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
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
|
<html><head><title>XLISP catch, throw</title>
<link rel="stylesheet" type="text/css" href="reference.css">
</head>
<body>
<a href="../start.htm">Nyquist / XLISP 2.0</a> -
<a href="../manual/contents.htm">Contents</a> |
<a href="../tutorials/tutorials.htm">Tutorials</a> |
<a href="../examples/examples.htm">Examples</a> |
<a href="reference-index.htm">Reference</a>
<hr>
<h1>catch, throw</h1>
<hr>
<p><table cellpadding="0" cellspacing="0" style="margin-left:10px"><tbody>
<tr valign="top">
<td><nobr>Type:</nobr></td>
<td><nobr> - </nobr></td>
<td width="100%"><nobr>special form (fsubr)</nobr></td>
</tr>
<tr valign="top">
<td><nobr>Source:</nobr></td>
<td><nobr> - </nobr></td>
<td width="100%"><nobr>xlcont.c, xljump.c</nobr></td>
</tr>
</tbody></table></p>
<h2>Syntax</h2>
<p><div class="box">
<dl>
<dt>(<b>catch</b> <i>tag-symbol</i> [<i>expr</i> ... ])</dt>
<dd><i>tag-symbol</i> - an expression that evaluates to a symbol<br>
<i>expr</i> - an optional series of expressions to be evaluated<br>
returns - the value of the last expression the
<a href="throw.htm">throw</a> expression</dd>
</dl>
<dl>
<dt>(<b>throw</b> <i>tag-symbol</i> [<i>expr</i>])</dt>
<dd><i>tag-symbol</i> - an expression that evaluates to a symbol<br>
<i>expr</i> - an optional expression to be returned<br>
returns - never returns</dd>
</dl>
</div></p>
<h2>Description</h2>
<p>The 'catch' and 'throw' special forms allow for non-local exits and traps
without going through the intermediate evaluations and function returns:</p>
<pre class="example">
(catch <font color="#0000CC">tag-symbol</font>
[<font color="#0000CC">expr</font> <font color="#008844">...</font>]
(throw <font color="#0000CC">tag-symbol</font> [<font color="#0000CC">expr</font>]))
</pre>
<p>If there is a 'catch' for a '<nobr>tag-symbol</nobr>' that has no 'throw'
performed to it, 'catch' returns the value returned from 'expr':</p>
<pre class="example">
> (catch 'mytag
(+ 1 (+ 2 3)))
6
</pre>
<p>If there is no 'expr', <nobr><a href="nil.htm">NIL</a> is</nobr>
returned:</p>
<pre class="example">
> (catch 'mytag)
NIL
</pre>
<p>The 'expr' in 'throw' specifies what value is to be returned by the
corresponding 'catch':</p>
<pre class="example">
> (catch 'mytag
(+ 1 (throw 'mytag 55)))
55
</pre>
<p>If there is no 'expr' in 'throw', <a href="nil.htm">NIL</a> is returned
to the corresponding 'catch':</p>
<pre class="example">
> (catch 'mytag
(throw 'mytag))
NIL
</pre>
<p>If more than one 'catch' is set up for the same
'<nobr>tag-symbol</nobr>', the most recently evaluated
'<nobr>tag-symbol</nobr>' will be the one that does the actual catching:</p>
<pre class="example">
> (catch 'mytag
(catch 'mytag
(throw 'mytag))
(print 'hello))
HELLO
</pre>
<p>If a 'throw' is evaluated with no corresponding 'catch', an error is
signalled:</p>
<pre class="example">
> (catch 'mytag
(throw 'foo))
<font color="#AA0000">error: no target for THROW</font>
</pre>
<p></p>
<h2>Examples</h2>
<pre class="example">
(defun in (x)
(if (numberp x) <font color="#008844">; if X is a number</font>
(+ x x) <font color="#008844">; then double X</font>
(throw 'math 42))) <font color="#008844">; else throw 42</font>
(defun out (x)
(princ "<")
(princ (* (in x) 2)) <font color="#008844">; double via multiply</font>
(princ ">")
"there")
(defun main (x)
(catch 'math (out x))) <font color="#008844">; catch the throw from IN</font>
> (in 5)
10 <font color="#008844">; return value</font>
> (out 5)
<20> <font color="#008844">; screen output of PRINC</font>
"there" <font color="#008844">; return value</font>
> (main 5)
<20> <font color="#008844">; screen output of PRINC</font>
"there" <font color="#008844">; return value</font>
> (main 'a)
< <font color="#008844">; screen output of PRINC</font>
42 <font color="#008844">; return value</font>
</pre>
<p>See <nobr><a href="addition.htm"> + </a></nobr>,
<nobr><a href="multiplication.htm"> * </a></nobr>,
<a href="defun.htm">defun</a>,
<nobr><a href="if.htm"> if </a></nobr>,
<a href="numberp.htm">numberp</a>,
<a href="princ.htm">princ</a>.</p>
<p><div class="box">
<p><b>Note:</b> 'catch' and 'throw' accept not only symbols as
'<nobr>tag-symbol</nobr>', but if a '<nobr>tag-symbol</nobr>' cannot be
compared via <a href="eql.htm">eql</a>, an error is signalled:</p>
<pre class="example">
> (catch "mytag"
(throw "mytag"))
<font color="#AA0000">error: no target for THROW</font>
</pre>
<p>This was reproduced with <nobr>Nyquist 3.03</nobr> in <nobr>December
2010</nobr>.</p>
</div></p>
<p>See also:</p>
<ul>
<li><nobr>Contents → <a href="../manual/contents.htm#control-constructs">Control Constructs</a></nobr></li>
</ul>
<p><nobr> <a href="#top">Back to Top</nobr></a></p>
<hr>
<a href="../start.htm">Nyquist / XLISP 2.0</a> -
<a href="../manual/contents.htm">Contents</a> |
<a href="../tutorials/tutorials.htm">Tutorials</a> |
<a href="../examples/examples.htm">Examples</a> |
<a href="reference-index.htm">Reference</a>
</body></html>
|
