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| author | Tom Faulhaber <git_net@infolace.com> | 2009-08-16 18:17:48 -0700 |
|---|---|---|
| committer | Tom Faulhaber <git_net@infolace.com> | 2009-08-16 18:17:48 -0700 |
| commit | e69a3d7ade498c867d234949b22c885286b44fa6 (patch) | |
| tree | 644abfdbf9f76ef86a1ff5ebf170c372048061b1 | |
| parent | c6648c454f7e6e83bfabdfc2180b534cdbbeb7be (diff) | |
Manual (forced) documentation build for commit c2f2b43fa22e3759a1844a1715fb8a32b18e9f6f
| -rw-r--r-- | doc/pprint/CommonLispFormat.html | 304 | ||||
| -rw-r--r-- | doc/pprint/PrettyPrinting.html | 361 |
2 files changed, 665 insertions, 0 deletions
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Not all formatting is done. + Contact Tom Faulhaber (tomfaulhaber on github, gmail, etc.) + with any questions. + </div> + <div id="right-sidebar"></div> + <div id="content-tag"><html><body><h1>Common Lisp-compatible Format function</h1> + +<p>cl-format is an implementation of the incredibly baroque Common Lisp format function as specified +in <a href="http://www.cs.cmu.edu/afs/cs.cmu.edu/project/ai-repository/ai/html/cltl/clm/node200.html#SECTION002633000000000000000">Common Lisp, the Language, 2nd edition, Chapter 22</a>.</p> + +<p>Format gives you an easy and powerful way to format text and data for output. It supports rich +formatting of strings and numbers, loops, conditionals, embedded formats, etc. It is really a +domain-specific language for formatting.</p> + +<p>This implementation for clojure has the following goals:</p> + +<ul> +<li>Support the full feature set of the Common Lisp format function (including the X3J13 extensions) with the only exception being concepts that make no sense or are differently interpreted in Clojure.</li> +<li>Make porting code from Common Lisp easier.</li> +<li>Provide a more native feeling solution for Clojure programmers than the Java format method and its relatives.</li> +<li>Be fast. This includes the ability to precompile formats that are going to be used reptitively.</li> +<li>Include useful error handling and comprehensive documentation.</li> +</ul> + +<h2>Why would I use cl-format?</h2> + +<p>For some people the answer to this question is that they are used to +Common Lisp and, therefore, they already know the syntax of format +strings and all the directives.</p> + +<p>A more interesting answer is that cl-format provides a way of +rendering strings that is much more suited to Lisp and its data +structures. </p> + +<p>Because iteration and conditionals are built into the directive +structure of cl-format, it is possible to render sequences and other +complex data structures directly without having to loop over the data +structure. </p> + +<p>For example, to print the elements of a sequence separated by commas, +you simply say:</p> + +<pre><code>(cl-format true "~{~a~^, ~}" aseq) +</code></pre> + +<p>(This example is taken from +<a href="http://www.gigamonkeys.com/book/">Practical Common Lisp</a> +by Peter Seibel.)</p> + +<p>The corresponding output using Clojure's Java-based <em>format</em> function +would involve a nasty loop/recur with some code to figure out about +the commas. Yuck!</p> + +<h2>Current Status of cl-format</h2> + +<p>cl-format is 100% compatible with the Common Lisp standard as +specified in CLtLv2. +This includes all of the functionality of Common +Lisp's format function including iteration, conditionals, +text justification and rich +options for displaying real and integer values. It also includes the +directives to support pretty printing structured output.</p> + +<p>If you find a bug in a directive, drop me a line +with a chunk of code that exhibits the bug and the version of +cl-format you found it in and I'll try to get it fixed.</p> + +<p>I also intend to have good built-in documentation for the directives, +but I haven't built that yet.</p> + +<p>The following directives are +not yet supported: ~:T and ~@:T (but all other forms of ~T work) +and extensions with ~/. </p> + +<p>The pretty printer interface is similar, but not identical to the +interface in Common Lisp.</p> + +<p>The custom dispatch table functionality is not fully fleshed out yet.</p> + +<p>Next up: </p> + +<ul> +<li>Support for ~/</li> +<li>Restructure unit tests into modular chunks.</li> +<li>Import tests from CLISP and SBCL.</li> +<li>Unit tests for exception conditions.</li> +<li>Interactive documentation</li> +</ul> + +<h2>How to use cl-format</h2> + +<h3>Loading cl-format in your program</h3> + +<p>Once cl-format is in your path, adding it to your code is easy:</p> + +<pre><code>(ns your-namespace-here + (:use clojure.contrib.pprint)) +</code></pre> + +<p>If you want to refer to the cl-format function as "format" (rather +than using the clojure function of that name), you can use this idiom:</p> + +<pre><code>(ns your-namespace-here + (:refer-clojure :exclude [format]) + (:use clojure.contrib.pprint)) + +(def format cl-format) +</code></pre> + +<p>You might want to do this in code that you've ported from Common Lisp, +for instance, or maybe just because old habits die hard.</p> + +<p>From the REPL, you can grab it using (use):</p> + +<pre><code>(use 'clojure.contrib.pprint) +</code></pre> + +<h3>Calling cl-format</h3> + +<p>cl-format is a standard clojure function that takes a variable number +of arguments. You call it like this:</p> + +<pre><code>(cl-format stream format args...) +</code></pre> + +<p><em>stream</em> can be any Java Writer (that is java.io.Writer) or the values +<em>true</em>, <em>false</em>, or <em>nil</em>. The argument <em>true</em> is identical to using +<code>*</code>out<code>*</code> while <em>false</em> or <em>nil</em> indicate that cl-format should return +its result as a string rather than writing it to a stream.</p> + +<p><em>format</em> is either a format string or a compiled format (see +below). The format string controls the output that's written in a way +that's similar to (but much more powerful than) the standard Clojure +API format function (which is based on Java's +java.lang.String.Format).</p> + +<p>Format strings consist of characters that are to be written to the +output stream plus directives (which are marked by ~) as in "The +answer is ~,2f". Format strings are documented in detail in +<a href="http://www.cs.cmu.edu/afs/cs.cmu.edu/project/ai-repository/ai/html/cltl/clm/node200.html#SECTION002633000000000000000"><em>Common Lisp the Language</em>, 2nd edition, Chapter 22</a>.</p> + +<p><em>args</em> is a set of arguments whose use is defined by the format.</p> + +<h3>Compiled formats</h3> + +<p>When you use a format string many times (for example, when you're outputting +in a loop), you can improve your performance by compiling the format +with <em>compile-format</em>. The result of compile format can be passed to +<em>cl-format</em> just like a format string but it doesn't need to be +parsed.</p> + +<p>For example:</p> + +<pre><code>(def log-format (compile-format "~2,'0D/~2,'0D/~D ~2D:~2,'0D ~:[PM,AM]: ~A~%")) + +(defn log [msg] + (let [[m d y h min am?] (some-date-decomposition-fn)] + (cl-format log-format m d y h min am? msg))) +</code></pre> + +<h2>Using column aware streams across format invocations</h2> + +<p>Writers in Java have no real idea of current column or device page width, so the format +directives that want to work relative to the current position on the +page have nothing to work with. To deal with this, cl-format contains +an extension to writer called PrettyWriter. PrettyWriter watches the +output and keeps track of what column the current output is going to.</p> + +<p>When you call format and your format includes a directive that cares +about what column it's in (~T, ~&, ~<...~>), cl-format will +automatically wrap the Writer you passed in with a PrettyWriter. This +means that by default all cl-format statements act like they begin on +a fresh line and have a page width of 72.</p> + +<p>For many applications, these assumptions are fine and you need to do +nothing more. But sometimes you want to use multiple cl-format calls +that output partial lines. You may also want to mix cl-format calls +with the native clojure calls like print. If you want stay +column-aware while doingg this you need to create a PrettyWriter of +your own (and possibly bind it to <code>*</code>out<code>*</code>).</p> + +<p>As an example of this, this function takes a nested list and prints it +as a table (returning the result as a string):</p> + +<pre><code>(defn list-to-table [aseq column-width] + (let [stream (PrettyWriter. (java.io.StringWriter.))] + (binding [*out* stream] + (doseq [row aseq] + (doseq [col row] + (cl-format true "~4D~7,vT" col column-width)) + (prn))) + (.toString (.getWriter stream)))) +</code></pre> + +<p>(In reality, you'd probably do this as a single call to cl-format.)</p> + +<p>The constructor to PrettyWriter takes the Writer it's wrapping and +(optionally) the page width (in columns) for use with ~<...~>. </p> + +<h2>Examples</h2> + +<p>The following function uses cl-format to dump a columnized table of the Java system properties:</p> + +<pre><code>(defn show-props [stream] + (let [p (mapcat + #(vector (key %) (val %)) + (sort-by key (System/getProperties)))] + (cl-format stream "~30A~A~%~{~20,,,'-A~10A~}~%~{~30A~S~%~}" + "Property" "Value" ["" "" "" ""] p))) +</code></pre> + +<p>There are some more examples in the clojure.contrib.pprint.examples +package:</p> + +<ul> +<li>hexdump - a program that uses cl-format to create a standard formatted hexdump of the requested stream.</li> +<li>multiply - a function to show a formatted multipication table in a very "first-order" way.</li> +<li>props - the show-props example shown above.</li> +<li>show_doc - some utilities for showing documentation from various name spaces.</li> +</ul> + +<h2>Differences from the Common Lisp format function</h2> + +<p>The floating point directives that show exponents (~E, ~G) show E for +the exponent character in all cases (unless overridden with an +<em>exponentchar</em>). Clojure does not distinguish between floats and +doubles in its printed representation and neither does cl-format.</p> + +<p>The ~A and ~S directives accept the colon prefix, but ignore it since +() and nil are not equivalent in Clojure.</p> + +<p>Clojure has 3 different reader syntaxes for characters. The ~@c +directive to cl-format has an argument extension to let you choose:</p> + +<ul> +<li>~@c (with no argument) prints "\c" (backslash followed by the printed representation of the character or \newline, \space, \tab, \backspace, \return)</li> +<li>~'o@c prints "\oDDD" where DDD are the octal digits representing the character. </li> +<li>~'u@c prints "\uXXXX" prints the hex Unicode representation of the character. </li> +</ul> +</body></html></div> + </div> + </div> + </div> + </div> + <div id="foot"> + <div style="text-align: center;"> + Copyright 2008-2009 Rich Hickey and the various contributors + </div> + </div> + </div> + <div id="DesignedBy">Logo & site design by <a title="Visit Tom Hickey's website." href="http://www.tomhickey.com">Tom Hickey</a>.</div> + </div> + <!-- /AllContentContainer --> + </body> + +</html>
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Not all formatting is done. + Contact Tom Faulhaber (tomfaulhaber on github, gmail, etc.) + with any questions. + </div> + <div id="right-sidebar"></div> + <div id="content-tag"><html><body><h1>A Pretty Printer for Clojure</h1> + +<p>To use pretty printing in clojure.contrib, it is necessary to build +clojure.contrib with compiled classes. To do this, you must tell +ant where to find clojure.jar. For me, this looks like:</p> + +<pre><code> ant -Dclojure.jar=../clojure/clojure.jar +</code></pre> + +<p>because I keep clojure source and clojure.contrib source right next to +each other. Just point the pathname to wherever you keep clojure.jar.</p> + +<h2>Overview</h2> + +<p>This library adds a new feature to Clojure: a generalized pretty +printer.</p> + +<p>The pretty printer is easy to use:</p> + +<pre><code>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)) +nil +user=> (use 'clojure.contrib.pprint) +nil +user=> (pprint (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)) +nil +user=> +</code></pre> + +<p>The pretty printer supports two modes: <em>code</em> which has special +formatting for special forms and core macros and <em>simple</em> (the +default) which formats the various Clojure data structures as +appropriate for raw data. In the future, the pretty printer will be +highly customizable, but right now it is pretty simple.</p> + +<p>All the functions and variables described here are in the +clojure.contrib.pprint namespace. Using them is as simple as including +clojure-contrib.jar to your classpath and adding a (:use clojure.contrib.pprint) to +your namespace declarations.</p> + +<p>pprint is being developed by Tom Faulhaber (to mail me you can use +my first name at my domain which is infolace.com).</p> + +<p>As part of clojure.contrib, the pretty printer is licensed under the +[http://opensource.org/licenses/eclipse-1.0.php Eclipse Public License 1.0].</p> + +<p>clojure.contrib is hosted on google code at [http://code.google.com/p/clojure-contrib/]</p> + +<p>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.</p> + +<h2>Pretty Printing Basics</h2> + +<p>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.</p> + +<p>Generally, the defaults are fine for pretty printing and you can +simply use:</p> + +<pre><code>(pprint obj) +</code></pre> + +<p>to print your object. If you wish to write to +another stream besides <code>*</code>out<code>*</code>, you can use:</p> + +<pre><code>(write obj :pretty true :stream foo) +</code></pre> + +<p>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.)</p> + +<p>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:</p> + +<pre><code>user=> (pp) +</code></pre> + +<p>and you'll get a pretty printed version of the last thing output (the +magic variable <code>*</code>1).</p> + +<h2>Dispatch tables and code formatting</h2> + +<p>The behavior of the pretty printer can be finely controlled through +the use of <em>dispatch tables</em> that contain descriptions for how +different structures should be formatted. </p> + +<p>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 +[#Custom<em>Dispatch</em>Functions Custom Dispatch Functions].</p> + +<p>The pretty printer comes with two pre-defined dispatch tables to cover +the most common situations:</p> + +<p><code>*</code>simple-dispatch<code>*</code> - 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. <code>*</code>simple-dispatch<code>*</code> is +the default and is good from showing the output of most operations.</p> + +<p><code>*</code>code-dispatch<code>*</code> - 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.</p> + +<p>An example formatted with code dispatch:</p> + +<pre><code>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/code +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) + format-in) + navigator (init-navigator args)] + (execute-format stream compiled-format navigator))) +nil +user=> +</code></pre> + +<p>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.</p> + +<h2>Control variables</h2> + +<p>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:</p> + +<p><em><code>*</code>print-pretty<code>*</code></em>: Default: <em>true</em> </p> + +<p>Bind to true if you want write to use pretty printing. (pprint and pp automatically +bind this to true.)</p> + +<p><em><code>*</code>print-right-margin<code>*</code></em>: Default: <em>72</em></p> + +<p>Pretty printing will try to avoid anything going beyond this column.</p> + +<p><em><code>*</code>print-miser-width<code>*</code></em>: Default: <em>40</em></p> + +<p>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.</p> + +<p><em><code>*</code>print-suppress-namespaces<code>*</code></em>: Default: <em>false</em></p> + +<p>Don't print namespaces with symbols. This is particularly useful when +pretty printing the results of macro expansions</p> + +<p><em><code>*</code>print-level<code>*</code></em>: Default: <em>nil</em></p> + +<p>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 <code>*</code>print-level<code>*</code>, a hash sign (#) is +printed.</p> + +<p>For example:</p> + +<pre><code>user=> (binding [*print-level* 2] (pprint '(a b (c d) ((e) ((f d) g))))) +(a b (c d) (# #)) +nil +user=> +</code></pre> + +<p><em><code>*</code>print-length<code>*</code></em>: Default: <em>nil</em></p> + +<p>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. </p> + +<p>For example:</p> + +<pre><code>user=> (defn foo [x] (for [i (range x) ] (range 1 (- x (dec i))))) +#'user/foo +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) + ...) +nil +user=> +</code></pre> + +<h2>Custom Dispatch Functions</h2> + +<p>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.</p> + +<h3>Basic Concepts of Pretty Printing</h3> + +<p>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].</p> + +<p>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."</p> + +<p>There are three key concepts to understand when creating custom pretty +printing functions: <em>logical blocks</em>, <em>conditional newlines</em>, and +<em>indentation</em>.</p> + +<p>A <em>logical block</em> 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.</p> + +<p><em>Conditional newlines</em> 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:</p> + +<ul> +<li>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, <em>all</em> the linear +newlines are emitted as actual newlines.</li> +<li>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.</li> +<li>Mandatory newlines tell the pretty printer to emit a newline +regardless of where it is in the output line. </li> +<li>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 <code>*</code>pprint-miser-width<code>*</code>). This allows you to +define special behavior as the output gets heavily nested near the +right margin.</li> +</ul> + +<p><em>Indentation</em> 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. </p> + +<p>(This section is still incomplete...)</p> + +<h2>Current limitations and future plans</h2> + +<p>This is an early version release of the pretty printer and there is +plenty that is yet to come.</p> + +<p>Here are some examples:</p> + +<ul> +<li>Support all the types and forms in |