type t = Point
       | Pixel
       | Circle
       | TriangleDown
       | TriangleUp
       | TriangleLeft
       | TriangleRight
       | Custom of string

exception Invalid_marker_length
let of_string s = 
    if String.length s <> 1
    then raise Invalid_marker_length
    else Custom(s)

let pp channel n =
    (match n with
        | Point         -> "."
        | Pixel         -> ","
        | Circle        -> "o"
        | TriangleDown  -> "v"
        | TriangleUp    -> "^"
        | TriangleLeft  -> ">"
        | TriangleRight -> "<"
        | Custom(s)     -> s) 
    |> Printf.fprintf channel "%s"