Upper and Lower Half of UInt16 in Enum? - enums

Question

I need to make an F# enum where the upper and lower bytes of the enum's value have meaning.
To make the code easily readable and maintainable in the future, I want to show each half of the uint16. Documentation refers to the number as the first 8 bits and the second 8 bits. Endianness is outside the scope of my question.
I have tried this, but it does not work.
type headers : uint16 =
| firstHeader = (0 <<< 8) + (0)
| secondHeader = (1 <<< 8) + (4)
| thirdHeader = (2 <<< 8) + (10)
| fourthHeader = (3 <<< 8) + (1)
The following works, but it obfuscates the number's meaning and introduces the possibility of calculating the composite number incorrectly.
type headers =
| firstHeader = 0us
| secondHeader = 260us
| thirdHeader = 522us
| fourthHeader = 769us
It does not need to be an enum, but I do need the same functionality (ie. being able to refer to the value as headers.firstHeader and get the underlying value let x = uint16 headers.firstHeader).
Does anyone know how to accomplish something like this?

Answer 1

You can't do this with an enum in F# because enum values must be literals. However, you could simply declare these as values within a module instead:
[<RequireQualifiedAccess>]
module headers =
let firstHeader = (0 <<< 8) + (0)
let secondHeader = (1 <<< 8) + (4)
let thirdHeader = (2 <<< 8) + (10)
let fourthHeader = (3 <<< 8) + (1)
The RequireQualifiedAccess attribute forces uses of the values to explicitly refer to headers, just like an enum (e.g. headers.firstHeader).