Arc Forum | It's for Common Lisp, not Arc, but if you want to play with it, send me an e-mai...

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2 points by Johnny_65701 6020 days ago | link | parent

It's for Common Lisp, not Arc, but if you want to play with it, send me an e-mail: jjuniman at ll dot mit dot edu. I tried to post it but the formatting got all mangled up and it became unreadable. It's not a lot of code, only a dozen lines or so.

2 points by absz 6020 days ago | link

To post code that looks like code, surround it by blank lines and indent each line with two spaces—that should allow the code to display in a sane manner.

  (When you do that,
   it comes out like this.)

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2 points by Johnny_65701 6020 days ago | link

I did that and the ends of the lines got all mutilated. Dunno why, I'm copying & pasting from emacs, I'm not doing anything exotic.

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2 points by absz 6020 days ago | link

Are you on Windows/editing files created there? If so, is it possible that Windows's "\r\n" line endings are being pasted in literally?

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3 points by Johnny_65701 6019 days ago | link

Nope, Linux. But let me try again, since I have a different macro now that's considerably simpler, so the loss of newlines isn't as bad.

The original one I wrote wasn't working quite right. Then I found something in On Lisp, a macro called with-gensyms that I renamed with-hygeine due to a symbol conflict with something in Clisp:

  (defmacro with-hygiene (syms &body body)
   `(let ,(mapcar #'(lambda (s)
                    `(,s (gensym)))
                syms)
    ,@body))

You're supposed to use it like this:

  (defmacro my-stupid-macro (x y z)
    (with-hygeine (a b c)
      `(do-something-with ,x ,y ,z ,a ,b ,c)))

This gets me most of the way there, but what I really want is this:

  (hyg my-stupid-macro (x y z) (a b c)
    `(do-something-with ,x ,y ,z ,a ,b ,c))

We can achieve that like this:

  (defmacro hyg (name args syms &body body)
    `(defmacro ,name ,args
       (with-hygiene ,syms ,@body)))

Now I can do this:

  (hyg swap (a b) (temp) 
    `(progn 
      (setf ,temp ,a) 
      (setf ,a ,b) 
      (setf ,b ,temp)))

Which expands to:

  (DEFMACRO SWAP (A B) 
    (WITH-HYGIENE (TEMP) 
     `(PROGN 
        (SETF ,TEMP ,A) 
        (SETF ,A ,B) 
        (SETF ,B ,TEMP))))

which expands to:

  (PROGN 
    (SETF #:G3089 A) 
     (SETF A B) 
     (SETF B #:G3089))

PS: I'm a Lisp noob, so apologies in advance if something about this is a little naive, or contains a major oversight.

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2 points by absz 6019 days ago | link

Are you sure that you're indenting the lines with two extra spaces each (even the first)? That might fix the formatting.

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2 points by Johnny_65701 6019 days ago | link

Ooops, you are correct, sir. I indented all the lines except the first. Indenting the first line fixed the formatting (I went back & edited it)

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