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Comment by ambrop7 | Hacker News Reader

What is this? It claims to be a (GNU99) C library, but I don't see how this can be the case, considering all the non-C constructs in the sample code ($(), try/catch, foreach). So it this just a language of its own that is compiled into C?

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  #define foreach(x, xs) \
    for(var x = iter_start(xs); x = iter_next(xs); x != iter_end(xs))

It's all C: http://libcello.org/documentation/hacking

Imagine what happens when someone says

    foreach(x, get_next(queue)) f(x);

To solve this problem, with GCC, you have the option of defining foreach as

    { typeof (xs) _xs = xs; \
      for (var x = iter_start(_xs); x = iter_next(_xs); x != iter_end(_xs))

but then you need a corresponding endforeach to apply the matching }. If you just leave off the opening {, then you end up with a mysterious compile error on

    if (ready(queue)) foreach(x, get_next(queue)) f(x);

since the declaration of _xs is not a statement.

And this kind of thing is why a "macro system" means something very different in C and in Lisp, and why Brad Cox wrote a compiler in 1983 instead of a macro library.

You could do

    #define foreach(x, xs) \
        for (var _xs = xs, *x = iter_start(_xs); x = iter_next(_xs); x != iter_end(_xs))

Because var is just void* and anything you can iterate over will also be a void*.

Oh, that's excellent! I don't know why the C9x for-declaration didn't occur to me as a possible solution, particularly since the macro was already using it. You should submit this as a fix!

$ is a valid function name according to gcc. try/catch/foreach are #define'd as part of it. As an example, foreach is defined to a for loop over an iterator, which is any Cello class with a couple of functions defined.

You can do pretty much anything if you rely on preprocessor. The $ is a define: http://libcello.org/documentation/hacking

check out the Prelude.h file on their github - it's essentially all implemented as preprocessor nameswaps and macros. It's quite clever actually!

I'm guessing that the GNU99 qualification is important. GCC supports most if not all of the claimed features, or at least the low-level constructs required to create them.

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