forked from mmcgrana/gobyexample
-
Notifications
You must be signed in to change notification settings - Fork 0
/
Copy patherrors
300 lines (238 loc) · 14.5 KB
/
errors
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
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Go by Example: Errors</title>
<link rel=stylesheet href="site.css">
</head>
<script>
onkeydown = (e) => {
if (e.key == "ArrowLeft") {
window.location.href = 'embedding';
}
if (e.key == "ArrowRight") {
window.location.href = 'goroutines';
}
}
</script>
<body>
<div class="example" id="errors">
<h2><a href="./">Go by Example</a>: Errors</h2>
<table>
<tr>
<td class="docs">
<p>In Go it’s idiomatic to communicate errors via an
explicit, separate return value. This contrasts with
the exceptions used in languages like Java and Ruby and
the overloaded single result / error value sometimes
used in C. Go’s approach makes it easy to see which
functions return errors and to handle them using the
same language constructs employed for any other,
non-error tasks.</p>
</td>
<td class="code empty leading">
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<a href="http://play.golang.org/p/NiJOpCPO3L0"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
<pre class="chroma"><span class="kn">package</span> <span class="nx">main</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"><span class="kn">import</span> <span class="p">(</span>
<span class="s">"errors"</span>
<span class="s">"fmt"</span>
<span class="p">)</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>By convention, errors are the last return value and
have type <code>error</code>, a built-in interface.</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="kd">func</span> <span class="nf">f1</span><span class="p">(</span><span class="nx">arg</span> <span class="kt">int</span><span class="p">)</span> <span class="p">(</span><span class="kt">int</span><span class="p">,</span> <span class="kt">error</span><span class="p">)</span> <span class="p">{</span>
<span class="k">if</span> <span class="nx">arg</span> <span class="o">==</span> <span class="mi">42</span> <span class="p">{</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p><code>errors.New</code> constructs a basic <code>error</code> value
with the given error message.</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="k">return</span> <span class="o">-</span><span class="mi">1</span><span class="p">,</span> <span class="nx">errors</span><span class="p">.</span><span class="nf">New</span><span class="p">(</span><span class="s">"can't work with 42"</span><span class="p">)</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"> <span class="p">}</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>A <code>nil</code> value in the error position indicates that
there was no error.</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="k">return</span> <span class="nx">arg</span> <span class="o">+</span> <span class="mi">3</span><span class="p">,</span> <span class="kc">nil</span>
<span class="p">}</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>It’s possible to use custom types as <code>error</code>s by
implementing the <code>Error()</code> method on them. Here’s a
variant on the example above that uses a custom type
to explicitly represent an argument error.</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="kd">type</span> <span class="nx">argError</span> <span class="kd">struct</span> <span class="p">{</span>
<span class="nx">arg</span> <span class="kt">int</span>
<span class="nx">prob</span> <span class="kt">string</span>
<span class="p">}</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"><span class="kd">func</span> <span class="p">(</span><span class="nx">e</span> <span class="o">*</span><span class="nx">argError</span><span class="p">)</span> <span class="nf">Error</span><span class="p">()</span> <span class="kt">string</span> <span class="p">{</span>
<span class="k">return</span> <span class="nx">fmt</span><span class="p">.</span><span class="nf">Sprintf</span><span class="p">(</span><span class="s">"%d - %s"</span><span class="p">,</span> <span class="nx">e</span><span class="p">.</span><span class="nx">arg</span><span class="p">,</span> <span class="nx">e</span><span class="p">.</span><span class="nx">prob</span><span class="p">)</span>
<span class="p">}</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"><span class="kd">func</span> <span class="nf">f2</span><span class="p">(</span><span class="nx">arg</span> <span class="kt">int</span><span class="p">)</span> <span class="p">(</span><span class="kt">int</span><span class="p">,</span> <span class="kt">error</span><span class="p">)</span> <span class="p">{</span>
<span class="k">if</span> <span class="nx">arg</span> <span class="o">==</span> <span class="mi">42</span> <span class="p">{</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>In this case we use <code>&argError</code> syntax to build
a new struct, supplying values for the two
fields <code>arg</code> and <code>prob</code>.</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="k">return</span> <span class="o">-</span><span class="mi">1</span><span class="p">,</span> <span class="o">&</span><span class="nx">argError</span><span class="p">{</span><span class="nx">arg</span><span class="p">,</span> <span class="s">"can't work with it"</span><span class="p">}</span>
<span class="p">}</span>
<span class="k">return</span> <span class="nx">arg</span> <span class="o">+</span> <span class="mi">3</span><span class="p">,</span> <span class="kc">nil</span>
<span class="p">}</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"><span class="kd">func</span> <span class="nf">main</span><span class="p">()</span> <span class="p">{</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>The two loops below test out each of our
error-returning functions. Note that the use of an
inline error check on the <code>if</code> line is a common
idiom in Go code.</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="k">for</span> <span class="nx">_</span><span class="p">,</span> <span class="nx">i</span> <span class="o">:=</span> <span class="k">range</span> <span class="p">[]</span><span class="kt">int</span><span class="p">{</span><span class="mi">7</span><span class="p">,</span> <span class="mi">42</span><span class="p">}</span> <span class="p">{</span>
<span class="k">if</span> <span class="nx">r</span><span class="p">,</span> <span class="nx">e</span> <span class="o">:=</span> <span class="nf">f1</span><span class="p">(</span><span class="nx">i</span><span class="p">);</span> <span class="nx">e</span> <span class="o">!=</span> <span class="kc">nil</span> <span class="p">{</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"f1 failed:"</span><span class="p">,</span> <span class="nx">e</span><span class="p">)</span>
<span class="p">}</span> <span class="k">else</span> <span class="p">{</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"f1 worked:"</span><span class="p">,</span> <span class="nx">r</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="k">for</span> <span class="nx">_</span><span class="p">,</span> <span class="nx">i</span> <span class="o">:=</span> <span class="k">range</span> <span class="p">[]</span><span class="kt">int</span><span class="p">{</span><span class="mi">7</span><span class="p">,</span> <span class="mi">42</span><span class="p">}</span> <span class="p">{</span>
<span class="k">if</span> <span class="nx">r</span><span class="p">,</span> <span class="nx">e</span> <span class="o">:=</span> <span class="nf">f2</span><span class="p">(</span><span class="nx">i</span><span class="p">);</span> <span class="nx">e</span> <span class="o">!=</span> <span class="kc">nil</span> <span class="p">{</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"f2 failed:"</span><span class="p">,</span> <span class="nx">e</span><span class="p">)</span>
<span class="p">}</span> <span class="k">else</span> <span class="p">{</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"f2 worked:"</span><span class="p">,</span> <span class="nx">r</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>If you want to programmatically use the data in
a custom error, you’ll need to get the error as an
instance of the custom error type via type
assertion.</p>
</td>
<td class="code">
<pre class="chroma">
<span class="nx">_</span><span class="p">,</span> <span class="nx">e</span> <span class="o">:=</span> <span class="nf">f2</span><span class="p">(</span><span class="mi">42</span><span class="p">)</span>
<span class="k">if</span> <span class="nx">ae</span><span class="p">,</span> <span class="nx">ok</span> <span class="o">:=</span> <span class="nx">e</span><span class="p">.(</span><span class="o">*</span><span class="nx">argError</span><span class="p">);</span> <span class="nx">ok</span> <span class="p">{</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="nx">ae</span><span class="p">.</span><span class="nx">arg</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="nx">ae</span><span class="p">.</span><span class="nx">prob</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
</pre>
</td>
</tr>
</table>
<table>
<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"><span class="gp">$</span> go run errors.go
<span class="go">f1 worked: 10
</span><span class="go">f1 failed: can't work with 42
</span><span class="go">f2 worked: 10
</span><span class="go">f2 failed: 42 - can't work with it
</span><span class="go">42
</span><span class="go">can't work with it</span></pre>
</td>
</tr>
<tr>
<td class="docs">
<p>See this <a href="http://blog.golang.org/2011/07/error-handling-and-go.html">great post</a>
on the Go blog for more on error handling.</p>
</td>
<td class="code empty">
</td>
</tr>
</table>
<p class="next">
Next example: <a href="goroutines">Goroutines</a>.
</p>
<p class="footer">
by <a href="https://markmcgranaghan.com">Mark McGranaghan</a> and <a href="https://eli.thegreenplace.net">Eli Bendersky</a> | <a href="https://github.com/mmcgrana/gobyexample">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
</p>
</div>
<script>
var codeLines = [];
codeLines.push('');codeLines.push('package main\u000A');codeLines.push('import (\u000A \"errors\"\u000A \"fmt\"\u000A)\u000A');codeLines.push('func f1(arg int) (int, error) {\u000A if arg \u003D\u003D 42 {\u000A');codeLines.push(' return -1, errors.New(\"can\'t work with 42\")\u000A');codeLines.push(' }\u000A');codeLines.push(' return arg + 3, nil\u000A}\u000A');codeLines.push('type argError struct {\u000A arg int\u000A prob string\u000A}\u000A');codeLines.push('func (e *argError) Error() string {\u000A return fmt.Sprintf(\"%d - %s\", e.arg, e.prob)\u000A}\u000A');codeLines.push('func f2(arg int) (int, error) {\u000A if arg \u003D\u003D 42 {\u000A');codeLines.push(' return -1, \u0026argError{arg, \"can\'t work with it\"}\u000A }\u000A return arg + 3, nil\u000A}\u000A');codeLines.push('func main() {\u000A');codeLines.push(' for _, i :\u003D range []int{7, 42} {\u000A if r, e :\u003D f1(i); e !\u003D nil {\u000A fmt.Println(\"f1 failed:\", e)\u000A } else {\u000A fmt.Println(\"f1 worked:\", r)\u000A }\u000A }\u000A for _, i :\u003D range []int{7, 42} {\u000A if r, e :\u003D f2(i); e !\u003D nil {\u000A fmt.Println(\"f2 failed:\", e)\u000A } else {\u000A fmt.Println(\"f2 worked:\", r)\u000A }\u000A }\u000A');codeLines.push(' _, e :\u003D f2(42)\u000A if ae, ok :\u003D e.(*argError); ok {\u000A fmt.Println(ae.arg)\u000A fmt.Println(ae.prob)\u000A }\u000A}\u000A');codeLines.push('');codeLines.push('');
</script>
<script src="site.js" async></script>
</body>
</html>