So changing the emacs mode has no effect on an existing buffer, but changing the file name does. I guess there's something about a .arc extension that turns off copy/paste from emacs.

edit: another datapoint - everything works fine when running emacs w/o X via: emacs -nw arc.arc

While experimenting, I received the following error after entering the following from the command line:

emacs bja.arc

Anyone have an idea of the problem? I've been looking through news.arc, and came across this:

  (mac newsop args
    `(do (pushnew ',(car args) newsop-names*)
         (opexpand defop ,@args)))

  (newsop news () (newspage user))

  (newsop ||   () (newspage user))

I prefer the word encourage :) I know it's nitpicky, but the idea was to not simply use a native print. Of course, had I required the 'pp' Ruby lib, I could have just said:

  pp stack   # => [ 1, 2, 3, ... , 8 ]

Your point is my point. I wanted to see how well Arc handles formatting something that didn't fit neatly into an expected pattern.

Ruby has several formatting techniques that are quite nice:

1) String interpolation. You can insert an expression into a spring by using #{expression}

2) String formatting. You can use sprintf style patterns in strings via format or the % operator. For example:

  puts "%4.1f hours" % hours
  puts "%4.1f hours and %4.1f minutes" % [hours, minutes]

Apparently folks have fixated on a particular algorithm in the wikipedia article. I only linked there for a general description of the problem. Instead, I should have simply said, "write a program that produces all the solutions to the problem of placing 8 queens on a chess board so that no queen is attacking any other queen and prints the solutions as follows (i.e. the output in the OP)"

For this challenge, it's important to have the output match exactly - that's why the rev is necessary and the complicated print. I do string interpolation all the time in Ruby, so I wanted to see how easy/hard it was in Arc to produce lines such as [ 1, 5, 8, 6, 3, 7, 2, 4 ]

I might as well limit the code to ArcN also.

I think the following may be the only working version of Arc code that passes this challenge so far.

Having to use point is a bit awkward, but not too bad. I particularly like the function composition such as abs:- and the more concise len.stack notation. The [ string sep _ ] notation is definitely a winner also. I'm pretty pleased with the core language so far - kudos to pg & rm.

  (def valid? (stack)
    (let q2 0
      (point return
             (each q1 (range 1 (- len.stack 1))
                   (if (or (is stack.q1 stack.q2)
                           (is (abs:- q1 q2) (abs:- stack.q1 stack.q2)))
                       (return nil)))
             t)))

  (def joinstr (lst (o sep " "))
    (if lst
        (string (car lst) (apply string (map [string sep _] (cdr lst))))
        ""))

  (def queens (stack n)
    (if (is n 8)
        (prn "[ " (joinstr (rev stack) ", ") " ]")
        (each rank (range 1 8)
              (push rank stack)
              (if (valid? stack)
                  (queens stack (+ n 1)))
              (pop stack))))

  (queens '() 0)

  arc> (queens '() 0)
  [ 1, 5, 8, 6, 3, 7, 2, 4 ]
  [ 1, 6, 8, 3, 7, 4, 2, 5 ]
  [ 1, 7, 4, 6, 8, 2, 5, 3 ]
  [ 1, 7, 5, 8, 2, 4, 6, 3 ]
  [ 2, 4, 6, 8, 3, 1, 7, 5 ]
  ...
  [ 8, 3, 1, 6, 2, 5, 7, 4 ]
  [ 8, 4, 1, 3, 6, 2, 7, 5 ]

  (def valid? (stack)
    (let i 0
      (if (or (pos stack.0 cdr.stack)
              (some (fn(x)(++ i)(is i (abs:- x stack.0))) cdr.stack))
        nil
        t)))

  (def queens (stack n)
    (if (is n 8)
      (and (prall rev.stack "[ ") (prn " ]"))
      (for rank 1 8
        (push rank stack)
        (if (valid? stack)
            (queens stack (+ n 1)))
        (pop stack))))

  (queens '() 0)

  (def valid? (stack)
    (let i 0
      (if (or (pos stack.0 cdr.stack)
              (some [do (++ i) (is i (abs:- _ stack.0))] cdr.stack))
        nil
        t)))

  (def valid? (stack)
    (let i 0
      (if (or (pos stack.0 cdr.stack)
              (some [is ++.i (abs:- stack.0 _)] cdr.stack))
        nil
        t)))

  (def queens (stack n)
    (if (is n 8)
      (do (prall rev.stack "[ ") (prn " ]"))
      (for rank 1 8
        (push rank stack)
        (if (valid? stack)
            (queens stack (+ n 1)))
        (pop stack))))

  (queens '() 0)

  arc> (help prall)
  (from "arc.arc")
  [fn]  (prall elts (o init ) (o sep , ))
   Prints several arguments with an initial header and separated by a
      given separator. 

Instead of (point return ...(return ...)) try using (catch ...(throw ...)).

  arc> (help prall)
  Error: "reference to undefined identifier: _help"

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