[cfe-dev] Nacro: A better C/C++ macro extension implemented in Clang plugins

[Madhur Amilkanthwar via cfe-dev]

Sounds interesting. It would be a good addition if you can support a bit of
type system and ensuring type sanity too somehow.

On Fri, Jun 12, 2020, 12:02 AM Min-Yih Hsu via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Hi Clang folks,
>
> Recently I have been working on a side project, Nacro (
> https://github.com/mshockwave/nacro), that you might find interesting.
>
> Nacro is a small DSL aim to provide a better C/C++ macro experience. Here
> is an example:
> ```
>
> #pragma nacro rule myPrint
> (val:$expr) -> $stmt {
>     printf("%d\n", val * 2)
> }
>
> #pragma nacro rule genColors
> (cases:$expr*) -> {
>     $loop(c in cases) {
>         case c:
>         printf("the color is %s\n", $str(c));
>         break;
>     }
> }
> enum Color { RED, BLUE, YELLOW };void printColor(enum Color color) {
>     switch(color) genColors(RED, BLUE, YELLOW)
> }
> int main() {
>     myPrint(1 + 3)   // print out '8'
>     printColor(RED); // print 'the color is RED'
>     return 0;
> }
>
> ```
> You can create a nacro rule with a pragma directive and follows with
> definitions written a simple DSL. The defined nacro rule works just like a
> normal macro function (i.e. expanded during preprocessing and called like a
> function), but with some extra features:
>  - Multiline definitions without ‘\’.
>  - Expression Protection: As shown in the ‘myPrint’ nacro above, you don’t
> need to add parens around expressions anymore, the nacro preprocessor will
> do that for you.
>  - Loops, one of the most exciting features in nacro: You can use “$loop”
> directive to ‘unroll’ a list of argument (argument type trailing with ‘*’,
> like ‘$expr*’. It’s equivalent to VA_ARGS) during preprocessing.
>  - Detecting invalid capturing. This is still an unstable feature, but
> basically it’s aiming to detect issues like this:
> ```
>
> #define foo(arg) {\
>     int x = 0;  \
>     return arg + x; \
> }
> // ERROR: caller will always return 0 // regardless the argument valueint caller(int x) {
>     foo(x)
> }
>
>
> ```
> Please checkout the its wiki page (
> https://github.com/mshockwave/nacro/wiki/Invalid-Capture-Detection) for
> more info.
>
> Another important thing is that nacro _doesn’t require a custom Clang /
> LLVM_. All of the features are implemented in a Clang plugin. So all you
> need is a Clang 10 by your hand and the plugin.
>
> Though not modifying Clang actually put a tons of restrictions and force
> me to create many hacky workarounds that might be broken in newer version
> of Clang. For example, as a preprocessor plugin, all i can do is mutating
> and injecting tokens. But the preprocessor doesn’t like it, or in other
> words, current preprocessor doesn’t have a good support on this kind of
> actions. Especially while I was dealing with SourceLocation: there are many
> time i need to use an imprecise one or even fake one.
>
> Another hard part is the Invalid Capture Detection. My original plan was
> to automatically fix it rather than throwing an error message. However,
> that will require symbol table and scope info that merely exist in AST.
> Though Parser/Sema has those info but currently we don’t have any plugin
> support on Parser/Sema.
>
> Hope you find this project interesting, or at least find it amusing.
>
> -Min
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