A Pretty Printer for Clojure


This namespace adds a new feature to Clojure: a generalized pretty printer.

The pretty printer is easy to use:

user=> (println (for [x (range 10)] (range x)))
(() (0) (0 1) (0 1 2) (0 1 2 3) (0 1 2 3 4) (0 1 2 3 4 5) (0 1 2 3 4 5 6) (0 1 2 3 4 5 6 7) (0 1 2 3 4 5 6 7 8))
user=> (use 'clojure.pprint)             
user=> (pprint (for [x (range 10)] (range x)))         
 (0 1)
 (0 1 2)
 (0 1 2 3)
 (0 1 2 3 4)
 (0 1 2 3 4 5)
 (0 1 2 3 4 5 6)
 (0 1 2 3 4 5 6 7)
 (0 1 2 3 4 5 6 7 8))

The pretty printer supports two modes: code which has special formatting for special forms and core macros and simple (the default) which formats the various Clojure data structures as appropriate for raw data. In fact, the pretty printer is highly customizable, but basic use is pretty simple.

All the functions and variables described here are in the clojure.pprint namespace. Using them is as simple as adding a (:use clojure.pprint) to your namespace declarations. Or, better practice would be (:use [clojure.pprint :only (<functions you wish to use>)]).

pprint is being developed by Tom Faulhaber (to mail me you can use my first name at my domain which is infolace.com).

As with the rest of Clojure, the pretty printer is licensed under the [http://opensource.org/licenses/eclipse-1.0.php Eclipse Public License 1.0].

Future development is guided by those using it, so send feedback about what's working and not working for you and what you'd like to see in the pretty printer.

Pretty Printing Basics

Pretty printing is primarily implemented with the function pprint. pprint takes a single argument and formats it according to the settings of several special variables.

Generally, the defaults are fine for pretty printing and you can simply use:

(pprint obj)

to print your object. If you wish to write to another stream besides *out*, you can use:

(write obj :pretty true :stream foo)

where foo is the stream to which you wish to write. (The write function has a lot more options which are not yet documented. Stay tuned.)

When at the REPL, the pp macro pretty prints the last output value. This is useful when you get something too complex to read comfortably. Just type:

user=> (pp)

and you'll get a pretty printed version of the last thing output (the magic variable *1).

Dispatch tables and code formatting

The behavior of the pretty printer can be finely controlled through the use of dispatch tables that contain descriptions for how different structures should be formatted.

Using custom dispatch tables, the pretty printer can create formatted output for data structures that is customized for the application. This allows pretty printing to be baked into any structured output. For information and examples, see below in [#CustomDispatchFunctions Custom Dispatch Functions].

The pretty printer comes with two pre-defined dispatch tables to cover the most common situations:

*simple-dispatch* - supports basic representation of data in various Clojure structures: seqs, maps, vectors, etc. in a fairly statndard way. When structures need to be broken across lines, following lines are indented to line up with the first element. *simple-dispatch* is the default and is good for showing the output of most operations.

*code-dispatch* - has special representation for various structures found in code: defn, condp, binding vectors, anonymous functions, etc. This dispatch indents following lines of a list one more space as appropriate for a function/argument type of list.

An example formatted with code dispatch:

user=> (def code '(defn cl-format 
"An implementation of a Common Lisp compatible format function"
[stream format-in & args] (let [compiled-format (if (string? format-in) 
(compile-format format-in) format-in) navigator (init-navigator args)] 
(execute-format stream compiled-format navigator))))
user=> (with-pprint-dispatch *code-dispatch* (pprint code))
(defn cl-format
  "An implementation of a Common Lisp compatible format function"
  [stream format-in & args]
  (let [compiled-format (if (string? format-in)
                          (compile-format format-in)
        navigator (init-navigator args)]
    (execute-format stream compiled-format navigator)))

There are three ways to set the current dispatch: set it to a specific table permanantly with set-pprint-dispatch, bind it with with-pprint-dispatch (as shown in the example above), or use the :dispatch keyword argument to write.

Control variables

The operation of pretty printing is also controlled by a set of variables that control general parameters of how the pretty printer makes decisions. The current list is as follows:

*print-pretty*: Default: true

Bind to true if you want write to use pretty printing. (pprint and pp automatically bind this to true.)

*print-right-margin*: Default: 72

Pretty printing will try to avoid anything going beyond this column.

*print-miser-width*: Default: 40

The column at which to enter miser style. Depending on the dispatch table, miser style add newlines in more places to try to keep lines short allowing for further levels of nesting. For example, in the code dispatch table, the pretty printer will insert a newline between the "if" and its condition when in miser style.

*print-suppress-namespaces*: Default: false

Don't print namespaces with symbols. This is particularly useful when pretty printing the results of macro expansions

*print-level*: Default: nil

As with the regular Clojure print function, this variable controls the depth of structure that is printed. The argument itself is level 0, the first level of a collection is level 1, etc. When the structure gets deeper than the specified *print-level*, a hash sign (#) is printed.

For example:

user=> (binding [*print-level* 2] (pprint '(a b (c d) ((e) ((f d) g)))))
(a b (c d) (# #))

*print-length*: Default: nil

As with the regular Clojure print function, this variable controls the number of items that are printed at each layer of structure. When a layer has too many items, elipses (...) are displayed.

For example:

user=> (defn foo [x] (for [i (range x) ] (range 1 (- x (dec i)))))
user=> (binding [*print-length* 6] (pprint (foo 10)))
((1 2 3 4 5 6 ...)
 (1 2 3 4 5 6 ...)
 (1 2 3 4 5 6 ...)
 (1 2 3 4 5 6 ...)
 (1 2 3 4 5 6)
 (1 2 3 4 5)

Custom Dispatch Functions

Using custom dispatch, you can easily create your own formatted output for structured data. Examples included with the pretty printer show how to use custom dispatch to translate simple Clojure structures into nicely formatted JSON and XML.

Basic Concepts of Pretty Printing

In order to create custom dispatch functions, you need to understand the fundamentals of pretty printing. The clojure pretty printer is based on the XP pretty printer algorithm (used in many Lisps including Common Lisp) which supports sophisticated decision-making about line breaking and indentation with reasonable performance even for very large structures. The XP algorithm is documented in the paper, [http://dspace.mit.edu/handle/1721.1/6504 XP. A Common Lisp Pretty Printing System].

The Clojure implementation of XP is similar in spirit to the Common Lisp implementation, but the details of the interface are somewhat different. The result is that writing custom dispatch in Clojure is more "Clojure-y."

There are three key concepts to understand when creating custom pretty printing functions: logical blocks, conditional newlines, and indentation.

A logical block marks a set of output that should be thought about as a single unit by the pretty printer. Logical blocks can contain other logical blocks (that is, they nest). As a simple example, when printing list structure, every sublist will typically be a logical block.

Conditional newlines tell the pretty printer where it can insert line breaks and how to make the decisions about when to do it. There are four types of conditional newline:

  • Linear newlines tell the pretty printer to insert a newline in a place whenever the enclosing logical block won't fit on a single line. Linear newlines are an all-or-nothing proposition; if the logical block doesn't fit on a single line, all the linear newlines are emitted as actual newlines.
  • Fill newlines tell the pretty printer that it should fit as many chunks of the logical block as possible on this line and then emit a newline.
  • Mandatory newlines tell the pretty printer to emit a newline regardless of where it is in the output line.
  • Miser newlines tell the pretty printer to emit a newline if the output column is in the miser region (as defined by the pretty printer variable *pprint-miser-width*). This allows you to define special behavior as the output gets heavily nested near the right margin.

Indentation commands allow you to specify how wrapped lines should be indented. Indentation can be relative to either the start column of the current logical block or the current column position of the output.

(This section is still incomplete...)

Current limitations and future plans

This is an early version release of the pretty printer and there is plenty that is yet to come.

Here are some examples:

  • Support all the types and forms in Clojure (most of the way there now).
  • Support for limiting pretty printing based on line counts.
  • Support for circular and shared substructure detection.
  • Finishing the integration with the format function (support for ~/ and tabular pretty printing).
  • Performance! (Not much thought has been made to making this go fast, but there are a bunch of pretty obvious speedups to be had.)
  • Handle Java objects intelligently

Please let me know about anything that's not working right, anything that should work differently, or the feature you think should be at the top of my list.

Logo & site design by Tom Hickey.
Clojure auto-documentation system by Tom Faulhaber.