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24 | </div> | |
25 | <div class="section"> | |
26 | <div class="titlepage"><div><div><h2 class="title" style="clear: both"> | |
27 | <a name="phoenix.basics"></a><a class="link" href="basics.html" title="Basics">Basics</a> | |
28 | </h2></div></div></div> | |
29 | <p> | |
30 | Almost everything is a function in the Phoenix library that can be evaluated | |
31 | 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 | |
32 | arity, or number of arguments that the function expects. Operators are also | |
33 | functions. For example, <code class="computeroutput"><span class="identifier">a</span> <span class="special">+</span> <span class="identifier">b</span></code> is just | |
34 | a function with arity == 2 (or binary). <code class="computeroutput"><span class="identifier">a</span> | |
35 | <span class="special">+</span> <span class="identifier">b</span></code> | |
36 | 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> | |
37 | <span class="special">+</span> <span class="identifier">b</span> <span class="special">+</span> <span class="identifier">c</span></code> is the | |
38 | 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> | |
39 | <span class="identifier">c</span><span class="special">)</span></code>. | |
40 | </p> | |
41 | <div class="note"><table border="0" summary="Note"> | |
42 | <tr> | |
43 | <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td> | |
44 | <th align="left">Note</th> | |
45 | </tr> | |
46 | <tr><td align="left" valign="top"><p> | |
47 | Amusingly, functions may even return functions. We shall see what this means | |
48 | in a short while. | |
49 | </p></td></tr> | |
50 | </table></div> | |
51 | <h4> | |
52 | <a name="phoenix.basics.h0"></a> | |
53 | <span><a name="phoenix.basics.partial_function_application"></a></span><a class="link" href="basics.html#phoenix.basics.partial_function_application">Partial | |
54 | Function Application</a> | |
55 | </h4> | |
56 | <p> | |
57 | Think of a function as a black box. You pass arguments and it returns something | |
58 | back. The figure below depicts the typical scenario. | |
59 | </p> | |
60 | <p> | |
61 | <span class="inlinemediaobject"><img src="../images/fbox.png"></span> | |
62 | </p> | |
63 | <p> | |
64 | A fully evaluated function is one in which all the arguments are given. All | |
65 | 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 | |
66 | 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> | |
67 | function, you have to pass two numbers x and y. The function will return the | |
68 | sum of the two numbers. The figure below is a fully evaluated <code class="computeroutput"><span class="identifier">add</span></code> function. | |
69 | </p> | |
70 | <p> | |
71 | <span class="inlinemediaobject"><img src="../images/adder.png"></span> | |
72 | </p> | |
73 | <p> | |
74 | A partially applied function, on the other hand, is one in which not all the | |
75 | 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. | |
76 | In doing so, the function does not have all the required information it needs | |
77 | to perform its task to compute and return a result. What it returns instead | |
78 | is another function, a lambda function. Unlike the original <code class="computeroutput"><span class="identifier">add</span></code> | |
79 | function which has an arity of 2, the resulting lambda function has an arity | |
80 | of 1. Why? because we already supplied part of the input: <code class="computeroutput"><span class="number">2</span></code> | |
81 | </p> | |
82 | <p> | |
83 | <span class="inlinemediaobject"><img src="../images/add2.png"></span> | |
84 | </p> | |
85 | <p> | |
86 | Now, when we shove in a number into our lambda function, it will return 2 plus | |
87 | whatever we pass in. The lambda function essentially remembers 1) the original | |
88 | function, <code class="computeroutput"><span class="identifier">add</span></code>, and 2) the partial | |
89 | input, 2. The figure below illustrates a case where we pass 3 to our lambda | |
90 | function, which then returns 5: | |
91 | </p> | |
92 | <p> | |
93 | <span class="inlinemediaobject"><img src="../images/add2_call.png"></span> | |
94 | </p> | |
95 | <p> | |
96 | Obviously, partially applying the <code class="computeroutput"><span class="identifier">add</span></code> | |
97 | function, as we see above, cannot be done directly in C++ where we are expected | |
98 | to supply all the arguments that a function expects. That's where the Phoenix | |
99 | library comes in. The library provides the facilities to do partial function | |
100 | application. And even more, with Phoenix, these resulting functions won't be | |
101 | black boxes anymore. | |
102 | </p> | |
103 | <h4> | |
104 | <a name="phoenix.basics.h1"></a> | |
105 | <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 | |
106 | and higher order functions</a> | |
107 | </h4> | |
108 | <p> | |
109 | So, what's all the fuss? What makes partial function application so useful? | |
110 | Recall our original example in the <a class="link" href="starter_kit/lazy_operators.html" title="Lazy Operators">previous | |
111 | section</a>: | |
112 | </p> | |
113 | <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> | |
114 | </pre> | |
115 | <p> | |
116 | The expression <code class="computeroutput"><span class="identifier">arg1</span> <span class="special">%</span> | |
117 | <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 | |
118 | be supplied later. Hence, since there's only one unsupplied argument, the lambda | |
119 | function has an arity 1. It just so happens that <code class="computeroutput"><span class="identifier">find_if</span></code> | |
120 | 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> | |
121 | to <code class="computeroutput"><span class="identifier">c</span><span class="special">.</span><span class="identifier">end</span><span class="special">()</span></code>. | |
122 | </p> | |
123 | <div class="note"><table border="0" summary="Note"> | |
124 | <tr> | |
125 | <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td> | |
126 | <th align="left">Note</th> | |
127 | </tr> | |
128 | <tr><td align="left" valign="top"><p> | |
129 | Higher order functions are functions which can take other functions as arguments, | |
130 | and may also return functions as results. Higher order functions are functions | |
131 | that are treated like any other objects and can be used as arguments and | |
132 | return values from functions. | |
133 | </p></td></tr> | |
134 | </table></div> | |
135 | <h4> | |
136 | <a name="phoenix.basics.h2"></a> | |
137 | <span><a name="phoenix.basics.lazy_evaluation"></a></span><a class="link" href="basics.html#phoenix.basics.lazy_evaluation">Lazy | |
138 | Evaluation</a> | |
139 | </h4> | |
140 | <p> | |
141 | In Phoenix, to put it more accurately, function evaluation has two stages: | |
142 | </p> | |
143 | <div class="orderedlist"><ol class="orderedlist" type="1"> | |
144 | <li class="listitem"> | |
145 | Partial application | |
146 | </li> | |
147 | <li class="listitem"> | |
148 | Final evaluation | |
149 | </li> | |
150 | </ol></div> | |
151 | <p> | |
152 | The first stage is handled by a set of generator functions. These are your | |
153 | front ends (in the client's perspective). These generators create (through | |
154 | partial function application), higher order functions that can be passed on | |
155 | just like any other function pointer or function object. The second stage, | |
156 | the actual function call, can be invoked or executed anytime in the future, | |
157 | or not at all; hence <span class="emphasis"><em>"lazy"</em></span>. | |
158 | </p> | |
159 | <p> | |
160 | If we look more closely, the first step involves partial function application: | |
161 | </p> | |
162 | <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> | |
163 | </pre> | |
164 | <p> | |
165 | 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 | |
166 | (often, the final invocation is never actually seen by the client). In our | |
167 | example, the <code class="computeroutput"><span class="identifier">find_if</span></code>, if we | |
168 | take a look inside, we'll see something like: | |
169 | </p> | |
170 | <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> | |
171 | <span class="identifier">InputIterator</span> | |
172 | <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> | |
173 | <span class="special">{</span> | |
174 | <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> | |
175 | <span class="special">++</span><span class="identifier">first</span><span class="special">;</span> <span class="comment">// passing in *first</span> | |
176 | <span class="keyword">return</span> <span class="identifier">first</span><span class="special">;</span> | |
177 | <span class="special">}</span> | |
178 | </pre> | |
179 | <p> | |
180 | Again, typically, we, as clients, see only the first step. However, in this | |
181 | document and in the examples and tests provided, don't be surprised to see | |
182 | the first and second steps juxtaposed in order to illustrate the complete semantics | |
183 | of Phoenix expressions. Examples: | |
184 | </p> | |
185 | <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> | |
186 | <span class="keyword">int</span> <span class="identifier">y</span> <span class="special">=</span> <span class="number">2</span><span class="special">;</span> | |
187 | ||
188 | <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> | |
189 | <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> | |
190 | </pre> | |
191 | <h4> | |
192 | <a name="phoenix.basics.h3"></a> | |
193 | <span><a name="phoenix.basics.forwarding_function_problem"></a></span><a class="link" href="basics.html#phoenix.basics.forwarding_function_problem">Forwarding | |
194 | Function Problem</a> | |
195 | </h4> | |
196 | <p> | |
197 | Usually, we, as clients, write the call-back functions while libraries (such | |
198 | as STL) provide the callee (e.g. <code class="computeroutput"><span class="identifier">find_if</span></code>). | |
199 | In case the role is reversed, e.g. if you have to write an STL algorithm that | |
200 | takes in a predicate, or develop a GUI library that accepts event handlers, | |
201 | 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 | |
202 | Function Problem</a>". | |
203 | </p> | |
204 | <p> | |
205 | Look again at the code above: | |
206 | </p> | |
207 | <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> | |
208 | </pre> | |
209 | <p> | |
210 | Notice that, in the second-stage (the final evaluation), we used a variable | |
211 | <code class="computeroutput"><span class="identifier">x</span></code>. | |
212 | </p> | |
213 | <p> | |
214 | In Phoenix we emulated perfect forwarding through preprocessor macros generating | |
215 | code to allow const and non-const references. | |
216 | </p> | |
217 | <p> | |
218 | We generate these second-stage overloads for Phoenix expression up to <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_PERFECT_FORWARD_LIMIT</span></code> | |
219 | </p> | |
220 | <div class="note"><table border="0" summary="Note"> | |
221 | <tr> | |
222 | <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td> | |
223 | <th align="left">Note</th> | |
224 | </tr> | |
225 | <tr><td align="left" valign="top"><p> | |
226 | You can set <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_PERFECT_FORWARD_LIMIT</span></code>, | |
227 | the predefined maximum perfect forward arguments an actor can take. By default, | |
228 | <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_PERFECT_FORWARDLIMIT</span></code> | |
229 | is set to 3. | |
230 | </p></td></tr> | |
231 | </table></div> | |
232 | <h4> | |
233 | <a name="phoenix.basics.h4"></a> | |
234 | <span><a name="phoenix.basics.polymorphic_functions"></a></span><a class="link" href="basics.html#phoenix.basics.polymorphic_functions">Polymorphic | |
235 | Functions</a> | |
236 | </h4> | |
237 | <p> | |
238 | Unless otherwise noted, Phoenix generated functions are fully polymorphic. | |
239 | For instance, the <code class="computeroutput"><span class="identifier">add</span></code> example | |
240 | above can apply to integers, floating points, user defined complex numbers | |
241 | or even strings. Example: | |
242 | </p> | |
243 | <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> | |
244 | <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> | |
245 | <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> | |
246 | </pre> | |
247 | <p> | |
248 | 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 | |
249 | World"</span><span class="special">)</span></code>. The observant | |
250 | reader might notice that this function call in fact takes in heterogeneous | |
251 | arguments where <code class="computeroutput"><span class="identifier">arg1</span></code> is of | |
252 | 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> | |
253 | 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> | |
254 | 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> | |
255 | 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> | |
256 | and <code class="computeroutput"><span class="identifier">b</span></code> is a <code class="computeroutput"><span class="keyword">char</span> | |
257 | <span class="keyword">const</span><span class="special">*</span></code>. | |
258 | </p> | |
259 | </div> | |
260 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
261 | <td align="left"></td> | |
262 | <td align="right"><div class="copyright-footer">Copyright © 2002-2005, 2010, 2014, 2015 Joel de Guzman, Dan Marsden, Thomas | |
263 | Heller, John Fletcher<p> | |
264 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
265 | 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>) | |
266 | </p> | |
267 | </div></td> | |
268 | </tr></table> | |
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