N-Queens in Lua and Fennel

queens.fnl

(global N 12)

(fn place-ok? [a n c]
  (var ok? true)
  (var i 1)
  (while (and ok? (< i n))
    (set ok? (not (or (= (. a i) c)
                      (= (- (. a i) i) (- c n))
                      (= (+ (. a i) i) (+ c n)))))
    (set i (+ 1 i)))
  ok?)

(fn print-solution [a]
  (for [i 1 N]
    (for [j 1 N]
      (io.write (if (= (. a i) j) "X" "-") " "))
    (print))
  (print))

(fn add-queen [a n]
  (if (> n N)
      (do
        (print-solution a))
      (for [c 1 N]
        (when (place-ok? a n c)
          (do
            (tset a n c)
            (add-queen a (+ 1 n)))))))

(add-queen [] 1)

queens.lua

-- Copied verbatim from Programming in Lua, 4th edition

-- board size
N = 12

-- check whether position (n,c) is free from attacks
function isplaceok(a, n, c)
  for i = 1, n - 1 do
    if (a[i] == c) or           -- same column?
       (a[i] - i == c - n) or      -- same diagonal?
       (a[i] + i == c + n) then -- same diagonal?
         return false
    end
  end
  return true
end

function printsolution(a)
  for i=1,N do
    for j=1,N do
      io.write(a[i] == j and "X" or "-", " ")
    end
    io.write("\n")
  end
  io.write("\n")
end

-- add to board 'a' all queens from 'n' to 'N'
function addqueen(a, n)
  if n > N then
    printsolution(a)
  else
    for c = 1, N do
      if isplaceok(a, n, c) then
        a[n] = c
        addqueen(a, n + 1)
      end
    end
  end
end

-- run the program
addqueen({}, 1)