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| author | Loïc Hoguin <[email protected]> | 2016-03-28 15:36:42 +0200 |
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| committer | Loïc Hoguin <[email protected]> | 2016-03-28 15:36:42 +0200 |
| commit | fe3492a98de29942477b061cd02c92246f4bf85a (patch) | |
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diff --git a/articles/xerl-0.3-atomic-expressions/index.html b/articles/xerl-0.3-atomic-expressions/index.html new file mode 100644 index 00000000..eca9f70f --- /dev/null +++ b/articles/xerl-0.3-atomic-expressions/index.html @@ -0,0 +1,306 @@ +<!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"> + + <meta name="generator" content="Hugo 0.15" /> + + <title>Nine Nines: Xerl: atomic expressions</title> + + <link href='http://fonts.googleapis.com/css?family=Open+Sans:400,700,400italic' rel='stylesheet' type='text/css'> + + <link href="/css/bootstrap.min.css" rel="stylesheet"> + <link href="/css/99s.css" 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="Keep in touch!" href="http://twitter.com/lhoguin"><img src="/img/ico_microblog.png" data-hover="/img/ico_microblog_alt.png"></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="day">18</span> + <span class="month">Feb</span> + </p> +</header> + +<div class="paragraph"><p>We will be adding atomic integer expressions to our language.
+These look as follow in Erlang:</p></div>
+<div class="listingblock">
+<div class="content"><!-- Generator: GNU source-highlight 3.1.8
+by Lorenzo Bettini
+http://www.lorenzobettini.it
+http://www.gnu.org/software/src-highlite -->
+<pre><tt><span style="color: #993399">42</span><span style="color: #990000">.</span></tt></pre></div></div>
+<div class="paragraph"><p>And the result of this expression is of course 42.</p></div>
+<div class="paragraph"><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></div>
+<div class="paragraph"><p>First, we must of course accept integers in the tokenizer.</p></div>
+<div class="listingblock">
+<div class="content"><!-- Generator: GNU source-highlight 3.1.8
+by Lorenzo Bettini
+http://www.lorenzobettini.it
+http://www.gnu.org/software/src-highlite -->
+<pre><tt>{<span style="color: #009900">D</span>}<span style="color: #990000">+</span> <span style="color: #990000">:</span> {<span style="color: #FF6600">token</span>, {<span style="font-weight: bold"><span style="color: #000080">integer</span></span>, <span style="color: #009900">TokenLine</span>, <span style="font-weight: bold"><span style="color: #000080">list_to_integer</span></span>(<span style="color: #009900">TokenChars</span>)}}<span style="color: #990000">.</span></tt></pre></div></div>
+<div class="paragraph"><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>
+<div class="listingblock">
+<div class="content"><!-- Generator: GNU source-highlight 3.1.8
+by Lorenzo Bettini
+http://www.lorenzobettini.it
+http://www.gnu.org/software/src-highlite -->
+<pre><tt><span style="color: #FF6600">expr</span> <span style="color: #990000">-></span> <span style="color: #FF6600">integer</span> <span style="color: #990000">:</span> <span style="color: #FF6600">'$1'</span><span style="color: #990000">.</span></tt></pre></div></div>
+<div class="paragraph"><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>
+<div class="listingblock">
+<div class="content"><!-- Generator: GNU source-highlight 3.1.8
+by Lorenzo Bettini
+http://www.lorenzobettini.it
+http://www.gnu.org/software/src-highlite -->
+<pre><tt>[{<span style="font-weight: bold"><span style="color: #000080">integer</span></span>,<span style="color: #993399">1</span>,<span style="color: #993399">42</span>}]</tt></pre></div></div>
+<div class="paragraph"><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>
+<div class="listingblock">
+<div class="content"><!-- Generator: GNU source-highlight 3.1.8
+by Lorenzo Bettini
+http://www.lorenzobettini.it
+http://www.gnu.org/software/src-highlite -->
+<pre><tt><span style="font-weight: bold"><span style="color: #000000">execute</span></span>(<span style="color: #009900">Filename</span>, [{<span style="font-weight: bold"><span style="color: #000080">integer</span></span>, <span style="color: #990000">_</span>, <span style="color: #009900">Int</span>}|<span style="color: #009900">Tail</span>], <span style="color: #009900">Modules</span>) <span style="color: #990000">-></span>
+ <span style="font-weight: bold"><span style="color: #000000">io:format</span></span>(<span style="color: #FF0000">"integer ~p~n"</span>, [<span style="color: #009900">Int</span>]),
+ <span style="font-weight: bold"><span style="color: #000000">execute</span></span>(<span style="color: #009900">Filename</span>, <span style="color: #009900">Tail</span>, <span style="color: #009900">Modules</span>)<span style="color: #990000">.</span></tt></pre></div></div>
+<div class="paragraph"><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></div>
+<div class="ulist"><ul>
+<li>
+<p>
+What happens if a file contains two integers?
+</p>
+</li>
+<li>
+<p>
+Can we live without expression separators?
+</p>
+</li>
+<li>
+<p>
+Do we need an interpreter for the compile step?
+</p>
+</li>
+</ul></div>
+<div class="paragraph"><p>This is what happens when we create a file that contains two
+integers on two separate lines:</p></div>
+<div class="listingblock">
+<div class="content"><!-- Generator: GNU source-highlight 3.1.8
+by Lorenzo Bettini
+http://www.lorenzobettini.it
+http://www.gnu.org/software/src-highlite -->
+<pre><tt>[{<span style="font-weight: bold"><span style="color: #000080">integer</span></span>,<span style="color: #993399">1</span>,<span style="color: #993399">42</span>},{<span style="font-weight: bold"><span style="color: #000080">integer</span></span>,<span style="color: #993399">2</span>,<span style="color: #993399">43</span>}]</tt></pre></div></div>
+<div class="paragraph"><p>And on the same lines:</p></div>
+<div class="listingblock">
+<div class="content"><!-- Generator: GNU source-highlight 3.1.8
+by Lorenzo Bettini
+http://www.lorenzobettini.it
+http://www.gnu.org/software/src-highlite -->
+<pre><tt>[{<span style="font-weight: bold"><span style="color: #000080">integer</span></span>,<span style="color: #993399">1</span>,<span style="color: #993399">42</span>},{<span style="font-weight: bold"><span style="color: #000080">integer</span></span>,<span style="color: #993399">1</span>,<span style="color: #993399">43</span>}]</tt></pre></div></div>
+<div class="paragraph"><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>
+<div class="listingblock">
+<div class="content"><!-- Generator: GNU source-highlight 3.1.8
+by Lorenzo Bettini
+http://www.lorenzobettini.it
+http://www.gnu.org/software/src-highlite -->
+<pre><tt><span style="color: #993399">42</span> <span style="color: #990000">-</span> <span style="color: #993399">12</span></tt></pre></div></div>
+<div class="paragraph"><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>
+<div class="listingblock">
+<div class="content"><!-- Generator: GNU source-highlight 3.1.8
+by Lorenzo Bettini
+http://www.lorenzobettini.it
+http://www.gnu.org/software/src-highlite -->
+<pre><tt><span style="color: #009900">Var</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">some_module:some_function</span></span>() <span style="color: #990000">+</span> <span style="font-weight: bold"><span style="color: #000000">some_module:other_function</span></span>()
+ <span style="color: #990000">+</span> <span style="font-weight: bold"><span style="color: #000000">another_module:another_function</span></span>()</tt></pre></div></div>
+<div class="paragraph"><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></div>
+<div class="paragraph"><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></div>
+<div class="paragraph"><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></div>
+<div class="paragraph"><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></div>
+<div class="paragraph"><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></div>
+<div class="paragraph"><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></div>
+<div class="ulist"><ul>
+<li>
+<p>
+<a href="https://github.com/extend/xerl/blob/0.3/">View the source</a>
+</p>
+</li>
+</ul></div>
+ +</article> +</div> + +<div class="span3 sidecol"> +<h3>More articles</h3> +<ul id="articles-nav" class="extra_margin"> + + <li><a href="http://ninenines.eu/articles/erlanger-playbook-september-2015-update/">The Erlanger Playbook September 2015 Update</a></li> + + <li><a href="http://ninenines.eu/articles/erlanger-playbook/">The Erlanger Playbook</a></li> + + <li><a href="http://ninenines.eu/articles/erlang-validate-utf8/">Validating UTF-8 binaries with Erlang</a></li> + + <li><a href="http://ninenines.eu/articles/on-open-source/">On open source</a></li> + + <li><a href="http://ninenines.eu/articles/the-story-so-far/">The story so far</a></li> + + <li><a href="http://ninenines.eu/articles/cowboy2-qs/">Cowboy 2.0 and query strings</a></li> + + <li><a href="http://ninenines.eu/articles/january-2014-status/">January 2014 status</a></li> + + <li><a href="http://ninenines.eu/articles/farwest-funded/">Farwest got funded!</a></li> + + <li><a href="http://ninenines.eu/articles/erlang.mk-and-relx/">Build Erlang releases with Erlang.mk and Relx</a></li> + + <li><a href="http://ninenines.eu/articles/xerl-0.5-intermediate-module/">Xerl: intermediate module</a></li> + + <li><a href="http://ninenines.eu/articles/xerl-0.4-expression-separator/">Xerl: expression separator</a></li> + + <li><a href="http://ninenines.eu/articles/erlang-scalability/">Erlang Scalability</a></li> + + <li><a href="http://ninenines.eu/articles/xerl-0.3-atomic-expressions/">Xerl: atomic expressions</a></li> + + <li><a href="http://ninenines.eu/articles/xerl-0.2-two-modules/">Xerl: two modules</a></li> + + <li><a href="http://ninenines.eu/articles/xerl-0.1-empty-modules/">Xerl: empty modules</a></li> + + <li><a href="http://ninenines.eu/articles/ranch-ftp/">Build an FTP Server with Ranch in 30 Minutes</a></li> + + <li><a href="http://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 2012-2016</p> + </div> + </div> + </div> + </footer> + + + <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script> + <script src="/js/bootstrap-carousel.js"></script> + <script src="/js/bootstrap-dropdown.js"></script> + <script src="/js/custom.js"></script> + </body> +</html> + |