Upper and Lower Half of UInt16 in Enum?

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).