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| author | Anas Nashif <anas.nashif@intel.com> | 2013-08-26 08:15:55 -0400 |
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| committer | Anas Nashif <anas.nashif@intel.com> | 2013-08-26 08:15:55 -0400 |
| commit | bb4dd8289b351fae6b55e303f189127a394a1edd (patch) | |
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diff --git a/libs/phoenix/doc/html/phoenix/basics.html b/libs/phoenix/doc/html/phoenix/basics.html new file mode 100644 index 0000000000..ed7a403faa --- /dev/null +++ b/libs/phoenix/doc/html/phoenix/basics.html @@ -0,0 +1,273 @@ +<html> +<head> +<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> +<title>Basics</title> +<link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css"> +<meta name="generator" content="DocBook XSL Stylesheets V1.76.1"> +<link rel="home" href="../index.html" title="Chapter 1. Phoenix 3.0"> +<link rel="up" href="../index.html" title="Chapter 1. Phoenix 3.0"> +<link rel="prev" href="starter_kit/more.html" title="More"> +<link rel="next" href="organization.html" title="Organization"> +</head> +<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> +<table cellpadding="2" width="100%"><tr> +<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../boost.png"></td> +<td align="center"><a href="../../../../../index.html">Home</a></td> +<td align="center"><a href="../../../../../libs/libraries.htm">Libraries</a></td> +<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> +<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> +<td align="center"><a href="../../../../../more/index.htm">More</a></td> +</tr></table> +<hr> +<div class="spirit-nav"> +<a accesskey="p" href="starter_kit/more.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="organization.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> +</div> +<div class="section"> +<div class="titlepage"><div><div><h2 class="title" style="clear: both"> +<a name="phoenix.basics"></a><a class="link" href="basics.html" title="Basics">Basics</a> +</h2></div></div></div> +<p> + Almost everything is a function in the Phoenix library that can be evaluated + as <code class="computeroutput"><span class="identifier">f</span><span class="special">(</span><span class="identifier">a1</span><span class="special">,</span> <span class="identifier">a2</span><span class="special">,</span> <span class="special">...,</span> a/n/<span class="special">)</span></code>, where <span class="emphasis"><em>n</em></span> is the function's + arity, or number of arguments that the function expects. Operators are also + functions. For example, <code class="computeroutput"><span class="identifier">a</span> <span class="special">+</span> <span class="identifier">b</span></code> is just + a function with arity == 2 (or binary). <code class="computeroutput"><span class="identifier">a</span> + <span class="special">+</span> <span class="identifier">b</span></code> + is the same as <code class="computeroutput"><span class="identifier">add</span><span class="special">(</span><span class="identifier">a</span><span class="special">,</span> <span class="identifier">b</span><span class="special">)</span></code>, <code class="computeroutput"><span class="identifier">a</span> + <span class="special">+</span> <span class="identifier">b</span> <span class="special">+</span> <span class="identifier">c</span></code> is the + same as <code class="computeroutput"><span class="identifier">add</span><span class="special">(</span><span class="identifier">add</span><span class="special">(</span><span class="identifier">a</span><span class="special">,</span> <span class="identifier">b</span><span class="special">),</span> + <span class="identifier">c</span><span class="special">)</span></code>. + </p> +<div class="note"><table border="0" summary="Note"> +<tr> +<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td> +<th align="left">Note</th> +</tr> +<tr><td align="left" valign="top"><p> + Amusingly, functions may even return functions. We shall see what this means + in a short while. + </p></td></tr> +</table></div> +<h4> +<a name="phoenix.basics.h0"></a> + <span><a name="phoenix.basics.partial_function_application"></a></span><a class="link" href="basics.html#phoenix.basics.partial_function_application">Partial + Function Application</a> + </h4> +<p> + Think of a function as a black box. You pass arguments and it returns something + back. The figure below depicts the typical scenario. + </p> +<p> + <span class="inlinemediaobject"><img src="../images/fbox.png" alt="fbox"></span> + </p> +<p> + A fully evaluated function is one in which all the arguments are given. All + functions in plain C++ are fully evaluated. When you call the <code class="computeroutput"><span class="identifier">sin</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span></code> function, you have to pass a number x. The + function will return a result in return: the sin of x. When you call the <code class="computeroutput"><span class="identifier">add</span><span class="special">(</span><span class="identifier">x</span><span class="special">,</span> <span class="identifier">y</span><span class="special">)</span></code> + function, you have to pass two numbers x and y. The function will return the + sum of the two numbers. The figure below is a fully evaluated <code class="computeroutput"><span class="identifier">add</span></code> function. + </p> +<p> + <span class="inlinemediaobject"><img src="../images/adder.png" alt="adder"></span> + </p> +<p> + A partially applied function, on the other hand, is one in which not all the + arguments are supplied. If we are able to partially apply the function <code class="computeroutput"><span class="identifier">add</span></code> above, we may pass only the first argument. + In doing so, the function does not have all the required information it needs + to perform its task to compute and return a result. What it returns instead + is another function, a lambda function. Unlike the original <code class="computeroutput"><span class="identifier">add</span></code> + function which has an arity of 2, the resulting lambda function has an arity + of 1. Why? because we already supplied part of the input: <code class="computeroutput"><span class="number">2</span></code> + </p> +<p> + <span class="inlinemediaobject"><img src="../images/add2.png" alt="add2"></span> + </p> +<p> + Now, when we shove in a number into our lambda function, it will return 2 plus + whatever we pass in. The lambda function essentially remembers 1) the original + function, <code class="computeroutput"><span class="identifier">add</span></code>, and 2) the partial + input, 2. The figure below illustrates a case where we pass 3 to our lambda + function, which then returns 5: + </p> +<p> + <span class="inlinemediaobject"><img src="../images/add2_call.png" alt="add2_call"></span> + </p> +<p> + Obviously, partially applying the <code class="computeroutput"><span class="identifier">add</span></code> + function, as we see above, cannot be done directly in C++ where we are expected + to supply all the arguments that a function expects. That's where the Phoenix + library comes in. The library provides the facilities to do partial function + application. And even more, with Phoenix, these resulting functions won't be + black boxes anymore. + </p> +<h4> +<a name="phoenix.basics.h1"></a> + <span><a name="phoenix.basics.stl_and_higher_order_functions"></a></span><a class="link" href="basics.html#phoenix.basics.stl_and_higher_order_functions">STL + and higher order functions</a> + </h4> +<p> + So, what's all the fuss? What makes partial function application so useful? + Recall our original example in the <a class="link" href="starter_kit/lazy_operators.html" title="Lazy Operators">previous + section</a>: + </p> +<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">find_if</span><span class="special">(</span><span class="identifier">c</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">c</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">arg1</span> <span class="special">%</span> <span class="number">2</span> <span class="special">==</span> <span class="number">1</span><span class="special">)</span> +</pre> +<p> + The expression <code class="computeroutput"><span class="identifier">arg1</span> <span class="special">%</span> + <span class="number">2</span> <span class="special">==</span> <span class="number">1</span></code> evaluates to a lambda function. <code class="computeroutput"><span class="identifier">arg1</span></code> is a placeholder for an argument to + be supplied later. Hence, since there's only one unsupplied argument, the lambda + function has an arity 1. It just so happens that <code class="computeroutput"><span class="identifier">find_if</span></code> + supplies the unsupplied argument as it loops from <code class="computeroutput"><span class="identifier">c</span><span class="special">.</span><span class="identifier">begin</span><span class="special">()</span></code> + to <code class="computeroutput"><span class="identifier">c</span><span class="special">.</span><span class="identifier">end</span><span class="special">()</span></code>. + </p> +<div class="note"><table border="0" summary="Note"> +<tr> +<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td> +<th align="left">Note</th> +</tr> +<tr><td align="left" valign="top"><p> + Higher order functions are functions which can take other functions as arguments, + and may also return functions as results. Higher order functions are functions + that are treated like any other objects and can be used as arguments and + return values from functions. + </p></td></tr> +</table></div> +<h4> +<a name="phoenix.basics.h2"></a> + <span><a name="phoenix.basics.lazy_evaluation"></a></span><a class="link" href="basics.html#phoenix.basics.lazy_evaluation">Lazy + Evaluation</a> + </h4> +<p> + In Phoenix, to put it more accurately, function evaluation has two stages: + </p> +<div class="orderedlist"><ol class="orderedlist" type="1"> +<li class="listitem"> + Partial application + </li> +<li class="listitem"> + Final evaluation + </li> +</ol></div> +<p> + The first stage is handled by a set of generator functions. These are your + front ends (in the client's perspective). These generators create (through + partial function application), higher order functions that can be passed on + just like any other function pointer or function object. The second stage, + the actual function call, can be invoked or executed anytime in the future, + or not at all; hence <span class="emphasis"><em>"lazy"</em></span>. + </p> +<p> + If we look more closely, the first step involves partial function application: + </p> +<pre class="programlisting"><span class="identifier">arg1</span> <span class="special">%</span> <span class="number">2</span> <span class="special">==</span> <span class="number">1</span> +</pre> +<p> + The second step is the actual function invocation (done inside the <code class="computeroutput"><span class="identifier">find_if</span></code> function. These are the back-ends + (often, the final invocation is never actually seen by the client). In our + example, the <code class="computeroutput"><span class="identifier">find_if</span></code>, if we + take a look inside, we'll see something like: + </p> +<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">InputIterator</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Predicate</span><span class="special">></span> +<span class="identifier">InputIterator</span> +<span class="identifier">find_if</span><span class="special">(</span><span class="identifier">InputIterator</span> <span class="identifier">first</span><span class="special">,</span> <span class="identifier">InputIterator</span> <span class="identifier">last</span><span class="special">,</span> <span class="identifier">Predicate</span> <span class="identifier">pred</span><span class="special">)</span> +<span class="special">{</span> + <span class="keyword">while</span> <span class="special">(</span><span class="identifier">first</span> <span class="special">!=</span> <span class="identifier">last</span> <span class="special">&&</span> <span class="special">!</span><span class="identifier">pred</span><span class="special">(*</span><span class="identifier">first</span><span class="special">))</span> <span class="comment">// <--- The lambda function is called here</span> + <span class="special">++</span><span class="identifier">first</span><span class="special">;</span> <span class="comment">// passing in *first</span> + <span class="keyword">return</span> <span class="identifier">first</span><span class="special">;</span> +<span class="special">}</span> +</pre> +<p> + Again, typically, we, as clients, see only the first step. However, in this + document and in the examples and tests provided, don't be surprised to see + the first and second steps juxtaposed in order to illustrate the complete semantics + of Phoenix expressions. Examples: + </p> +<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span> +<span class="keyword">int</span> <span class="identifier">y</span> <span class="special">=</span> <span class="number">2</span><span class="special">;</span> + +<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="special">(</span><span class="identifier">arg1</span> <span class="special">%</span> <span class="number">2</span> <span class="special">==</span> <span class="number">1</span><span class="special">)(</span><span class="identifier">x</span><span class="special">)</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="comment">// prints 1 or true</span> +<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="special">(</span><span class="identifier">arg1</span> <span class="special">%</span> <span class="number">2</span> <span class="special">==</span> <span class="number">1</span><span class="special">)(</span><span class="identifier">y</span><span class="special">)</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="comment">// prints 0 or false</span> +</pre> +<h4> +<a name="phoenix.basics.h3"></a> + <span><a name="phoenix.basics.forwarding_function_problem"></a></span><a class="link" href="basics.html#phoenix.basics.forwarding_function_problem">Forwarding + Function Problem</a> + </h4> +<p> + Usually, we, as clients, write the call-back functions while libraries (such + as STL) provide the callee (e.g. <code class="computeroutput"><span class="identifier">find_if</span></code>). + In case the role is reversed, e.g. if you have to write an STL algorithm that + takes in a predicate, or develop a GUI library that accepts event handlers, + you have to be aware of a little known problem in C++ called the "<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2002/n1385.htm" target="_top">Forwarding + Function Problem</a>". + </p> +<p> + Look again at the code above: + </p> +<pre class="programlisting"><span class="special">(</span><span class="identifier">arg1</span> <span class="special">%</span> <span class="number">2</span> <span class="special">==</span> <span class="number">1</span><span class="special">)(</span><span class="identifier">x</span><span class="special">)</span> +</pre> +<p> + Notice that, in the second-stage (the final evaluation), we used a variable + <code class="computeroutput"><span class="identifier">x</span></code>. + </p> +<p> + In Phoenix we emulated perfect forwarding through preprocessor macros generating + code to allow const and non-const references. + </p> +<p> + We generate these second-stage overloads for Phoenix expression up to <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_PERFECT_FORWARD_LIMIT</span></code> + </p> +<div class="note"><table border="0" summary="Note"> +<tr> +<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td> +<th align="left">Note</th> +</tr> +<tr><td align="left" valign="top"><p> + You can set <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_PERFECT_FORWARD_LIMIT</span></code>, + the predefined maximum perfect forward arguments an actor can take. By default, + <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_PERFECT_FORWARDLIMIT</span></code> + is set to 3. + </p></td></tr> +</table></div> +<h4> +<a name="phoenix.basics.h4"></a> + <span><a name="phoenix.basics.polymorphic_functions"></a></span><a class="link" href="basics.html#phoenix.basics.polymorphic_functions">Polymorphic + Functions</a> + </h4> +<p> + Unless otherwise noted, Phoenix generated functions are fully polymorphic. + For instance, the <code class="computeroutput"><span class="identifier">add</span></code> example + above can apply to integers, floating points, user defined complex numbers + or even strings. Example: + </p> +<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">h</span><span class="special">(</span><span class="string">"Hello"</span><span class="special">);</span> +<span class="keyword">char</span> <span class="keyword">const</span><span class="special">*</span> <span class="identifier">w</span> <span class="special">=</span> <span class="string">" World"</span><span class="special">;</span> +<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">r</span> <span class="special">=</span> <span class="identifier">add</span><span class="special">(</span><span class="identifier">arg1</span><span class="special">,</span> <span class="identifier">arg2</span><span class="special">)(</span><span class="identifier">h</span><span class="special">,</span> <span class="identifier">w</span><span class="special">);</span> +</pre> +<p> + evaluates to <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">(</span><span class="string">"Hello + World"</span><span class="special">)</span></code>. The observant + reader might notice that this function call in fact takes in heterogeneous + arguments where <code class="computeroutput"><span class="identifier">arg1</span></code> is of + type <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code> and <code class="computeroutput"><span class="identifier">arg2</span></code> + is of type <code class="computeroutput"><span class="keyword">char</span> <span class="keyword">const</span><span class="special">*</span></code>. <code class="computeroutput"><span class="identifier">add</span></code> + still works because the C++ standard library allows the expression <code class="computeroutput"><span class="identifier">a</span> <span class="special">+</span> <span class="identifier">b</span></code> + where <code class="computeroutput"><span class="identifier">a</span></code> is a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code> + and <code class="computeroutput"><span class="identifier">b</span></code> is a <code class="computeroutput"><span class="keyword">char</span> + <span class="keyword">const</span><span class="special">*</span></code>. + </p> +</div> +<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> +<td align="left"></td> +<td align="right"><div class="copyright-footer">Copyright © 2002-2005, 2010 Joel de Guzman, Dan Marsden, Thomas Heller<p> + Distributed under the Boost Software License, Version 1.0. (See accompanying + file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) + </p> +</div></td> +</tr></table> +<hr> +<div class="spirit-nav"> +<a accesskey="p" href="starter_kit/more.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="organization.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> +</div> +</body> +</html> |