190 lines
6.8 KiB
Markdown
190 lines
6.8 KiB
Markdown
<b>P</b>ump is <b>U</b>seful for <b>M</b>eta <b>P</b>rogramming.
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<!-- GOOGLETEST_CM0035 DO NOT DELETE -->
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# The Problem
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Template and macro libraries often need to define many classes, functions, or
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macros that vary only (or almost only) in the number of arguments they take.
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It's a lot of repetitive, mechanical, and error-prone work.
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Our experience is that it's tedious to write custom scripts, which tend to
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reflect the structure of the generated code poorly and are often hard to read
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and edit. For example, a small change needed in the generated code may require
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some non-intuitive, non-trivial changes in the script. This is especially
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painful when experimenting with the code.
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This script may be useful for generating meta code, for example a series of
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macros of FOO1, FOO2, etc. Nevertheless, please make it your last resort
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technique by favouring C++ template metaprogramming or variadic macros.
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# Our Solution
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Pump (for Pump is Useful for Meta Programming, Pretty Useful for Meta
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Programming, or Practical Utility for Meta Programming, whichever you prefer) is
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a simple meta-programming tool for C++. The idea is that a programmer writes a
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`foo.pump` file which contains C++ code plus meta code that manipulates the C++
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code. The meta code can handle iterations over a range, nested iterations, local
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meta variable definitions, simple arithmetic, and conditional expressions. You
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can view it as a small Domain-Specific Language. The meta language is designed
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to be non-intrusive (s.t. it won't confuse Emacs' C++ mode, for example) and
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concise, making Pump code intuitive and easy to maintain.
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## Highlights
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* The implementation is in a single Python script and thus ultra portable: no
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build or installation is needed and it works cross platforms.
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* Pump tries to be smart with respect to
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[Google's style guide](https://github.com/google/styleguide): it breaks long
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lines (easy to have when they are generated) at acceptable places to fit
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within 80 columns and indent the continuation lines correctly.
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* The format is human-readable and more concise than XML.
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* The format works relatively well with Emacs' C++ mode.
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## Examples
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The following Pump code (where meta keywords start with `$`, `[[` and `]]` are
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meta brackets, and `$$` starts a meta comment that ends with the line):
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```
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$var n = 3 $$ Defines a meta variable n.
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$range i 0..n $$ Declares the range of meta iterator i (inclusive).
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$for i [[
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$$ Meta loop.
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// Foo$i does blah for $i-ary predicates.
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$range j 1..i
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template <size_t N $for j [[, typename A$j]]>
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class Foo$i {
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$if i == 0 [[
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blah a;
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]] $elif i <= 2 [[
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blah b;
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]] $else [[
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blah c;
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]]
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};
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]]
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```
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will be translated by the Pump compiler to:
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```cpp
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// Foo0 does blah for 0-ary predicates.
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template <size_t N>
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class Foo0 {
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blah a;
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};
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// Foo1 does blah for 1-ary predicates.
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template <size_t N, typename A1>
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class Foo1 {
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blah b;
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};
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// Foo2 does blah for 2-ary predicates.
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template <size_t N, typename A1, typename A2>
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class Foo2 {
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blah b;
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};
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// Foo3 does blah for 3-ary predicates.
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template <size_t N, typename A1, typename A2, typename A3>
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class Foo3 {
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blah c;
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};
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```
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In another example,
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```
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$range i 1..n
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Func($for i + [[a$i]]);
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$$ The text between i and [[ is the separator between iterations.
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```
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will generate one of the following lines (without the comments), depending on
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the value of `n`:
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```cpp
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Func(); // If n is 0.
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Func(a1); // If n is 1.
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Func(a1 + a2); // If n is 2.
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Func(a1 + a2 + a3); // If n is 3.
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// And so on...
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```
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## Constructs
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We support the following meta programming constructs:
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| `$var id = exp` | Defines a named constant value. `$id` is |
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: : valid until the end of the current meta :
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: : lexical block. :
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| :------------------------------- | :--------------------------------------- |
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| `$range id exp..exp` | Sets the range of an iteration variable, |
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: : which can be reused in multiple loops :
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: : later. :
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| `$for id sep [[ code ]]` | Iteration. The range of `id` must have |
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: : been defined earlier. `$id` is valid in :
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: : `code`. :
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| `$($)` | Generates a single `$` character. |
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| `$id` | Value of the named constant or iteration |
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: : variable. :
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| `$(exp)` | Value of the expression. |
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| `$if exp [[ code ]] else_branch` | Conditional. |
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| `[[ code ]]` | Meta lexical block. |
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| `cpp_code` | Raw C++ code. |
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| `$$ comment` | Meta comment. |
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**Note:** To give the user some freedom in formatting the Pump source code, Pump
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ignores a new-line character if it's right after `$for foo` or next to `[[` or
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`]]`. Without this rule you'll often be forced to write very long lines to get
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the desired output. Therefore sometimes you may need to insert an extra new-line
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in such places for a new-line to show up in your output.
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## Grammar
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```ebnf
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code ::= atomic_code*
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atomic_code ::= $var id = exp
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| $var id = [[ code ]]
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| $range id exp..exp
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| $for id sep [[ code ]]
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| $($)
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| $id
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| $(exp)
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| $if exp [[ code ]] else_branch
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| [[ code ]]
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| cpp_code
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sep ::= cpp_code | empty_string
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else_branch ::= $else [[ code ]]
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| $elif exp [[ code ]] else_branch
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| empty_string
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exp ::= simple_expression_in_Python_syntax
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```
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## Code
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You can find the source code of Pump in [scripts/pump.py](../scripts/pump.py).
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It is still very unpolished and lacks automated tests, although it has been
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successfully used many times. If you find a chance to use it in your project,
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please let us know what you think! We also welcome help on improving Pump.
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## Real Examples
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You can find real-world applications of Pump in
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[Google Test](https://github.com/google/googletest/tree/master/googletest) and
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[Google Mock](https://github.com/google/googletest/tree/master/googlemock). The
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source file `foo.h.pump` generates `foo.h`.
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## Tips
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* If a meta variable is followed by a letter or digit, you can separate them
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using `[[]]`, which inserts an empty string. For example `Foo$j[[]]Helper`
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generate `Foo1Helper` when `j` is 1.
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* To avoid extra-long Pump source lines, you can break a line anywhere you
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want by inserting `[[]]` followed by a new line. Since any new-line
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character next to `[[` or `]]` is ignored, the generated code won't contain
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this new line.
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