I know that discriminated unions can refer to themselves, e.g.
type Tree=
| Node of Tree
| Leaf
but is there any means to refer to other cases in the type signatures? Both of the following raise errors that "The type 'Year' is not defined" & "The type 'Month' is not defined"
type Period =
| Year of int
| Month of Year * int
| Day of Month * int
type Period' =
| Year of int
| Month of Period'.Year * int
| Day of Period'.Month * int
Is there some form of annotation or a keyword I've yet to encounter (analogous to rec) that would permit such a usage?
I think you are confused about what constitutes a union case. You cannot reference a union case as a type. I think what you're looking for is single case DUs like this:
type Year = Year of int
type Month = | Month of Year * int
type Day = Month * int
type Period =
| Year of Year
| Month of Month
| Day of Day