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<h1>- <a href="http://pauillac.inria.fr/~ddr/camlp5">Camlp5</a> -</h1>
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<h1 class="top">Extensible printers</h1>
<p>This chapter describes the syntax and semantics of the extensible
printers of Camlp5.</p>
<p>Similar to the <a href="grammars.html">extensible grammars</a>, the
extensible printers allow to define and extend printers of data or
programs. A specific statement "<tt>EXTEND_PRINTER</tt>" allow to
define these extensions.</p>
<div id="tableofcontents">
<ol>
<li><a href="#a:Getting-started">Getting started</a></li>
<li><a href="#a:Syntax-of-the-EXTEND_PRINTER-statement">Syntax of the EXTEND_PRINTER statement</a></li>
<li><a href="#a:Semantics-of-EXTEND_PRINTER">Semantics of EXTEND_PRINTER</a>
<ul>
<li><a href="#b:Printers-definition-list">Printers definition list</a></li>
<li><a href="#b:Rules-insertion">Rules insertion</a></li>
<li><a href="#b:Semantic-action">Semantic action</a></li>
</ul>
</li>
<li><a href="#a:The-Eprinter-module">The Eprinter module</a></li>
<li><a href="#a:Examples">Examples</a>
<ul>
<li><a href="#b:Parser-and-Printer-of-expressions">Parser and Printer of expressions</a></li>
<li><a href="#b:Printing-OCaml-programs">Printing OCaml programs</a></li>
</ul>
</li>
</ol>
</div>
<h2 id="a:Getting-started">Getting started</h2>
<p>A printer is a value of type "<tt>Eprinter.t a</tt>" where
"<tt>a</tt>" is the type of the item to be printed. When applied, a
printer returns a string, representing the printed item.</p>
<p>To create a printer, one must use the function
"<tt>Eprinter.make</tt>" with, as parameter, the name of the
printer, (used in error messages). A printer is created empty,
i.e. it fails if it is applied.</p>
<p>As with grammar entries, printers may have several levels. When the
function "<tt>Eprinter.apply</tt>" is applied to a printer, the
first level is called. The function "<tt>Eprinter.apply_level</tt>"
allows to call a printer at some specific level possibly different
from the first one. When a level does not match any value of the
printed item, the next level is tested. If there is no more levels,
the printer fails.</p>
<p>In semantic actions of printers, functions are provided to
recursively call the current level and the next level. Moreover,
a <em>printing context</em> variable is also given, giving the
current indentation, what should be printed before in the same line
and what should be printed after in the same line (it is not
mandatory to use them).</p>
<p>The extension of printers can be done with the
"<tt>EXTEND_PRINTER</tt>" statement added by the <em>parsing
kit</em> "<tt>pa_extprint.cmo</tt>".</p>
<h2 id="a:Syntax-of-the-EXTEND_PRINTER-statement">Syntax of the EXTEND_PRINTER statement</h2>
<pre>
expression ::= extend-statement
extend-statement ::= "EXTEND_PRINTER" extend-body "END"
extend-body ::= extend-printers
extend-printers ::= extend-printer extend-printers
| <nothing>
extend-printer ::= printer-name ":" position-opt "[" levels "]"
position-opt ::= "FIRST"
| "LAST"
| "BEFORE" label
| "AFTER" label
| "LEVEL" label
| <nothing>
levels ::= level "|" levels
| level
level ::= label-opt "[" rules "]"
label-opt ::= label
| <nothing>
rules ::= rule "|" rules
| rule
rule ::= pattern "->" expression
| pattern "when" expression "->" expression
printer-name ::= qualid
qualid ::= qualid "." qualid
| uident
| lident
uident ::= 'A'-'Z' ident
lident ::= ('a'-'z' | '_' | misc-byte) ident
ident ::= ident-char*
ident-char ::= ('a'-'a' | 'A'-'Z' | '0'-'9' | '_' | ''' | misc-byte)
misc-byte ::= '\128'-'\255'
</pre>
<h2 id="a:Semantics-of-EXTEND_PRINTER">Semantics of EXTEND_PRINTER</h2>
<h3 id="b:Printers-definition-list">Printers definition list</h3>
<p>All printers are extended according to their corresponding
definitions which start with an optional "position" and follow with
the "levels" definition.</p>
<h4>Optional position</h4>
<p>After the colon, it is possible to specify where to insert the
defined levels:</p>
<ul>
<li>The identifier "<tt>FIRST</tt>" (resp. "<tt>LAST</tt>")
indicates that the level must be inserted before (resp. after) all
possibly existing levels of the entry. They become their first
(resp. last) levels.</li>
<li>The identifier "<tt>BEFORE</tt>" (resp. "<tt>AFTER</tt>")
followed by a level label (a string) indicates that the levels
must be inserted before (resp. after) that level, if it exists. If
it does not exist, the extend statement fails at run time.</li>
<li>The identifier "<tt>LEVEL</tt>" followed by a label indicates
that the first level defined in the extend statement must be
inserted at the given level, extending and modifying it. The other
levels defined in the statement are inserted after this level, and
before the possible levels following this level. If there is no
level with this label, the extend statement fails at run
time.</li>
<li>By default, if the entry has no level, the levels defined in the
statement are inserted in the entry. Otherwise the first defined
level is inserted at the first level of the entry, extending or
modifying it. The other levels are inserted afterwards (before the
possible second level which may previously exist in the entry).</li>
</ul>
<h4>Levels</h4>
<p>After the optional "position", the <em>level</em> list follow. The
levels are separated by vertical bars, the whole list being between
brackets.</p>
<p>A level starts with an optional label, which corresponds to its
name. This label is useful to specify this level in case of future
extensions, using the <em>position</em> (see previous section) or
for possible direct calls to this specific level.</p>
<h4>Rules</h4>
<p>A level is a list of <em>rules</em> separated by vertical bars, the
whole list being between brackets.</p>
<p>A rule is an usual pattern association (in a function or in the
"match" statement), i.e. a pattern, an arrow and an expression. The
expression is the semantic action which must be of type
"<tt>string</tt>".</p>
<h3 id="b:Rules-insertion">Rules insertion</h3>
<p>The rules are sorted by their patterns, according to the rules of
the <a href="extfun.html">extensible functions</a>.</p>
<h3 id="b:Semantic-action">Semantic action</h3>
<p>The semantic action, i.e. the expression following the right arrow
in rules, contains in its environment the variables bound by the
pattern and three more variables:</p>
<ul>
<li>The variable "<tt>curr</tt>" which is a function which can be
called to recursively invoke the printer at the current
level,</li>
<li>The variable "<tt>next</tt>" which is a function which can be
called to invoke the printer at the next level,</li>
<li>The variable "<tt>pc</tt>" which contains the printing context
of type "<tt>Pprintf.pr_context</tt>" (see
chapter <a href="pprintf.html">Pprintf</a>).</li>
</ul>
<p>The variables "<tt>curr</tt>" and "<tt>next</tt>" are of type:</p>
<pre>
pr_context -> t -> string
</pre>
<p>where "<tt>t</tt>" is the type of the printer (i.e. the type of its
patterns).</p>
<p>The variable "<tt>curr</tt>", "<tt>next</tt>" and "<tt>pc</tt>"
have predefined names and can hide the possible identifiers having
the same names in the pattern or in the environment of the
"<tt>EXTEND_PRINTER</tt>" statement.</p>
<h2 id="a:The-Eprinter-module">The Eprinter module</h2>
<p>See its <a href="library.html#a:Eprinter-module">section</a> in the
chapter "Library".</p>
<h2 id="a:Examples">Examples</h2>
<h3 id="b:Parser-and-Printer-of-expressions">Parser and Printer of expressions</h3>
<p>This example illustrates the symmetry between parsers and
printers. A simple type of expressions is defined. A parser converts
a string to a value of this type, and a printer converts a value of
this type to a string.</p>
<p>In the printer, there is no use of the "<tt>pc</tt>" parameter and
no use of the "<tt>Pretty</tt>" module. The strings are printed on
a single line.</p>
<p>Here is the source (file "<tt>foo.ml</tt>"):</p>
<pre>
#load "pa_extend.cmo";
#load "pa_extprint.cmo";
open Printf;
type expr =
[ Op of string and expr and expr
| Int of int
| Var of string ]
;
value g = Grammar.gcreate (Plexer.gmake ());
value pa_e = Grammar.Entry.create g "expr";
value pr_e = Eprinter.make "expr";
EXTEND
pa_e:
[ [ x = SELF; "+"; y = SELF -> Op "+" x y
| x = SELF; "-"; y = SELF -> Op "-" x y ]
| [ x = SELF; "*"; y = SELF -> Op "*" x y
| x = SELF; "/"; y = SELF -> Op "/" x y ]
| [ x = INT -> Int (int_of_string x)
| x = LIDENT -> Var x
| "("; x = SELF; ")" -> x ] ]
;
END;
EXTEND_PRINTER
pr_e:
[ [ Op "+" x y -> sprintf "%s + %s" (curr pc x) (next pc y)
| Op "-" x y -> sprintf "%s - %s" (curr pc x) (next pc y) ]
| [ Op "*" x y -> sprintf "%s * %s" (curr pc x) (next pc y)
| Op "/" x y -> sprintf "%s / %s" (curr pc x) (next pc y) ]
| [ Int x -> string_of_int x
| Var x -> x
| x -> sprintf "(%s)" (Eprinter.apply pr_e pc x) ] ]
;
END;
value parse s = Grammar.Entry.parse pa_e (Stream.of_string s);
value print e = Eprinter.apply pr_e Eprinter.empty_pc e;
if Sys.interactive.val then ()
else print_endline (print (parse Sys.argv.(1)));
</pre>
<p>Remark on the use of "curr" and "next" while printing operators:
due to left associativity, the first operand uses "curr" and the
second operand uses "next". For right associativity operators, they
should be inverted. For no associativity, both should use
"next".</p>
<p>The last line of the file allows use in either the OCaml toplevel
or as standalone program, taking the string to be printed as
parameter. It can be compiled this way:</p>
<pre>
ocamlc -pp camlp5r -I +camlp5 gramlib.cma foo.ml
</pre>
<p>Examples of use (notice the redundant parentheses automatically
removed by the printing algorithm):</p>
<pre>
$ ./a.out "(3 * x) + (2 / y)"
3 * x + 2 / y
$ ./a.out "(x+y)*(x-y)"
(x + y) * (x - y)
$ ./a.out "x + y - z"
x + y - z
$ ./a.out "(x + y) - z"
x + y - z
$ ./a.out "x + (y - z)"
x + (y - z)
</pre>
<h3 id="b:Printing-OCaml-programs">Printing OCaml programs</h3>
<p>Complete examples of usage of extensible printers are the printers
in syntaxes and extended syntaxes provided by Camlp5 in the pretty
printing <em>kits</em>:</p>
<ul>
<li><tt>pr_r.cmo</tt>: pretty print in revised syntax</li>
<li><tt>pr_o.cmo</tt>: pretty print in normal syntax</li>
<li><tt>pr_rp.cmo</tt>: also pretty print the parsers in revised
syntax</li>
<li><tt>pr_op.cmo</tt>: also pretty print the parsers in normal
syntax</li>
</ul>
<p>See the chapter entitled "<a href="opretty.html">Printing
programs</a>".</p>
<a class="toplink" href="printers.html">↑</a>
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<hr style="margin:0" /><div style="font-size: 80%"><em>Copyright 2007
Daniel de Rauglaudre (INRIA)</em></div>
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