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authorDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-07 18:45:59 +0000
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+
+
+<b>P</b>ump is <b>U</b>seful for <b>M</b>eta <b>P</b>rogramming.
+
+# The Problem #
+
+Template and macro libraries often need to define many classes,
+functions, or macros that vary only (or almost only) in the number of
+arguments they take. It's a lot of repetitive, mechanical, and
+error-prone work.
+
+Variadic templates and variadic macros can alleviate the problem.
+However, while both are being considered by the C++ committee, neither
+is in the standard yet or widely supported by compilers. Thus they
+are often not a good choice, especially when your code needs to be
+portable. And their capabilities are still limited.
+
+As a result, authors of such libraries often have to write scripts to
+generate their implementation. However, our experience is that it's
+tedious to write such scripts, which tend to reflect the structure of
+the generated code poorly and are often hard to read and edit. For
+example, a small change needed in the generated code may require some
+non-intuitive, non-trivial changes in the script. This is especially
+painful when experimenting with the code.
+
+# Our Solution #
+
+Pump (for Pump is Useful for Meta Programming, Pretty Useful for Meta
+Programming, or Practical Utility for Meta Programming, whichever you
+prefer) is a simple meta-programming tool for C++. The idea is that a
+programmer writes a `foo.pump` file which contains C++ code plus meta
+code that manipulates the C++ code. The meta code can handle
+iterations over a range, nested iterations, local meta variable
+definitions, simple arithmetic, and conditional expressions. You can
+view it as a small Domain-Specific Language. The meta language is
+designed to be non-intrusive (s.t. it won't confuse Emacs' C++ mode,
+for example) and concise, making Pump code intuitive and easy to
+maintain.
+
+## Highlights ##
+
+ * The implementation is in a single Python script and thus ultra portable: no build or installation is needed and it works cross platforms.
+ * 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.
+ * The format is human-readable and more concise than XML.
+ * The format works relatively well with Emacs' C++ mode.
+
+## Examples ##
+
+The following Pump code (where meta keywords start with `$`, `[[` and `]]` are meta brackets, and `$$` starts a meta comment that ends with the line):
+
+```
+$var n = 3 $$ Defines a meta variable n.
+$range i 0..n $$ Declares the range of meta iterator i (inclusive).
+$for i [[
+ $$ Meta loop.
+// Foo$i does blah for $i-ary predicates.
+$range j 1..i
+template <size_t N $for j [[, typename A$j]]>
+class Foo$i {
+$if i == 0 [[
+ blah a;
+]] $elif i <= 2 [[
+ blah b;
+]] $else [[
+ blah c;
+]]
+};
+
+]]
+```
+
+will be translated by the Pump compiler to:
+
+```
+// Foo0 does blah for 0-ary predicates.
+template <size_t N>
+class Foo0 {
+ blah a;
+};
+
+// Foo1 does blah for 1-ary predicates.
+template <size_t N, typename A1>
+class Foo1 {
+ blah b;
+};
+
+// Foo2 does blah for 2-ary predicates.
+template <size_t N, typename A1, typename A2>
+class Foo2 {
+ blah b;
+};
+
+// Foo3 does blah for 3-ary predicates.
+template <size_t N, typename A1, typename A2, typename A3>
+class Foo3 {
+ blah c;
+};
+```
+
+In another example,
+
+```
+$range i 1..n
+Func($for i + [[a$i]]);
+$$ The text between i and [[ is the separator between iterations.
+```
+
+will generate one of the following lines (without the comments), depending on the value of `n`:
+
+```
+Func(); // If n is 0.
+Func(a1); // If n is 1.
+Func(a1 + a2); // If n is 2.
+Func(a1 + a2 + a3); // If n is 3.
+// And so on...
+```
+
+## Constructs ##
+
+We support the following meta programming constructs:
+
+| `$var id = exp` | Defines a named constant value. `$id` is valid util the end of the current meta lexical block. |
+|:----------------|:-----------------------------------------------------------------------------------------------|
+| `$range id exp..exp` | Sets the range of an iteration variable, which can be reused in multiple loops later. |
+| `$for id sep [[ code ]]` | Iteration. The range of `id` must have been defined earlier. `$id` is valid in `code`. |
+| `$($)` | Generates a single `$` character. |
+| `$id` | Value of the named constant or iteration variable. |
+| `$(exp)` | Value of the expression. |
+| `$if exp [[ code ]] else_branch` | Conditional. |
+| `[[ code ]]` | Meta lexical block. |
+| `cpp_code` | Raw C++ code. |
+| `$$ comment` | Meta comment. |
+
+**Note:** To give the user some freedom in formatting the Pump source
+code, Pump ignores a new-line character if it's right after `$for foo`
+or next to `[[` or `]]`. Without this rule you'll often be forced to write
+very long lines to get the desired output. Therefore sometimes you may
+need to insert an extra new-line in such places for a new-line to show
+up in your output.
+
+## Grammar ##
+
+```
+code ::= atomic_code*
+atomic_code ::= $var id = exp
+ | $var id = [[ code ]]
+ | $range id exp..exp
+ | $for id sep [[ code ]]
+ | $($)
+ | $id
+ | $(exp)
+ | $if exp [[ code ]] else_branch
+ | [[ code ]]
+ | cpp_code
+sep ::= cpp_code | empty_string
+else_branch ::= $else [[ code ]]
+ | $elif exp [[ code ]] else_branch
+ | empty_string
+exp ::= simple_expression_in_Python_syntax
+```
+
+## Code ##
+
+You can find the source code of Pump in [scripts/pump.py](../scripts/pump.py). It is still
+very unpolished and lacks automated tests, although it has been
+successfully used many times. If you find a chance to use it in your
+project, please let us know what you think! We also welcome help on
+improving Pump.
+
+## Real Examples ##
+
+You can find real-world applications of Pump in [Google Test](http://www.google.com/codesearch?q=file%3A\.pump%24+package%3Ahttp%3A%2F%2Fgoogletest\.googlecode\.com) and [Google Mock](http://www.google.com/codesearch?q=file%3A\.pump%24+package%3Ahttp%3A%2F%2Fgooglemock\.googlecode\.com). The source file `foo.h.pump` generates `foo.h`.
+
+## Tips ##
+
+ * If a meta variable is followed by a letter or digit, you can separate them using `[[]]`, which inserts an empty string. For example `Foo$j[[]]Helper` generate `Foo1Helper` when `j` is 1.
+ * 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.