nullable_pointer.pony

struct NullablePointer[A]
  """
  A NullablePointer[A] is used to encode a possibly-null type. It should
  _only_ be used for structs that need to be passed to and from the C FFI.

  An optional type for anything that isn't a struct should be encoded as a
  union type, for example (A | None).
  """
  new create(that: A) =>
    """
    This re-encodes the type of `that` from A to NullablePointer[A], allowing
    `that` to be assigned to a field or variable of type NullablePointer[A]. It
    doesn't allocate a wrapper object: there is no containing object for `that`.
    """
    compile_intrinsic

  new none() =>
    """
    This returns a null pointer typed as a NullablePointer[A].
    """
    compile_intrinsic

  fun apply(): this->A ? =>
    """
    This re-encodes the type of `this` from NullablePointer[A] to A, allowing
    `this` to be assigned to a field of variable of type A. If `this` is a null
    pointer, an error is raised.
    """
    compile_intrinsic

  fun is_none(): Bool =>
    """
    Returns true if `this` is null (ie apply would raise an error).
    """
    compile_intrinsic