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diff --git a/docs/pump_manual.md b/docs/pump_manual.md new file mode 100644 index 00000000..9dbe15bb --- /dev/null +++ b/docs/pump_manual.md @@ -0,0 +1,190 @@ +<b>P</b>ump is <b>U</b>seful for <b>M</b>eta <b>P</b>rogramming. + +<!-- GOOGLETEST_CM0035 DO NOT DELETE --> + +# 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. + +Our experience is that it's tedious to write custom 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. + +This script may be useful for generating meta code, for example a series of +macros of FOO1, FOO2, etc. Nevertheless, please make it your last resort +technique by favouring C++ template metaprogramming or variadic macros. + +# 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]]); +$$ The text between i and [[ is the separator between iterations. +``` + +will generate one of the following lines (without the comments), depending on +the value of `n`: + +```cpp +Func(); // If n is 0. +Func(a1); // If n is 1. +Func(a1 + a2); // If n is 2. +Func(a1 + a2 + a3); // If n is 3. +// And so on... +``` + +## Constructs + +We support the following meta programming constructs: + +| `$var id = exp` | Defines a named constant value. `$id` is | +: : valid until the end of the current meta : +: : lexical block. : +| :------------------------------- | :--------------------------------------- | +| `$range id exp..exp` | Sets the range of an iteration variable, | +: : which can be reused in multiple loops : +: : later. : +| `$for id sep [[ code ]]` | Iteration. The range of `id` must have | +: : been defined earlier. `$id` is valid in : +: : `code`. : +| `$($)` | Generates a single `$` character. | +| `$id` | Value of the named constant or iteration | +: : variable. : +| `$(exp)` | Value of the expression. | +| `$if exp [[ code ]] else_branch` | Conditional. | +| `[[ code ]]` | Meta lexical block. | +| `cpp_code` | Raw C++ code. | +| `$$ 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 ]] + | $elif exp [[ code ]] else_branch + | empty_string +exp ::= simple_expression_in_Python_syntax +``` + +## Code + +You can find the source code of Pump in +[googlemock/scripts/pump.py](../googlemock/scripts/pump.py). It is still very +unpolished and lacks automated tests, although it has been successfully used +many times. If you find a chance to use it in your project, please let us know +what you think! We also welcome help on improving Pump. + +## Real Examples + +You can find real-world applications of Pump in +[Google Test](https://github.com/google/googletest/tree/master/googletest) and +[Google Mock](https://github.com/google/googletest/tree/master/googlemock). The +source file `foo.h.pump` generates `foo.h`. + +## Tips + +* If a meta variable is followed by a letter or digit, you can separate them + using `[[]]`, which inserts an empty string. For example `Foo$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. |
