Parameterizing/Extracting Discriminated Union cases

Question

Currently i'm working in a game and use Event/Observables much, one thing i run into was to eliminate some redundant code, and i didn't found a way to do it. To explain it, let's assume we have following DU and an Observable for this DU.

type Health =
    | Healed
    | Damaged
    | Died
    | Revived

let health = Event<Health>()
let pub    = health.Publish

I have a lot of this kind of structures. Having all "Health" Messages grouped together is helpfull and needed in some situations, but in some situations i only care for a special Message. Because that is still often needed i use Observable.choose to separate those message. I have then code like this.

let healed = pub |> Observable.choose (function 
    | Healed -> Some ()
    | _      -> None
)

let damaged = pub |> Observable.choose (function
    | Damaged -> Some ()
    | _       -> None
)

Writing this kind of code is actually pretty repetitive and annoying. I have a lot of those types and messages. So one "rule" of functional programming is "Parametrize all the things". So i wrote a function only to just help me.

let only msg pub = pub |> Observable.choose (function
    | x when x = msg -> Some ()
    | _              -> None
)

With such a function in place, now the code becomes a lot shorter and less annoying to write.

let healed  = pub |> only Healed
let damaged = pub |> only Damaged
let died    = pub |> only Died
let revived = pub |> only Revived

EDIT: The important thing to note. healed, damaged, died, revived are now of type IObservable<unit> not IObservable<Health>. The idea is not just to separate the messages. This can be easily achieved with Observable.filter. The idea is that the the data for each case additional get extracted. For DU case that don't carry any additional data this is easy, as i only have to write Some () in the Observable.choose function.

But this only works, as long the different cases in a DU don't expect additional values. Unlucky i also have a lot of cases that carry additional information. For example instead of Healed or Damaged i have HealedBy of int. So a message also contains additional how much something got healed. What i'm doing is something like this, in this case.

let healedBy = pub |> Observable.choose (function
    | HealedBy x -> Some x
    | _          -> None
)

But what i really want is to write it something like this

let healedBy = pub |> onlyWith HealeadBy

What i'm expecting is to get an Observable<int>. And i didn't found any way how to do it. I cannot write a function like only above. because when i try to evaluate msg inside a Pattern Matching then it is just seen as a variable to Pattern Match all cases. I cannot say something like: "Match on the case inside the variable."

I can check if a variable is of some specific case. I can do if x = HealedBy then but after that, i cannot extract any kind of data from x. What i'm really need is something like an "unsecure" extracting like option for example provide it with optional.Value. Does there exists any way to implement such a "onlyWith" function to remove the boilerplate?

---

EDIT: The idea is not just separating the different messages. This can be achieved through Observable.filter. Here healedBy is of type IObservable<int> NOT IObservable<Health> anymore. The big idea is to separate the messages AND extract the data it carries along AND doing it without much boilerplate. I already can separate and extract it in one go with Observable.choose currently. As long as a case don't have any additional data i can use the only function to get rid of the boilerplate.

But as soon a case has additional data i'm back at writing the repetitive Observable.Choose function and do all the Pattern Matching again. The thing is currently i have code like this.

let observ = pub |> Observable.choose (function 
    | X (a) -> Some a
    | _     -> None
)

And i have this kind of stuff for a lot of messages and different types. But the only thing that changes is the "X" in it. So i obviously want to Parameterize "X" so i don't have to write the whole construct again and again. At best it just should be

let observ = anyObservable |> onlyWith CaseIWantToSeparate

But the new Observable is of the type of the specific case i separated. Not the type of the DU itself.

Answer 1

The behaviour it appears you are looking for doesn't exist, it works fine in your first example because you can always consistently return a unit option.

let only msg pub = 
    pub |> Observable.choose (function
        | x when x = msg -> Some ()
        | _              -> None)

Notice that this has type: 'a -> IObservable<'a> -> IObservable<unit>

Now, let's imagine for the sake of creating a clear example that I define some new DU that can contain several types:

type Example =
    |String of string
    |Int of int
    |Float of float

Imagine, as a thought exercise, I now try to define some general function that does the same as the above. What might its type signature be?

Example -> IObservable<Example> -> IObservable<???>

??? can't be any of the concrete types above because the types are all different, nor can it be a generic type for the same reason.

Since it's impossible to come up with a sensible type signature for this function, that's a pretty strong implication that this isn't the way to do it.

The core of the problem you are experiencing is that you can't decide on a return type at runtime, returning a data type that can be of several different possible but defined cases is precisely the problem discriminated unions help you solve.

As such, your only option is to explicitly handle each case, you already know or have seen several options for how to do this. Personally, I don't see anything too horrible about defining some helper functions to use:

let tryGetHealedValue = function
    |HealedBy hp -> Some hp
    |None -> None

let tryGetDamagedValue = function
    |DamagedBy dmg -> Some dmg
    |None -> None