djedr/talk--general-c-preprocessor-featuring-javascript--2017-02-15-outline.md

## talk--general-c-preprocessor-featuring-javascript--2017-02-15-outline.md

      
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              talk--general-c-preprocessor-featuring-javascript--2017-02-15-outline.md
            
          
    General-purpose text preprocessing with the C preprocessor. Featuring JavaScript.

2017-02-15
Dariusz Jędrzejczak
http://djedr.github.io

Outline


Intro
Applications
Advantages
Disadvantages
Technical details
Related work


0. Intro


C
C++
Objective-C

Who ever used any of the above?

Common feature?


The same preprocessor!


The C preprocessor

aka cpp

What is it?


a separate program invoked by the compiler as the first part of translation

[0]

1. Applications

This talk's goal is to show potential applications of cpp outside of the context of C/C++/Objective-C compilation.
Perhaps not everyone is aware that cpp can be used in this manner.
Maybe you might find it useful.

Common uses of cpp


inclusion of header files, macro expansions, conditional compilation

[0]

All of these could work in any language


the language of preprocessor directives is only weakly related to the grammar of C

[0]

Real-world example:


Believe it or not, we used this technique in our production JS workflow at Y[ahoo]! Search ~2007. Worked well.

-- Ryan Grove[1][2][3]

What for?


macros

for sharing constants

(across all text files -- across languages, e.g. JS, Java, CSS)


for syntax extension
for inlining code (avoiding function-call cost)


conditional compilation of blocks of code
splitting code across files via #include

[2][4]

Macros and conditional compilation

// include.js
#if DEBUG_VERSION
function assert(condition, message) {
    ...
}
#define ASSERT(x, ...) assert(x, ## __VA_ARGS__)
#else
#define ASSERT(x, ...)
#endif
[2][4]

And then just #include and use

// foobar.js
#include "include.js"

function myFunction(arg) {
    ASSERT(YAHOO.lang.isString(argvar), "arg should be a string");
    ...
#if DEBUG_VERSION
    YAHOO.log("Log this in debug mode only");
#endif
    ...
}
[2]

That was 10 years ago and now we have way more advanced and modern JS workflows.

But.

It "worked well" then. And it would still work now. cpp didn't change.
And it's not going anywhere. Stability and reliability are advantages.

2. Advantages


Widely available


essential part of every *C* compiler

by default on UNIX machines (cpp or gcc -E)
on Windows machines using cygwin or via MSVC /P
so a lot programmers used it and know how


Simple


Just string-based macros

Easy to understand: basically copy-paste and find-replace (with parameters)


Less complicated and easier to reason about than AST-based macros (like in Scala or Lisps)


Predictable


When not abused
conditional compilation either includes code or not and its completely controllable

unlike automatic dead code elimination or tree shaking


macros can be #undefed or #ifdefed away to ensure they have absolutely no effect on production code[4]
code inlining with macros has no runtime cost and is guaranteed

unlike automatic inlining done by modern JS engines


3. Disadvantages


Limited


Less power than AST-based macros

No notion of structure of underlying language


Awkward syntax for multiline macros
Macros do little more than simple text substitution

E.g. can't execute code, which would dynamically generate the text to be substituted


Unpredictable


When abused
When used for complex things and/or without caution


Somewhat coupled with C semantics


It was designed specifically for this language


Has some gotchas


make sure you use the UNIX EOL character before preprocessing your files. Otherwise, you’ll have some issues with multi-line macros

[2]

No (or difficult) integration with IDEs


4. Technical details


How to invoke


Just configure your web server to pipe Javascript through GNU cpp:

/usr/bin/cpp -P -undef -Wundef -std=c99 -nostdinc -Wtrigraphs -fdollars-in-identifiers -C


before minifying and compressing it for transmission.

[4]

5. Related work


SweetJS, AST-based macros for JavaScript[6]
My project, codenamed jstpp: general-purpose JavaScript-based text preprocessor[7]

Perhaps for next week


Thank you!

Questions?

Links

[0] https://en.wikipedia.org/wiki/C_preprocessor
[1] https://twitter.com/yaypie/status/633062183585234944
[2] http://www.julienlecomte.net/blog/2007/09/16/
[3] https://rreverser.com/es6-modules-are-dead-long-live-c-preprocessor/
[4] http://www.nongnu.org/espresso/js-cpp.html
[6] http://sweetjs.org/
[7] https://github.com/djedr/jstpp
[8] https://linux.die.net/man/1/cpp
[9] https://gcc.gnu.org/onlinedocs/cpp/