Hi guys What do you think about languages with tagged unions

Question

Hi guys What do you think about languages with tagged unions aka sum types Did you managed to use such a language Kotlin has something that reminds me them sealed classes but they look very verbose and I am not sure if they are useful as tagged unions I have to check this my Kotlin experience is too short

scott · Answer

witoldsz care to share a good example

roberto.guerra · Answer

I ve used them in SML and scala They are very elegant in SML and very useful When I started looking at Kotlin I was hoping it was a port of SML to the JVM

witoldsz · Answer

So I have used tagged unions in Elm It was my first contact with this let me show an example of application structure A top level model of the application looks like this ``` type alias Model = page Page csrfToken String type Page = NoPage | AnnouncementListPage WebData List Announcement | AnnouncementItemPage WebData AnnouncementForm | PayoutCancelledListPage PayoutCancelledListForm | PayoutListPage PayoutListForm ```

witoldsz · Answer

`WebData` comes from a library it looks like this ``` type RemoteData e a = NotAsked | Loading | Failure e | Success a type alias WebData a = RemoteData Http Error a ``` reference <http package elm lang org packages ohanhi remotedata http latest>

witoldsz · Answer

In such a small amount of code one can see the application through My favorite features is that such a type system literally does not allow invalid state of an app to be modeled This brings lots of positive consequences

pniederw · Answer

modern oo languages achieve a similar effect with sealed classes or fancy enums kotlin s sealed classes are more verbose than sum types but that s life

scott · Answer

is the difference mainly just verbosity

witoldsz · Answer

I do not know yet

witoldsz · Answer

as far as I can tell the `WebData` could not be modeled with `enum` because enums are all singletons `NotAsked` and `Loading` would work but `Failure ` and `Success a` are tagged values

scott · Answer

if there is variable data use a sealed class heierachy if it s static no data use enum

witoldsz · Answer

`Success Hello from Backend ` produces an instance of `RemoteData`

witoldsz · Answer

In Kotlin `Success` would be a constructor with one argument but in the same time `Loading` is just a `Loading` Each `Loading` is equal to any other one

scott · Answer

would still be equal in kotlin in terms of value but not instance

scott · Answer

although you could use an object if you wanted only one of the data less versions although I don t see why unless allocating millions or something

witoldsz · Answer

So it looks like sealed classes are just a verbose version of tagged unions

witoldsz · Answer

Now when I think about it sealed class should be able to map 1 1 shouldn t it

witoldsz · Answer

It is BTW such a nice construct to model a property with a value fetched from remote location isn t it

witoldsz · Answer

I am thinking how could `WebData` alias in Kotlin represent a special version of `RemoteData` sealed class

witoldsz · Answer

For not familiar with Elm s syntax `WebData e a` `e` stands for any error `a` stands for well anything successful response The `WebData` is a RemoteData with `e` error of type `Http Error` and `a` is left as is

witoldsz · Answer

my first attempt looks like this ``` sealed class RemoteData lt E A gt object notAsked RemoteData lt Any Any gt object loading RemoteData lt Any Any gt data class Failure lt Any gt val error Any RemoteData lt Any Any gt data class Success val success Any RemoteData lt Any Any gt ``` Does not looks OK thogh the ` lt E A gt ` cannot be used in `Failure` and `Success` compiler complains it does not know what E and A are

witoldsz · Answer

second attempt looks promising ``` sealed class RemoteData lt E A gt object notAsked RemoteData lt Any Any gt object loading RemoteData lt Any Any gt data class Failure lt E gt val error E RemoteData lt E Any gt data class Success lt A gt val success A RemoteData lt Any A gt class HttpError whatever typealias WebData lt A gt = RemoteData lt HttpError A gt ``` I am not sure it it s working it just compiles

pniederw · Answer

I was mentioning enums because some oo langs e g swift have enums that are more powerful than kotlin s each enum value can carry different data also heard that java folks are considering to go in that direction

witoldsz · Answer

I guess the ``` object notAsked RemoteData lt Any Any gt object loading RemoteData lt Any Any gt data class Failure lt E gt val error E RemoteData lt E Any gt data class Success lt A gt val success A RemoteData lt Any A gt ``` Should be ``` object notAsked RemoteData lt Nothing Nothing gt object loading RemoteData lt Nothing Nothing gt data class Failure lt E gt val error E RemoteData lt E Nothing gt data class Success lt A gt val success A RemoteData lt Nothing A gt ``` It s even more to write though confused

cedric · Answer

< witoldsz> The cleanest modern language supporting sum and product types is Ceylon in my opinion take a look at it

witoldsz · Answer

I really started to like Kotlin slightly smiling face Will look at Ceylon anyway

witoldsz · Answer

At first look Ceylon does not look attractive I wish there was something like F for JVM so it could coop with Java like Kotlin

raulraja · Answer

< witoldsz> Scala models them similar to Kotlin as you may already know and they are getting improved syntax in Dotty A more important feature for Kotlin that would make sum types and other FP techniques more appealing would be Higher Kinded types Without these is hard to do any serious stuff in typed FP in Kotlin and emulating them is a real pain

raulraja · Answer

Another one is typeclasses I d take those over better syntax for sum types any time

witoldsz · Answer

If you have a free minute to spare I would strongly advise reading this little post <https becoming functional com higher level coding with elm union types 2502d1f5a615>

witoldsz · Answer

This guy has expressed my whole point of view at that matter

darkmoon_uk · Answer

Swift models these more concisely as it s parameterised enum implementation

darkmoon_uk · Answer

Kotlins sealed classes are a little more verbose but still invaluable say when representing variously branched states of a state machine

roberto.guerra · Answer

Nice blog it is doable in Kotlin but it is way more verbose I ve been adopting that technique but what drove me to that was not pretty code but weird errors at runtime when integrating with external services Modelling those interactions as state machines virtually eliminated those kinds of errors because the clients of the code were forced to think about what to do for each valid state and not just ignore it