Common Lisp Pretty Printing

Introduction

For some time, I found Common Lisp’s pretty printer to be intimidating, obscure, and complex, and, for as long as I’ve been comfortable with the basics of FORMAT I felt able to ignore it, as irrelevant to my interests.

However, I often find it convenient to arrange lisp data in aligned columns. For small tables, this isn’t so bad, but I end up doing it a lot and it would be nice to figure out how to make the lisp printer do that for me, and so I’ve recently found myself digging into the lisp pretty printer.

Here’s what I’ve learned.

Hyperspec resources

As usual, the first place to look for information on this subject is the Common Lisp Hyperspec. If you’ve been programming in Common Lisp for almost any length of time, you’ve probably already got bookmarks to sections in the Hyperspec like, http://l1sp.org/cl/22.3 on “Formatted Output” and perhaps deeper links in there, like to the subsection on “Control Flow”. If you’re like me you also regularly browse through “The Printer Dictionary” at http://l1sp.org/cl/22.4 where you refresh yourself in *print-readably* verses *print-escape*, and/or the options on the print-unreadable-object method.

To acquaint yourself with the pretty-printer options you’ll want to keep these handy:

of course:

22.3.7 FORMAT Control-Flow Operations
- especially 22.3.7.4 Tilde Left-Brace: Iteration

also:

Looking at it from one the upper links you might first think that 22.3.5.1 Tilde Underscore: Conditional Newline would be handy, and it kind of is. It would be nice if it had a link though, like this:

Without any modifiers, ~_ is the same as ([[http://l1sp.org/cl/pprint-newline][pprint-newline]] :linear). ~@_ is the same as (pprint-newline :miser). ~:_ is the same as (pprint-newline :fill). ~:@_ is the same as (pprint-newline :mandatory).

Logic and Control

The pretty-printer is used for printing lisp forms in a standard fashion. There are pretty printing functions for printing LET forms, DEFUN and DEFMACRO, as well as lisp standard data objects. One reason you might want to use it is to pretty-print a lisp form of a custom object you’ve been working on, which can be expressed in a lisp form. You use it by printing anything with *PRINT-PRETTY* set to T, typically with PPRINT or perhaps with (WRITE foo :PRETTY T).

There’s two ways of working with a custom pretty print settings. One way is to use PPRINT-LOGICAL-BLOCK and attendant functions, PPRINT-NEWLINE, PPRINT-INDENT, etc. The other way is to use the much more compact FORMAT control string ~<...~:>. Since they inform each other, we’re go over both.

One note, you might recognize the control string the format control used for text justification described in 22.3.6.2 Tilde Less-Than-Sign: Justification. To this I can only say, “good eye”, because that it is.

Pretty-print Logical Block

Format Logical Block

Example

(format t "~:@<~:{~
           ~:@<~A~12T~A~:>~:^~:@_~
           ~}~:>"
        '((foo bar)
          (baz qux)))

wobh/cl-pretty-print.org