5 ms·

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

• GregBuchholz 16y ago
  (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?"))))
  • makmanalp 16y ago
    I think Greg is trying to say "why the hell do we even need macros for this"?
    • mahmud 16y ago
      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.
• funptr 16y ago
  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'.)
  • chc 16y ago
    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.
    • funptr 16y ago
      You're right, it's not even valid C99. My mistake.
  • Xurinos 16y ago
    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.
    • draegtun 16y ago
      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 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.