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Comment by jfm3 | Hacker News Reader

Nice try blanco niño, but the following program runs fine. This doesn't detract from your overall point, but if you're trying to win over people who think the Boost macros are super awesome, this argument won't do it.

#include <stdio.h>

#define LOOP(n) \ int i; \ for (i = 0; i < n; i++)

#define LOOP10 \ LOOP(10) printf("%d\n", i)

int main(int argc, char *argv[]) { LOOP10; }

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Yesterday I was writing code that kept messing my Lisp's signal handling, so I wrote a FOR-DURATION macro that arms a timeout and makes sure whatever that runs in its body gets killed after N seconds.

Here it is:

  (defmacro for-duration ((seconds) &body body)
    `(handler-case  
         (bt:with-timeout (,seconds) 
	   ,@body)
       (bt:timeout () nil)))

Five lines to alter the evaluation model of your language. Not bad.

Use as:

  (for-duration (10)
    (loop 
      (print "Infinite loop! where is your Godel now?")))

"bt" is the bordeaux-threads package, a portable Lisp library for thread programming.

    (defun for-duration (seconds body)
        (handler-case  
             (bt:with-timeout seconds 
        	   (body))
           (bt:timeout () nil)))
    
    (for-duration 10
        (lambda ()
          (loop 
            (print "Infinite loop! where is your Godel now?"))))

I think Greg is trying to say "why the hell do we even need macros for this"?

With functions you have to make a conscious effort to quote arguments, otherwise the strict evaluation will evaluate its arguments before the function. Also, with macros you can introduce you new "implicit" control and structure semantics; it's very common for forms to have an "implicit progn", or a block named NIL or similar.

C99 + GCC extensions + POSIX:

  #define for_duration(seconds, body)                     \
    {                                                     \
      pthread_t tid_task, tid_watcher;                    \
                                                          \
      void* task(void* arg) {                             \
        body ;                                            \
        pthread_cancel(tid_watcher);                      \
        return NULL;                                      \
      }                                                   \
                                                          \
      void* watcher(void* arg) {                          \
        sleep((seconds));                                 \
        pthread_cancel(tid_task);                         \
        return NULL;                                      \
      }                                                   \
                                                          \
      pthread_create(&tid_task, NULL, &task, NULL);       \
      pthread_create(&tid_watcher, NULL, &watcher, NULL); \
      pthread_join(tid_task, NULL);                       \
    }

Use as:

  for_duration(10, {
      while (true) {
        printf("Infinite loop! where is your Godel now?\n");
      }
    });

Or with single brace-less statements:

  for_duration(10, while (true) printf("something\n") );

Lexical scope is sane:

  int i = 0;
  for_duration(10, {
      while (true) {
        printf("%d\n", i++);
      }
    });

My comparison isn't quite fair, because your threading library provides 'with-timeout', while pthreads doesn't. If you factored this out, say with a signature like:

  void with_timeout(unsigned int seconds, void (*func)(void));

(in analogy to the common lisp function), then the macro part becomes just four lines:

  #define for_duration(seconds, body) {			\
    void func() { body ; }				\
    with_timeout((seconds), &func);			\
  }

I acknowledge that the lisp solution is rather more elegant. Also, my C macro is potentially dangerous because it is unhygenic. (It shadows outer declarations of 'task()', 'watcher()', 'tid_task', 'tid_watcher', and 'arg', in the body of 'body'.)

I don't believe nested functions like you're using there are valid C. They're a nonstandard extension that some compilers implement because they're so handy. So this basically illustrates that C gets closer Lisp's usability when you add Lispy features to it.

You're right, it's not even valid C99. My mistake.

Is there a C macro that can do this?

    (defmacro execute-in-reverse (&body body)
      `(progn ,@(reverse body)))

    (execute-in-reverse (print "Hi") (print "Middle") (print "Bye"))

Prints...

    "Bye"
    "Middle"
    "Hi"

Point: You can do whatever you want with the symbols sent to the macro, whatever their contents. Mahmud's wrapper macro is trivial (it could be done with lambdas). Lisp's macro system allows you to create new syntax, including changes in flow control.

But my example is trivial, too. I could go on to swap individual parts of my forms around, remap them to other forms depending on various conditions, etc.

In addition, people often talk about Lisp macros, but they neglect to mention the power of reader macros, which allow you to go beyond Lisp's basic AST look-and-feel.

>Is there a C macro that can do this?

No, of course. C macros only see strings; you need to parse a syntax tree to do your reverse example (like lisp macros).

No argument from me.

>Lisp's macro system allows you to create new syntax, including changes in flow control.

Well, C macros seem to be able to manipulate control flow. You can't break up an expression to do that (not smart enough to parse), but you can rearrange expressions that are given whole.

Wonderful example of what macros can do.

Just for shit & giggles I'd thought I give it a go in Io (http://www.iolanguage.com):

    executeInReverse := method (
        m     := call argAt(0)
        stmts := list()              // list of statements (ie. messages)
    
        loop (
            rest := m next           // rest of messages after current 
            stmts append(m setNext)  // get current message
            m := rest
            if (m == nil, break)     // exhausted statements when "nil"
        )

        stmts reverseForeach (n, doMessage(n))
    )

    executeInReverse( writeln("Hi") writeln("Middle") writeln("Bye") )

And I think with a bit more work I can get it to amend its AST rather than rerunning the loop each time it sees executeInReverse.

GCC extensions make me want to hurt people.

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