Simple Maze Solver in F#

simpleMazeSolver.fs

let labyrinthString = """xxxxxxx
x     x
x x x x
  x x x
x xxx x
x     x
xxxxxxx"""

let labyrinth =
    labyrinthString.Split("\n")
    |> Array.map (fun line -> line.ToCharArray())

let dimensions =
    labyrinth.[0].Length, labyrinth.Length

let startPos = (fst dimensions /2, snd dimensions / 2)

let printLabyrinth (lab : char array array) =
    for row in lab do
        printfn "%s" (System.String(row))

printLabyrinth labyrinth

let markPosition lab (x, y) =
    lab
    |> Array.mapi (fun i row -> if i = y then row |> Array.mapi (fun j c -> if j = x then 'o' else c) else row)

type FieldType =
| Free
| Wall
| Visited
| Outside

let rec findWayOut lab (x, y) =
    let walkedLab = markPosition lab (x, y)
    let checkPos (checkX, checkY) =
        if checkX < 0 || checkX > fst dimensions ||
           checkY < 0 || checkY > snd dimensions then
           Outside
        else
           match lab.[checkY].[checkX] with
           | 'x' -> Wall
           | 'o' -> Visited
           | ' ' -> Free
           | _ -> failwith "Unkown labyrinth character"
    let neighbours =
        seq {
                yield (x-1, y)
                yield (x, y+1)
                yield (x+1, y)
                yield (x, y-1)
        }
    neighbours
    |> Seq.choose (fun field ->
        match checkPos field with
        | Outside -> Some walkedLab
        | Visited -> None
        | Wall -> None
        | Free -> findWayOut walkedLab field )
    |> Seq.tryHead

let maybeWayOut = findWayOut labyrinth startPos

match maybeWayOut with
| Some wayOut -> printLabyrinth wayOut
| None -> printfn "No way found!"