let rec r=function|'!'::t->(1-r t)|'0'::[]->0|_->1

​

///////////////////////////////////////

///////////////////////////////////////

​

let rec string_of_list = function

| [] -> ""

| h::t -> System.String([|h|]) + string_of_list t

​

let test (input,expected) = 

  let result = r input

  if result = expected then

    printfn "%s = %d" (string_of_list input) result

  else

    printfn "Error at %s" (string_of_list input)

​

[

(['0'], 0)

(['1'], 1)

(['0';'!'], 1)

(['1';'!'], 1)

(['!';'0'], 1)

(['!';'1'], 0)

(['!';'0';'!'], 0)

(['!';'1';'!'], 0)

(['0';'!';'!'], 1)

(['1';'!';'!'], 1)

(['!';'!';'0'], 0)

(['!';'!';'1'], 1)

(['!';'0';'!';'!'], 0)

(['!';'!';'!';'1'], 0)

(['!';'!';'!';'0';'!';'!';'!';'!'], 0)

(['!';'!';'!';'1';'!';'!';'!';'!'], 0)

] |> Seq.iter test