[Mono-list] 11 Queens problem slow on mono

[dtelford719]

Per your request, this is the code for the 11 queens problem:

open System

(* member of a list.  If have F# powerpack can use List.contains *)
let rec mem x l1 =
    if  l1 = []  then false
    else if x = (List.head l1) then true
    else (mem x (List.tail l1));;


(* are 2 queens safe from each other *)
let rec safe (x1, y1) (x2, y2) =
    x1 <> x2 && y1 <> y2 && x2 - x1 <> y2 - y1 && x1 - y2 <> x2 - y1 ;;

(* generate possible solutions *)
let ps n =
  [ for i in 1..n do
      for j in 1..n do
          yield (i,j) ] ;;

(* actually don't use print, commented out *)
let print n qs =
    for x in 1 .. n do
        for y in 1 .. n do
            printf "%s" (if mem (x, y ) qs then "Q" else "." )
        printf "\n" ;;

(* primary routine *)        
let rec search f n qs ps accu =
    match ps with
    | []  when List.length qs = n -> f qs accu
    | [] -> accu
    | q::ps ->
         search f n qs ps accu
         |> search f n (q::qs) (List.filter (safe q) ps)  ;;

(* solve for n queens on an n x n board *)
let solve n = 
    let f qs i = 
        (* print n qs  *) (* don't print solutions *)
        i + 1
    search f n [] (ps n) 0;;

(* time execution *)
let time f x =
  let timer = new System.Diagnostics.Stopwatch()
  timer.Start()
  try f x finally
  printf "Took %d milliseconds \n" timer.ElapsedMilliseconds

(* initial execution *)
let result = time  solve 11;;
printf "number of solutions = %d \n" result;;

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