mirror of
https://github.com/oxen-io/lokinet.git
synced 2024-11-15 12:13:24 +00:00
177 lines
6.9 KiB
Markdown
177 lines
6.9 KiB
Markdown
|
|
|
|
<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]]);
|
|
$$ The text between i and [[ is the separator between iterations.
|
|
```
|
|
|
|
will generate one of the following lines (without the comments), depending on the value of `n`:
|
|
|
|
```
|
|
Func(); // If n is 0.
|
|
Func(a1); // If n is 1.
|
|
Func(a1 + a2); // If n is 2.
|
|
Func(a1 + a2 + a3); // If n is 3.
|
|
// And so on...
|
|
```
|
|
|
|
## Constructs ##
|
|
|
|
We support the following meta programming constructs:
|
|
|
|
| `$var id = exp` | Defines a named constant value. `$id` is valid util the end of the current meta lexical block. |
|
|
|:----------------|:-----------------------------------------------------------------------------------------------|
|
|
| $range id exp..exp | Sets the range of an iteration variable, which can be reused in multiple loops later. |
|
|
| $for id sep [[code ](.md)] | Iteration. The range of `id` must have been defined earlier. `$id` is valid in `code`. |
|
|
| `$($)` | Generates a single `$` character. |
|
|
| `$id` | Value of the named constant or iteration variable. |
|
|
| `$(exp)` | Value of the expression. |
|
|
| `$if exp [[ code ]] else_branch` | Conditional. |
|
|
| `[[ code ]]` | Meta lexical block. |
|
|
| `cpp_code` | Raw C++ code. |
|
|
| `$$ 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 ]]
|
|
| $elif exp [[ code ]] else_branch
|
|
| empty_string
|
|
exp ::= simple_expression_in_Python_syntax
|
|
```
|
|
|
|
## Code ##
|
|
|
|
You can find the source code of Pump in [scripts/pump.py](http://code.google.com/p/googletest/source/browse/trunk/scripts/pump.py). It is still
|
|
very unpolished and lacks automated tests, although it has been
|
|
successfully used many times. If you find a chance to use it in your
|
|
project, please let us know what you think! We also welcome help on
|
|
improving Pump.
|
|
|
|
## Real Examples ##
|
|
|
|
You can find real-world applications of Pump in [Google Test](http://www.google.com/codesearch?q=file%3A\.pump%24+package%3Ahttp%3A%2F%2Fgoogletest\.googlecode\.com) and [Google Mock](http://www.google.com/codesearch?q=file%3A\.pump%24+package%3Ahttp%3A%2F%2Fgooglemock\.googlecode\.com). The source file `foo.h.pump` generates `foo.h`.
|
|
|
|
## Tips ##
|
|
|
|
* If a meta variable is followed by a letter or digit, you can separate them using `[[]]`, which inserts an empty string. For example `Foo$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. |