F# option computation expression

option.fs

[<Struct>]
type OptionalBuilder =
  member __.Bind(opt, binder) =
    match opt with
    | Some value -> binder value
    | None -> None
  member __.Return(value) =
    Some value
    
let optional = OptionalBuilder()

type Part = {
    Type: System.Type;
    Format: int option;
    Name: string;
    Part: int16;
    Number: float32;
    FileName: string option
}

type PartResult = {
    ty: System.Type;
    format: string;
    name: string;
    part: int16;
    number: float32;
    fileName: string
}

let truncateString length (s: string) =
    if s.Length > length then s.Substring(length) else s

let part = Some { Type = typeof<byte>; Format = Some 123; Name = "ABC"; Part = 456s; Number = 123.456f; FileName = Some "aaa.txt" }

let foo = optional {
  let! p = part
  let! f = p.Format
  let! fn = p.FileName
  return { ty = p.Type; format = f |> string; name = p.Name; part = p.Part; number = p.Number; fileName = fn |> truncateString 254 }
}