178 lines
6.8 KiB
Markdown
178 lines
6.8 KiB
Markdown
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<b>P</b>ump is <b>U</b>seful for <b>M</b>eta <b>P</b>rogramming.
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# The Problem #
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Template and macro libraries often need to define many classes,
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functions, or macros that vary only (or almost only) in the number of
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arguments they take. It's a lot of repetitive, mechanical, and
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error-prone work.
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Variadic templates and variadic macros can alleviate the problem.
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However, while both are being considered by the C++ committee, neither
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is in the standard yet or widely supported by compilers. Thus they
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are often not a good choice, especially when your code needs to be
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portable. And their capabilities are still limited.
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As a result, authors of such libraries often have to write scripts to
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generate their implementation. However, our experience is that it's
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tedious to write such scripts, which tend to reflect the structure of
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the generated code poorly and are often hard to read and edit. For
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example, a small change needed in the generated code may require some
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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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# 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
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prefer) is a simple meta-programming tool for C++. The idea is that a
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programmer writes a `foo.pump` file which contains C++ code plus meta
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code that manipulates the C++ code. The meta code can handle
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iterations over a range, nested iterations, local meta variable
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definitions, simple arithmetic, and conditional expressions. You can
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view it as a small Domain-Specific Language. The meta language is
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designed to be non-intrusive (s.t. it won't confuse Emacs' C++ mode,
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for example) and concise, making Pump code intuitive and easy to
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maintain.
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## Highlights ##
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* The implementation is in a single Python script and thus ultra portable: no build or installation is needed and it works cross platforms.
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* Pump tries to be smart with respect to [Google's style guide](https://github.com/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.
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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 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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```
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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 the value of `n`:
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```
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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 valid util the end of the current meta lexical block. |
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|:----------------|:-----------------------------------------------------------------------------------------------|
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| `$range id exp..exp` | Sets the range of an iteration variable, which can be reused in multiple loops later. |
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| `$for id sep [[ code ]]` | Iteration. The range of `id` must have been defined earlier. `$id` is valid in `code`. |
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| `$($)` | Generates a single `$` character. |
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| `$id` | Value of the named constant or iteration 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
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code, Pump ignores a new-line character if it's right after `$for foo`
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or next to `[[` or `]]`. Without this rule you'll often be forced to write
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very long lines to get the desired output. Therefore sometimes you may
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need to insert an extra new-line in such places for a new-line to show
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up in your output.
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## Grammar ##
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```
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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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| $elif exp [[ code ]] else_branch
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| empty_string
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exp ::= simple_expression_in_Python_syntax
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```
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## Code ##
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You can find the source code of Pump in [scripts/pump.py](../scripts/pump.py). It is still
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very unpolished and lacks automated tests, although it has been
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successfully used many times. If you find a chance to use it in your
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project, please let us know what you think! We also welcome help on
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improving Pump.
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## Real Examples ##
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You can find real-world applications of Pump in [Google Test](https://github.com/google/googletest/tree/master/googletest) and [Google Mock](https://github.com/google/googletest/tree/master/googlemock). The source file `foo.h.pump` generates `foo.h`.
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## Tips ##
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* If a meta variable is followed by a letter or digit, you can separate them using `[[]]`, which inserts an empty string. For example `Foo$j[[]]Helper` generate `Foo1Helper` when `j` is 1.
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* 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.
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