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| author | Owen Jacobson <owen.jacobson@grimoire.ca> | 2015-07-03 22:31:49 -0400 |
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| committer | Owen Jacobson <owen.jacobson@grimoire.ca> | 2015-07-03 22:35:09 -0400 |
| commit | 76aed6ef732de38d82245b3d674f70bab30221e5 (patch) | |
| tree | d50e9a296d91ef8a49bcb29c3e80096f200a3c26 /.html/java/kwargs.html | |
| parent | 92f66d3e3a0996bb1fad9dc83d7e184f92673e5d (diff) | |
Fuck it, serve the files directly.
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| -rw-r--r-- | .html/java/kwargs.html | 223 |
1 files changed, 223 insertions, 0 deletions
diff --git a/.html/java/kwargs.html b/.html/java/kwargs.html new file mode 100644 index 0000000..64cc16b --- /dev/null +++ b/.html/java/kwargs.html @@ -0,0 +1,223 @@ +<!DOCTYPE html> +<html> +<head> + <title> + The Codex » + Keyword Arguments in Java + </title> + + <link + rel='stylesheet' + type='text/css' + href='http://fonts.googleapis.com/css?family=Buenard:400,700&subset=latin,latin-ext'> + <link + rel="stylesheet" + type="text/css" + href="../media/css/reset.css"> + <link + rel="stylesheet" + type="text/css" + href="../media/css/grimoire.css"> +</head> +<body> + +<div id="shell"> + + <ol id="breadcrumbs"> + + <li class="crumb-0 not-last"> + + <a href="../">index</a> + + </li> + + <li class="crumb-1 not-last"> + + <a href="./">java</a> + + </li> + + <li class="crumb-2 last"> + + kwargs + + </li> + + </ol> + + + + <div id="article"> + <h1 id="keyword-arguments-in-java">Keyword Arguments in Java</h1> +<h2 id="what">What</h2> +<p>Java arguments are traditionally passed by position:</p> +<pre><code>void foo(int x, int y, int z) +</code></pre> +<p>matches the call</p> +<pre><code>foo(1, 2, 3) +</code></pre> +<p>and assigns <code>1</code> to <code>x</code>, <code>2</code> to <code>y</code>, and <code>3</code> to <code>z</code> in the resulting +activation. Keyword arguments assign values to formal parameters by matching +the parameter's name, instead.</p> +<h2 id="why">Why</h2> +<p>Fuck the builder pattern, okay? Patterns like</p> +<pre><code>Response r = Response + .status(200) + .entity(foo) + .header("X-Plane", "Amazing") + .build(); +</code></pre> +<p>(from JAX-RS) mean the creation and maintenance of an entire separate type +just to handle arbitrary ordering and presence/absence of options. Ordering +can be done using keywords; presence/absence can be done by providing one +method for each legal combination of arguments (or by adding optional +arguments to Java).</p> +<p>The keyword-argument version would be something like</p> +<pre><code>Response r = new Response( + .status = 200, + .entity = foo, + .headers = Arrays.asList(Header.of("X-Plane", "Amazing")) +); +</code></pre> +<p>and the <code>ResponseBuilder</code> class would not need to exist at all for this case. +(There are others in JAX-RS that would still make <code>ResponseBuilder</code> mandatory, +but the use case for it gets much smaller.)</p> +<p>As an added bonus, the necessary class metadata to make this work would also +allow reflective frameworks such as Spring to make sensible use of the +parameter names:</p> +<pre><code><bean class="com.example.Person"> + <constructor-arg name="name" value="Erica McKenzie" /> +</bean> +</code></pre> +<h2 id="other-languages">Other Languages</h2> +<p>Python, most recently:</p> +<pre><code>def foo(x, y, z): + pass + +foo(z=3, x=1, y=2) +</code></pre> +<p>Smalltalk (and ObjectiveC) use an interleaving convention that reads very much +like keyword arguments:</p> +<pre><code>Point atX: 5 atY: 8 +</code></pre> +<h2 id="challenges">Challenges</h2> +<ul> +<li>Minimize changes to syntax.<ul> +<li>Make keyword arguments unambiguous.</li> +</ul> +</li> +<li>Minimize changes to bytecode spec.</li> +</ul> +<h2 id="proposal">Proposal</h2> +<p>Given a method definition</p> +<pre><code>void foo(int x, int y, int z) +</code></pre> +<p>Allow calls written as</p> +<pre><code>foo( + SOME-SYNTAX(x, EXPR), + SOME-SYNTAX(y, EXPR), + SOME-SYNTAX(z, EXPR) +) +</code></pre> +<p><code>SOME-SYNTAX</code> is a production that is not already legal at that point in Java, +which is a surprisingly frustrating limitation. Constructs like</p> +<pre><code>foo(x = EXPR, y = EXPR, z = EXPR) +</code></pre> +<p>are already legal (assignment is an expression) and already match positional +arguments.</p> +<p>Keyword arguments match the name of the formal argument in the method +declaration. Passing a keyword argument that does not match a formal argument +is a compilation error.</p> +<p>Calls can mix keyword arguments and positional arguments, in the following +order:</p> +<ol> +<li>Positional arguments.</li> +<li>Varargs positional arguments.</li> +<li>Keyword arguments.</li> +</ol> +<p>Passing the same argument as both a positional and a keyword argument is a +compilation error.</p> +<p>Call sites must satisfy every argument the method/constructor has (i.e., this +doesn't imply optional arguments). This makes implementation easy and +unintrusive: the compiler can implement keyword arguments by transforming them +into positional arguments. Reflective calls (<code>Method.invoke</code> and friends) can +continue accepting arguments as a sequence.</p> +<p>The <code>Method</code> class would expose a new method:</p> +<pre><code>public List<String> getArgumentNames() +</code></pre> +<p>The indexes in <code>getArgumentNames</code> match the indexes in <code>getArgumentTypes</code> and +related methods.</p> +<p>Possibilities for syntax:</p> +<ul> +<li> +<p><code>foo(x := 5, y := 8, z := 2)</code> - <code>:=</code> is never a legal sequence of tokens in + Java. Introduces one new operator-like construct; the new sequence <code>:=</code> + “looks like” assignment, which is a useful mnemonic.</p> +</li> +<li> +<p><code>foo(x ~ 5, y ~ 8, z ~ 2)</code> - <code>~</code> is not a binary operator and this is never + legal right now. This avoids introducing new operators, but adds a novel + interpretation to an existing unary operator that's not related to its + normal use.</p> +</li> +<li> +<p><code>foo(.x = 5, .y = 8, .z = 2)</code> - using <code>=</code> as the keyword binding feels more + natural. Parameter names must be legal identifiers, which means the leading + dot is unambiguous. This syntax is not legal anywhere right now (the dot + always has a leading expression). The dot is a “namespace” symbol already.</p> +</li> +</ul> +<p>To support this, the class file format will need to record the names of +parameters, not just their order. This is a breaking change, and generated +names will need to be chosen for existing class files. (This may be derivable +from debug information, where present.)</p> +<h2 id="edge-cases">Edge Cases</h2> +<ul> +<li>Mixed positional and keyword arguments.<ul> +<li>Collisions (same argument passed by both) are, I think, detectable at + compile time. This should be an error.</li> +</ul> +</li> +<li>Inheritance. It is legal for a superclass to define <code>foo(a, b)</code> and for + subclasses to override it as <code>foo(x, y)</code>. Which argument names do you use + when?</li> +<li>Varargs.</li> +</ul> + </div> + + + +<div id="comments"> +<div id="disqus_thread"></div> +<script type="text/javascript"> + /* * * CONFIGURATION VARIABLES: EDIT BEFORE PASTING INTO YOUR WEBPAGE * * */ + var disqus_shortname = 'grimoire'; // required: replace example with your forum shortname + + /* * * DON'T EDIT BELOW THIS LINE * * */ + (function() { + var dsq = document.createElement('script'); dsq.type = 'text/javascript'; dsq.async = true; + dsq.src = 'http://' + disqus_shortname + '.disqus.com/embed.js'; + (document.getElementsByTagName('head')[0] || document.getElementsByTagName('body')[0]).appendChild(dsq); + })(); +</script> +<noscript>Please enable JavaScript to view the <a href="http://disqus.com/?ref_noscript">comments powered by Disqus.</a></noscript> +<a href="http://disqus.com" class="dsq-brlink">comments powered by <span class="logo-disqus">Disqus</span></a> +</div> + + + + <div id="footer"> + <p> + + The Codex — + + Powered by <a href="http://markdoc.org/">Markdoc</a>. + +<a href="https://bitbucket.org/ojacobson/grimoire.ca/src/master/wiki/java/kwargs.md">See this page on Bitbucket</a> (<a href="https://bitbucket.org/ojacobson/grimoire.ca/history-node/master/wiki/java/kwargs.md">history</a>). + + </p> + </div> + +</div> +</body> +</html>
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