datatype StringList
      = CONS of string * (unit -> StringList)
      | NIL

fun filter p NIL = NIL
    | filter p (CONS (x,xf)) =
        if p x then CONS (x, fn() => filter p (xf()))
               else filter p (xf())	  
	  
fun concat (xq, 0)          = ""
    | concat (NIL, n)         = ""
    | concat (CONS (x,xf), n) = x ^ concat (xf(),n-1)
	
fun binString 0 = ""
    | binString x = binString(x div 2)^(Int.toString (x mod 2))
	
val bin =
      let 
        fun genBin 0 = NIL 
			| genBin x = CONS(binString x, fn() => genBin(x+1)) 
      in 
        CONS("", fn() => genBin(0))
      end
	
	
fun main () =
      let 
        val filteredBin = bin 
      in
        print (concat(bin, 10))
      end