Pump is Useful for Meta Programming.

# 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](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.
*   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:

```cpp
// 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`:

```cpp
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

```ebnf
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](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`.

## 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.
