use Perl

• your macro (Score:1)

  by jjore (6662)

  You've just evaluated x twice when the person writing (++ x) expected x to be evaluated once.
  
  I consulted the manual and came up with this which doesn't work because the original x isn't getting updated anymore.
  
  (defmacro ++ (x)
      "Increment the value of symbol X, returning the new value"
      (let ((xt (make-symbol "quoted-x")))
          `(let ((,xt (quote ,x)))
                (set ,xt (+ 1 (eval ,xt))))))
  
  This fixes three bugs. x is ev
  • Re:your macro (Score:2)

    by TorgoX (1933)

    You've just evaluated x twice when the person writing (++ x) expected x to be evaluated once.

    Oh, I didn't think of that! I was only thinking of passing in a symbol constant. But now I see what you mean. [emacswiki.org]

    As to the general hardness of Lisp macros: I'm glad it's not just me that thinks that.

    Maybe there's just some kind of magic syntax-highlighty thing in (e)lisp mode that will turn on all the happy macro easiness or something.

    Emacs is a pathless land.
    • Re:your macro (Score:1)

      by jjore (6662)

      I goofed slightly. I'd written an earlier version which didn't work and it looks like I left the text that mentioned that it didn't work. The one I eventually posted does work.
      
      As for making macros easier, I'm sure the Scheme folks have stuff on this. I'm a novice at this but I could guess that you might write something to check that each value is only used once. Something to do with data flows maybe.
• incf? (Score:1)

  by arc (5008)

  See also the incf macro from cl.el:

  (incf x)
  (incf (car x))
  (incf x 23)

  It's definitely too hard to write such things from scratch in Lisp. cl.el also has callf and define-modify-macro macros which help somewhat.