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
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
|
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta name="description" content="">
<meta name="author" content="Loïc Hoguin based on a design from (Soft10) Pol Cámara">
<title>Nine Nines: Xerl: atomic expressions</title>
<link href='https://fonts.googleapis.com/css?family=Open+Sans:400,700,400italic' rel='stylesheet' type='text/css'>
<link href="/css/99s.css?r=7" rel="stylesheet">
<link rel="shortcut icon" href="/img/ico/favicon.ico">
<link rel="apple-touch-icon-precomposed" sizes="114x114" href="/img/ico/apple-touch-icon-114.png">
<link rel="apple-touch-icon-precomposed" sizes="72x72" href="/img/ico/apple-touch-icon-72.png">
<link rel="apple-touch-icon-precomposed" href="/img/ico/apple-touch-icon-57.png">
</head>
<body class="">
<header id="page-head">
<div id="topbar" class="container">
<div class="row">
<div class="span2">
<h1 id="logo"><a href="/" title="99s">99s</a></h1>
</div>
<div class="span10">
<div id="side-header">
<nav>
<ul>
<li class="active"><a title="Hear my thoughts" href="/articles">Articles</a></li>
<li><a title="Watch my talks" href="/talks">Talks</a></li>
<li><a title="Read the docs" href="/docs">Documentation</a></li>
<li><a title="Request my services" href="/services">Consulting & Training</a></li>
</ul>
</nav>
<ul id="social">
<li>
<a href="https://github.com/ninenines" title="Check my Github repositories"><img src="/img/ico_github.png" data-hover="/img/ico_github_alt.png" alt="Github"></a>
</li>
<li>
<a title="Contact me" href="mailto:[email protected]"><img src="/img/ico_mail.png" data-hover="/img/ico_mail_alt.png"></a>
</li>
</ul>
</div>
</div>
</div>
</div>
</header>
<div id="contents">
<div class="container">
<div class="row">
<div class="span9 maincol">
<article class="blog_item">
<header>
<h1 class="lined-header"><span>Xerl: atomic expressions</span></h1>
<p class="date">
<span class="year">2013</span>
<span class="day-month">18 Feb</span>
</p>
</header>
<p>We will be adding atomic integer expressions to our language. These look as follow in Erlang:</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><font color="#993399">42</font><font color="#990000">.</font></tt></pre>
</div></div>
<p>And the result of this expression is of course 42.</p>
<p>We will be running this expression at compile time, since we don't have the means to run code at runtime yet. This will of course result in no module being compiled, but that's OK, it will allow us to discuss a few important things we'll have to plan for later on.</p>
<p>First, we must of course accept integers in the tokenizer.</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt>{<font color="#009900">D</font>}<font color="#990000">+</font> <font color="#990000">:</font> {<font color="#FF6600">token</font>, {<b><font color="#000080">integer</font></b>, <font color="#009900">TokenLine</font>, <b><font color="#000080">list_to_integer</font></b>(<font color="#009900">TokenChars</font>)}}<font color="#990000">.</font></tt></pre>
</div></div>
<p>We must then accept atomic integer expressions in the parser. This is a simple change. The integer token is terminal so we need to add it to the list of terminals, and then we only need to add it as a possible expression.</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><font color="#FF6600">expr</font> <font color="#990000">-></font> <font color="#FF6600">integer</font> <font color="#990000">:</font> <font color="#FF6600">'$1'</font><font color="#990000">.</font></tt></pre>
</div></div>
<p>A file containing only the number 42 (with no terminating dot) will give the following result when parsing it. This is incidentally the same result as when tokenizing.</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt>[{<b><font color="#000080">integer</font></b>,<font color="#993399">1</font>,<font color="#993399">42</font>}]</tt></pre>
</div></div>
<p>We must then evaluate it. We're going to interpret it for now. Since the result of this expression is not stored in a variable, we are going to simply print it on the screen and discard it.</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><b><font color="#000000">execute</font></b>(<font color="#009900">Filename</font>, [{<b><font color="#000080">integer</font></b>, <font color="#990000">_</font>, <font color="#009900">Int</font>}|<font color="#009900">Tail</font>], <font color="#009900">Modules</font>) <font color="#990000">-></font>
<b><font color="#000000">io:format</font></b>(<font color="#FF0000">"integer ~p~n"</font>, [<font color="#009900">Int</font>]),
<b><font color="#000000">execute</font></b>(<font color="#009900">Filename</font>, <font color="#009900">Tail</font>, <font color="#009900">Modules</font>)<font color="#990000">.</font></tt></pre>
</div></div>
<p>You might think by now that what we've done so far this time is useless. It brings up many interesting questions though.</p>
<ul><li>What happens if a file contains two integers?
</li>
<li>Can we live without expression separators?
</li>
<li>Do we need an interpreter for the compile step?
</li>
</ul>
<p>This is what happens when we create a file that contains two integers on two separate lines:</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt>[{<b><font color="#000080">integer</font></b>,<font color="#993399">1</font>,<font color="#993399">42</font>},{<b><font color="#000080">integer</font></b>,<font color="#993399">2</font>,<font color="#993399">43</font>}]</tt></pre>
</div></div>
<p>And on the same lines:</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt>[{<b><font color="#000080">integer</font></b>,<font color="#993399">1</font>,<font color="#993399">42</font>},{<b><font color="#000080">integer</font></b>,<font color="#993399">1</font>,<font color="#993399">43</font>}]</tt></pre>
</div></div>
<p>Does this mean we do not need separators between expressions? Not quite. The <code>+</code> and <code>-</code> operators are an example of why we can't have nice things. They are ambiguous. They have two different meanings: make an atomic integer positive or negative, or perform an addition or a substraction between two integers. Without a separator you won't be able to know if the following snippet is one or two expressions:</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><font color="#993399">42</font> <font color="#990000">-</font> <font color="#993399">12</font></tt></pre>
</div></div>
<p>Can we use the line ending as an expression separator then? Some languages make whitespace important, often the line separator becomes the expression separator. I do not think this is the best idea, it can lead to errors. For example the following snippet would be two expressions:</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><font color="#009900">Var</font> <font color="#990000">=</font> <b><font color="#000000">some_module:some_function</font></b>() <font color="#990000">+</font> <b><font color="#000000">some_module:other_function</font></b>()
<font color="#990000">+</font> <b><font color="#000000">another_module:another_function</font></b>()</tt></pre>
</div></div>
<p>It is not obvious what would happen unless you are a veteran of the language, and so we will not go down that road. We will use an expression separator just like in Erlang: the comma. We will however allow a trailing comma to make copy pasting code easier, even if this means some old academics guy will go nuts about it later on. This trailing comma will be optional and simply discarded by the parser when encountered. We will implement this next.</p>
<p>The question as to how we will handle running expressions remains. We have two choices here: we can write an interpreter, or we can compile the code and run it. Writing an interpreter would require us to do twice the work, and we are lazy, so we will not do that.</p>
<p>You might already know that Erlang does not use the same code for compiling and for evaluating commands in the shell. The main reason for this is that in Erlang everything isn't an expression. Indeed, the compiler compiles forms which contain expressions, but you can't have forms in the shell.</p>
<p>How are we going to compile the code that isn't part of a module then? What do we need to run at compile-time, anyway? The body of the file itself, of course. The body of module declarations. That's about it.</p>
<p>For the file itself, we can simply compile it as a big function that will be executed. Then, everytime we encounter a module declaration, we will run the compiler on its body, making its body essentially a big function that will be executed. The same mechanism will be applied when we encounter a module declaration at runtime.</p>
<p>At runtime there's nothing else for us to do, the result of this operation will load all the compiled modules. At compile time we will also want to save them to a file. We'll see later how we can do that.</p>
<ul><li><a href="https://github.com/extend/xerl/blob/0.3/">View the source</a>
</li>
</ul>
</article>
</div>
<div class="span3 sidecol">
<h3>More articles</h3>
<ul id="articles-nav" class="extra_margin">
<li><a href="https://ninenines.eu/articles/merry-christmas-2019/">Merry Christmas 2019: New Beginnings</a></li>
<li><a href="https://ninenines.eu/articles/github-sponsors/">GitHub Sponsors</a></li>
<li><a href="https://ninenines.eu/articles/cowboy-2.7.0/">Cowboy 2.7</a></li>
<li><a href="https://ninenines.eu/articles/gun-2.0.0-pre.1/">Gun 2.0 pre-release 1</a></li>
<li><a href="https://ninenines.eu/articles/gun-2.0.0-pre.2/">Gun 2.0 pre-release 2</a></li>
<li><a href="https://ninenines.eu/articles/erlang-meetup-10-septembre-2019/">Erlang meetup: 10 septembre 2019</a></li>
<li><a href="https://ninenines.eu/articles/ranch-2.0.0-rc.1/">Ranch 2.0 release candidate 1</a></li>
<li><a href="https://ninenines.eu/articles/joe_the_rubber_duck/">Joe Armstrong the rubber duck</a></li>
<li><a href="https://ninenines.eu/articles/merry-christmas-2018/">Merry Christmas 2018: A Recap</a></li>
<li><a href="https://ninenines.eu/articles/cowboy-2.6.0/">Cowboy 2.6</a></li>
<li><a href="https://ninenines.eu/articles/ranch-1.7.0/">Ranch 1.7</a></li>
<li><a href="https://ninenines.eu/articles/cowboy-2.5.0/">Cowboy 2.5</a></li>
<li><a href="https://ninenines.eu/articles/gun-1.3.0/">Gun 1.3</a></li>
<li><a href="https://ninenines.eu/articles/gun-1.2.0/">Gun 1.2</a></li>
<li><a href="https://ninenines.eu/articles/ranch-1.6.0/">Ranch 1.6</a></li>
<li><a href="https://ninenines.eu/articles/gun-1.0.0/">Gun 1.0</a></li>
<li><a href="https://ninenines.eu/articles/asciideck/">Asciideck: Asciidoc for Erlang</a></li>
<li><a href="https://ninenines.eu/articles/gun-1.0.0-rc.1/">Gun 1.0 release candidate 1</a></li>
<li><a href="https://ninenines.eu/articles/cowboy-2.4.0/">Cowboy 2.4</a></li>
<li><a href="https://ninenines.eu/articles/cowboy-2.3.0/">Cowboy 2.3</a></li>
<li><a href="https://ninenines.eu/articles/cowboy-2.2.0/">Cowboy 2.2</a></li>
<li><a href="https://ninenines.eu/articles/cowboy-2.1.0/">Cowboy 2.1</a></li>
<li><a href="https://ninenines.eu/articles/cowboy-2.0.0/">Cowboy 2.0</a></li>
<li><a href="https://ninenines.eu/articles/cowboy-2.0.0-rc.2/">Cowboy 2.0 release candidate 2</a></li>
<li><a href="https://ninenines.eu/articles/cowboy-2.0.0-rc.1/">Cowboy 2.0 release candidate 1</a></li>
<li><a href="https://ninenines.eu/articles/the-elephant-in-the-room/">The elephant in the room</a></li>
<li><a href="https://ninenines.eu/articles/dont-let-it-crash/">Don't let it crash</a></li>
<li><a href="https://ninenines.eu/articles/cowboy-2.0.0-pre.4/">Cowboy 2.0 pre-release 4</a></li>
<li><a href="https://ninenines.eu/articles/ranch-1.3/">Ranch 1.3</a></li>
<li><a href="https://ninenines.eu/articles/ml-archives/">Mailing list archived</a></li>
<li><a href="https://ninenines.eu/articles/website-update/">Website update</a></li>
<li><a href="https://ninenines.eu/articles/erlanger-playbook-september-2015-update/">The Erlanger Playbook September 2015 Update</a></li>
<li><a href="https://ninenines.eu/articles/erlanger-playbook/">The Erlanger Playbook</a></li>
<li><a href="https://ninenines.eu/articles/erlang-validate-utf8/">Validating UTF-8 binaries with Erlang</a></li>
<li><a href="https://ninenines.eu/articles/on-open-source/">On open source</a></li>
<li><a href="https://ninenines.eu/articles/the-story-so-far/">The story so far</a></li>
<li><a href="https://ninenines.eu/articles/cowboy2-qs/">Cowboy 2.0 and query strings</a></li>
<li><a href="https://ninenines.eu/articles/january-2014-status/">January 2014 status</a></li>
<li><a href="https://ninenines.eu/articles/farwest-funded/">Farwest got funded!</a></li>
<li><a href="https://ninenines.eu/articles/erlang.mk-and-relx/">Build Erlang releases with Erlang.mk and Relx</a></li>
<li><a href="https://ninenines.eu/articles/xerl-0.5-intermediate-module/">Xerl: intermediate module</a></li>
<li><a href="https://ninenines.eu/articles/xerl-0.4-expression-separator/">Xerl: expression separator</a></li>
<li><a href="https://ninenines.eu/articles/erlang-scalability/">Erlang Scalability</a></li>
<li><a href="https://ninenines.eu/articles/xerl-0.3-atomic-expressions/">Xerl: atomic expressions</a></li>
<li><a href="https://ninenines.eu/articles/xerl-0.2-two-modules/">Xerl: two modules</a></li>
<li><a href="https://ninenines.eu/articles/xerl-0.1-empty-modules/">Xerl: empty modules</a></li>
<li><a href="https://ninenines.eu/articles/ranch-ftp/">Build an FTP Server with Ranch in 30 Minutes</a></li>
<li><a href="https://ninenines.eu/articles/tictactoe/">Erlang Tic Tac Toe</a></li>
</ul>
<h3>Feedback</h3>
<p>Feel free to <a href="mailto:[email protected]">email us</a>
if you found any mistake or need clarification on any of the
articles.</p>
</div>
</div>
</div>
</div>
<footer>
<div class="container">
<div class="row">
<div class="span6">
<p id="scroll-top"><a href="#">↑ Scroll to top</a></p>
<nav>
<ul>
<li><a href="mailto:[email protected]" title="Contact us">Contact us</a></li><li><a href="https://github.com/ninenines/ninenines.github.io" title="Github repository">Contribute to this site</a></li>
</ul>
</nav>
</div>
<div class="span6 credits">
<p><img src="/img/footer_logo.png"></p>
<p>Copyright © Loïc Hoguin 2012-2018</p>
</div>
</div>
</div>
</footer>
<script src="/js/custom.js"></script>
</body>
</html>
|
