チューリングを読む p.126

gistfile1.fs

// .01010101...

type Symbol =
  | None
  | Str of string
  | Num of int

type Op =
  | P of Symbol
  | R

let tape = Array.create 20 None
let mutable pos = 0

let showTape() =
  tape
  |> Array.map (function
    | None -> " "
    | Str x -> x
    | Num x -> string x)
  |> Array.iter (printf "%s")

let run opList m = 
    for op in opList do
      if pos < tape.Length then 
        match op with 
        | P symbol -> tape.[pos] <- symbol
        | R -> pos <- pos + 1
    if pos < tape.Length then
      m()
    else 
      showTape()

let P0 = P(Num 0)
let P1 = P(Num 1)

let rec b() =
  match tape.[pos] with
  | None -> run [P0; R] c
  | otherwise -> failwith <| string otherwise
and c() =
  match tape.[pos] with
  | None -> run [R] e
  | otherwise -> failwith <| string otherwise
and e() = 
  match tape.[pos] with
  | None -> run [P1; R] k
  | otherwise -> failwith <| string otherwise
and k() =
  match tape.[pos] with
  | None -> run [R] b
  | otherwise -> failwith <| string otherwise

run [] b