1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
|
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<!-- $Id: extfun.html,v 6.3 2012-01-09 14:22:20 deraugla Exp $ -->
<!-- Copyright (c) INRIA 2007-2012 -->
<title>Extensible functions</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<meta http-equiv="Content-Style-Type" content="text/css" />
<link rel="stylesheet" type="text/css" href="styles/base.css"
title="Normal" />
</head>
<body>
<div id="menu">
</div>
<div id="content">
<h1 class="top">Extensible functions</h1>
<p>Extensible functions allows the definition of pattern matching functions
which are extensible by adding new cases that are inserted
automatically at the proper place by comparing the patterns. The pattern
cases are ordered according to syntax trees representing them, "when"
statements being inserted before the cases without "when".</p>
<p>Notice that extensible functions are functional: when extending a
function, a new function is returned.</p>
<p>The extensible functions are used in
the <a href="pretty.html">pretty printing</a> system of Camlp5.</p>
<div id="tableofcontents">
</div>
<h2>Syntax</h2>
<p>The syntax of the extensible functions, when loading
"pa_extfun.cmo", is the following:</p>
<pre>
expression ::= extensible-function
extensible-function ::= "extfun" expression "with" "[" match-cases "]"
match-cases ::= match-case "|" match-cases
match-case ::= pattern "->" expression
| pattern "when" expression "->" expression
</pre>
<p>It is an extension of the same syntax as the "match" and "try"
constructions.</p>
<h2>Semantics</h2>
<p>The statement "extend" defined by the syntax takes an extensible
function and return another extensible function with the new match
cases inserted at the proper place within the initial extensible
function.</p>
<p>Extensible functions are of type "<code>Extfun.t a b</code>", which
is an abstract type, where "<code>a</code>" and "<code>b</code>" are
respectively the type of the patterns and the type of the
expressions. It corresponds to a function of type "<code>a ->
b</code>".</p>
<p>The function "<code>Extfun.apply</code>" takes an extensible
function as parameter and returns a function which can be applied
like a normal function.</p>
<p>The value "<code>Extfun.empty</code>" is an empty extensible
function, of type "<code>Extfun.t 'a 'b</code>". When applied with
"<code>Extfun.apply</code>" and a parameter, it raises the exception
"<code>Extfun.Failure</code>" whatever the parameter.</p>
<p>For debugging, it is possible to use the function
"<code>Extfun.print</code>" which displays the match cases of the
extensible functions. (Only the patterns are displayed in
clear text, the associated expressions are not.)</p>
<p>The match cases are inserted according to the following rules:</p>
<ul>
<li>The match cases are inserted in the order they are defined in
the syntax "<code>extend</code>"</li>
<li>A match case pattern with "when" is inserted before a match case
pattern without "when".</li>
<li>Two match cases patterns both with "when" or both without "when"
are inserted according to the alphabetic order of some internal
syntax tree of the patterns where bound variables names are
<span class="u">not</span> taken into account.</li>
<li>If two match cases patterns without "when" have the same
patterns internal syntax tree, the initial one is silently
removed.</li>
<li>If two match cases patterns with "when" have the same patterns
internal syntax tree, the new one is inserted before the old
one.</li>
</ul>
<div class="trailer">
</div>
</div>
</body>
</html>
|
