
- report an issue upstream
- if all else fails
  - scream loudly at your computer
  - take a break from your keyboard
  - clear your head and try again
- : unit = ()

Resources

As mentioned earlier, this Format guide can be a good starting point to understand the pretty-printing mechanics of Pp. Additionally, Format Unraveled is a great resource for understanding the core mental model of Format. And since Pp uses the same concepts as Format, it can be a good resource for Pp too.

Note that the Format Unraveled paper discuss some limitations of Format that are due to the fact that it never has the full document in-memory before rendering it. This does not apply to Pp since Pp clearly always construct the full document in-memory. However, since right now the only way to render a Pp.t is via the Format module, the same limitations that apply to Format apply to Pp as well. We might add another renderer in the future that does not have these limitations if there is sufficient incentive to do so.

History

This library comes from the dune build system. Initially, to construct the various messages displayed to the user on the terminal, dune was mostly using the Format module, and in particular the Format.fprintf style format strings. The Format API, its concepts and printf-like format strings are quite complicated and not easy to grasp at all. It requires quite a bit of learning and practice before one can be fluent with Format.

What is more, it is well known that programmers absolutely "love" spending time writing good error messages. Hint: this is sarcastic.

The result of all this was terrible and most messages printed by DUne where badly formatted. So to remedy to the situation we introduced a Pp module in stdune, the mini-standard library inside Dune. Pp is completely detached from Format, and there is no mention of formatter until the rendering stage. While in the Pp world, all we do is construct a document with various formatting hints.

In the end, the API of Pp just makes it easy for someone to do the right thing. And this makes all the difference. Since then, it has been easy to construct well formatted error messages for Dune and the formatting of existing error messages has generally improved.

Once Pp was mature enough, we extracted it into its own library so that it can benefit others.

Interoperability

It is easy to integrate Pp with Format. For that, simply use the Pp.to_fmt function. For instance, if you have a value pp of type _ Pp.t you can do:

Format.fprintf "... %a ..." Pp.to_fmt pp

If you are familiar with the fmt library, Pp.to_fmt basically allows you to go from a 'a Pp.t to a 'a Fmt.t. The opposite is not possible; it is not possible to inject arbitrary side-effecting formatting functions into a Pp.t.

If you want to convert Pp tags fot Format tags, you can use the function Pp.to_fmt_with_tags.

Comparison with other libraries

This is not an in-depth comparison as I haven't used these libraries much myself, so this is to be taken with a grain of salt. The below is basically what I can tell from a quick look at their API. If you know more and would like to contribute to this comparison, please do so by opening a PR :)

Comparison with fmt

The main difference with fmt is that Fmt.t is am alias for Format.formatter -> 'a -> unit, while Pp.t is an abstract type.

Comparison with easy-format

The easy-format library looks much higher-level than Pp. Pp still works with boxes and break hints like the Format module, while easy-format works with atoms, lists and labelled nodes.