Other sub typing methods less commonly talked about

[Xyncgas]

In tutorials, we often see a sub type first created then put into DU, which is something advertised to people from C# (doesn't have DU) for its greatness.

One example is :

    type ValidPassword = string
    type ValidEmail = string
    type ValidZip = uint
    type ErrorCode = uint
    
    type Result =
    | ValidPassword of ValidPassword
    | ValidEmail of ValidEmail
    | ValidZip of ValidZip
    | ErrorCode of ErrorCode

    let Process () = ErrorCode(uint(5))

[baronfel]

What's the actual request here? If it's a language suggestion it should be raised at fsharp/fslang-suggestions instead of here.

[Xyncgas]

can you remove this comment for now, I am still typing adding mroe stuff

[Xyncgas]

I am adding a new way (less talked about) for sub typing, it's not a new language feature

[Xyncgas]

There is another way to create sub-type that's putting all the possible cases in the discriminated union first, then create active pattern of the DU

For example :

    type Result = | Str of string | Num of uint
    let (|ValidPassword|_|) result = result |> function Str str -> ValidPassword str |> Some |_ -> None
    let (|ValidEmail|_|) result = result |> function Str str -> ValidEmail str |> Some |_ -> None
    let (|ValidZip|_|) result = result |> function Num num -> ValidZip num |> Some |_ -> None
    let (|ErrorCode|_|) result = result |> function Num num -> ErrorCode num |> Some |_ -> None
    let Process res =
        match res with
        | ValidPassword pwd -> 0
        | ValidEmail pwd -> 0
        | ValidZip pwd -> 0
        | ErrorCode pwd -> 0
        | _ -> 0