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proxygen/wangle/_build/gtest/src/gtest/googletest/docs/PumpManual.md
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1 
2 
3 <b>P</b>ump is <b>U</b>seful for <b>M</b>eta <b>P</b>rogramming.
4 
5 # The Problem #
6 
7 Template and macro libraries often need to define many classes,
8 functions, or macros that vary only (or almost only) in the number of
9 arguments they take. It's a lot of repetitive, mechanical, and
10 error-prone work.
11 
12 Variadic templates and variadic macros can alleviate the problem.
13 However, while both are being considered by the C++ committee, neither
14 is in the standard yet or widely supported by compilers. Thus they
15 are often not a good choice, especially when your code needs to be
16 portable. And their capabilities are still limited.
17 
18 As a result, authors of such libraries often have to write scripts to
19 generate their implementation. However, our experience is that it's
20 tedious to write such scripts, which tend to reflect the structure of
21 the generated code poorly and are often hard to read and edit. For
22 example, a small change needed in the generated code may require some
23 non-intuitive, non-trivial changes in the script. This is especially
24 painful when experimenting with the code.
25 
26 # Our Solution #
27 
28 Pump (for Pump is Useful for Meta Programming, Pretty Useful for Meta
29 Programming, or Practical Utility for Meta Programming, whichever you
30 prefer) is a simple meta-programming tool for C++. The idea is that a
31 programmer writes a `foo.pump` file which contains C++ code plus meta
32 code that manipulates the C++ code. The meta code can handle
33 iterations over a range, nested iterations, local meta variable
34 definitions, simple arithmetic, and conditional expressions. You can
35 view it as a small Domain-Specific Language. The meta language is
36 designed to be non-intrusive (s.t. it won't confuse Emacs' C++ mode,
37 for example) and concise, making Pump code intuitive and easy to
38 maintain.
39 
40 ## Highlights ##
41 
42  * The implementation is in a single Python script and thus ultra portable: no build or installation is needed and it works cross platforms.
43  * Pump tries to be smart with respect to [Google's style guide](http://code.google.com/p/google-styleguide/): it breaks long lines (easy to have when they are generated) at acceptable places to fit within 80 columns and indent the continuation lines correctly.
44  * The format is human-readable and more concise than XML.
45  * The format works relatively well with Emacs' C++ mode.
46 
47 ## Examples ##
48 
49 The following Pump code (where meta keywords start with `$`, `[[` and `]]` are meta brackets, and `$$` starts a meta comment that ends with the line):
50 
51 ```
52 $var n = 3 $$ Defines a meta variable n.
53 $range i 0..n $$ Declares the range of meta iterator i (inclusive).
54 $for i [[
55  $$ Meta loop.
56 // Foo$i does blah for $i-ary predicates.
57 $range j 1..i
58 template <size_t N $for j [[, typename A$j]]>
59 class Foo$i {
60 $if i == 0 [[
61  blah a;
62 ]] $elif i <= 2 [[
63  blah b;
64 ]] $else [[
65  blah c;
66 ]]
67 };
68 
69 ]]
70 ```
71 
72 will be translated by the Pump compiler to:
73 
74 ```
75 // Foo0 does blah for 0-ary predicates.
76 template <size_t N>
77 class Foo0 {
78  blah a;
79 };
80 
81 // Foo1 does blah for 1-ary predicates.
82 template <size_t N, typename A1>
83 class Foo1 {
84  blah b;
85 };
86 
87 // Foo2 does blah for 2-ary predicates.
88 template <size_t N, typename A1, typename A2>
89 class Foo2 {
90  blah b;
91 };
92 
93 // Foo3 does blah for 3-ary predicates.
94 template <size_t N, typename A1, typename A2, typename A3>
95 class Foo3 {
96  blah c;
97 };
98 ```
99 
100 In another example,
101 
102 ```
103 $range i 1..n
104 Func($for i + [[a$i]]);
105 $$ The text between i and [[ is the separator between iterations.
106 ```
107 
108 will generate one of the following lines (without the comments), depending on the value of `n`:
109 
110 ```
111 Func(); // If n is 0.
112 Func(a1); // If n is 1.
113 Func(a1 + a2); // If n is 2.
114 Func(a1 + a2 + a3); // If n is 3.
115 // And so on...
116 ```
117 
118 ## Constructs ##
119 
120 We support the following meta programming constructs:
121 
122 | `$var id = exp` | Defines a named constant value. `$id` is valid util the end of the current meta lexical block. |
123 |:----------------|:-----------------------------------------------------------------------------------------------|
124 | `$range id exp..exp` | Sets the range of an iteration variable, which can be reused in multiple loops later. |
125 | `$for id sep [[ code ]]` | Iteration. The range of `id` must have been defined earlier. `$id` is valid in `code`. |
126 | `$($)` | Generates a single `$` character. |
127 | `$id` | Value of the named constant or iteration variable. |
128 | `$(exp)` | Value of the expression. |
129 | `$if exp [[ code ]] else_branch` | Conditional. |
130 | `[[ code ]]` | Meta lexical block. |
131 | `cpp_code` | Raw C++ code. |
132 | `$$ comment` | Meta comment. |
133 
134 **Note:** To give the user some freedom in formatting the Pump source
135 code, Pump ignores a new-line character if it's right after `$for foo`
136 or next to `[[` or `]]`. Without this rule you'll often be forced to write
137 very long lines to get the desired output. Therefore sometimes you may
138 need to insert an extra new-line in such places for a new-line to show
139 up in your output.
140 
141 ## Grammar ##
142 
143 ```
144 code ::= atomic_code*
145 atomic_code ::= $var id = exp
146  | $var id = [[ code ]]
147  | $range id exp..exp
148  | $for id sep [[ code ]]
149  | $($)
150  | $id
151  | $(exp)
152  | $if exp [[ code ]] else_branch
153  | [[ code ]]
154  | cpp_code
155 sep ::= cpp_code | empty_string
156 else_branch ::= $else [[ code ]]
157  | $elif exp [[ code ]] else_branch
158  | empty_string
159 exp ::= simple_expression_in_Python_syntax
160 ```
161 
162 ## Code ##
163 
164 You can find the source code of Pump in [scripts/pump.py](../scripts/pump.py). It is still
165 very unpolished and lacks automated tests, although it has been
166 successfully used many times. If you find a chance to use it in your
167 project, please let us know what you think! We also welcome help on
168 improving Pump.
169 
170 ## Real Examples ##
171 
172 You can find real-world applications of Pump in [Google Test](http://www.google.com/codesearch?q=file%3A\.pump%24+package%3Ahttp%3A%2F%2Fgoogletest\.googlecode\.com) and [Google Mock](http://www.google.com/codesearch?q=file%3A\.pump%24+package%3Ahttp%3A%2F%2Fgooglemock\.googlecode\.com). The source file `foo.h.pump` generates `foo.h`.
173 
174 ## Tips ##
175 
176  * If a meta variable is followed by a letter or digit, you can separate them using `[[]]`, which inserts an empty string. For example `Foo$j[[]]Helper` generate `Foo1Helper` when `j` is 1.
177  * To avoid extra-long Pump source lines, you can break a line anywhere you want by inserting `[[]]` followed by a new line. Since any new-line character next to `[[` or `]]` is ignored, the generated code won't contain this new line.