open Function

  integer function open(filename, mode, iostat) result(u)
    !! version: experimental
    !!
    !! Opens a file
    !! ([Specification](../page/specs/stdlib_io.html#description_1))
    !!
    !!##### Behavior
    !!
    !!
    !! To open a file to read:
    !!
    !!```fortran
    !! u = open("somefile.txt")        ! The default `mode` is "rt"
    !! u = open("somefile.txt", "r")
    !!```
    !!
    !! To open a file to write:
    !!
    !!```fortran
    !! u = open("somefile.txt", "w")
    !!```
    !!
    !! To append to the end of the file if it exists:
    !!
    !!```fortran
    !! u = open("somefile.txt", "a")
    !!```

    character(*), intent(in) :: filename
    character(*), intent(in), optional :: mode
    integer, intent(out), optional :: iostat

    character(3) :: mode_
    character(:),allocatable :: action_, position_, status_, access_, form_


    mode_ = parse_mode(optval(mode, ""))

    select case (mode_(1:2))
    case('r')
      action_='read'
      position_='asis'
      status_='old'
    case('w')
      action_='write'
      position_='asis'
      status_='replace'
    case('a')
      action_='write'
      position_='append'
      status_='old'
    case('x')
      action_='write'
      position_='asis'
      status_='new'
    case('r+')
      action_='readwrite'
      position_='asis'
      status_='old'
    case('w+')
      action_='readwrite'
      position_='asis'
      status_='replace'
    case('a+')
      action_='readwrite'
      position_='append'
      status_='old'
    case('x+')
      action_='readwrite'
      position_='asis'
      status_='new'
    case default
      error stop "Unsupported mode: "//mode_(1:2)
    end select

    select case (mode_(3:3))
    case('t')
      form_='formatted'
    case('b')
      form_='unformatted'
    case default
      error stop "Unsupported mode: "//mode_(3:3)   
    end select

    access_ = 'stream'

    if (present(iostat)) then
      open(newunit=u, file=filename, &
          action = action_, position = position_, status = status_, &
          access = access_, form = form_, &
          iostat = iostat)
    else
      open(newunit=u, file=filename, &
          action = action_, position = position_, status = status_, &
          access = access_, form = form_)
    end if

  end function open