Oskar Hahn (May 31 2024 at 07:46): Oskar Hahn (May 31 2024 at 07:46):A platform can depend on a Model, that is specified by the app. For example the wasm4 and the kingfisher platform do this.
To import the Model, the platform requires the Model. It is specified in the headers: requires { Model } { main : _ }.
Is it currently possible for a module inside the platform (other then main.roc) to import the Model? As far as I can see, the required-header does not work. It is also not possible to import main.Model.
What I am trying to do is to create a Task, that takes the Model as an argument. For example updateModel : Model -> Task {} [UpdateFailed]. For this to work at the moment, the hosted module has to use the model to create the Effect.
Is this currently possible?
Oskar Hahn (Jun 05 2024 at 09:30):@Agus Zubiaga Is this something, that will be possible with the new module headers of platform modules?
Agus Zubiaga (Jun 05 2024 at 10:34):Yeah, I don’t think that’s quite possible. Can you show me an example where you’d use updateModel? There might be another way to get to your desired functionality
Oskar Hahn (Jun 05 2024 at 13:43):So here is the draft code. I did not update the host, just the roc part: https://github.com/ostcar/kingfisher/compare/task
The effect is defined here: https://github.com/ostcar/kingfisher/compare/task#diff-2f519f2cdd03e9c119f1f1da69ec5681b2d6e762a2d059d0cd49c788fc520640R27-R31
The Task is defined here: https://github.com/ostcar/kingfisher/compare/task#diff-2f519f2cdd03e9c119f1f1da69ec5681b2d6e762a2d059d0cd49c788fc520640R27-R31
An example, using it, is here: https://github.com/ostcar/kingfisher/compare/task#diff-81772da4aabf8aca762cb72425207ba62abd414dece0c57719f8a87574002a38R49
Agus Zubiaga (Jun 05 2024 at 13:49):Right. This is how I'd do it instead:
handleRequest : Request, Model -> Task (Model, Response) _
handleRequest = \request, model ->
when request.method is
Post ->
newModel =
if List.isEmpty request.body then
"World"
else
request.body
|> Str.fromUtf8
|> Result.withDefault "invalid body"
Task.ok (newModel, {
body: newModel |> Str.toUtf8,
headers: [],
status: 200,
})
_ ->
Task.ok (model, {
body: "Hello $(model)\n" |> Str.toUtf8,
headers: [],
status: 200,
})
Your task simply returns the new model in addition to the response
Agus Zubiaga (Jun 05 2024 at 13:50):The Task type itself doesn't have to care about the type of your Model
Agus Zubiaga (Jun 05 2024 at 13:51):It can be a requirement of the handleRequest function
Agus Zubiaga (Jun 05 2024 at 13:53):If you split a request handler into multiple functions, you have to thread the Model through, but that's also the case with the updateModel API because you want to use the most recent one to apply edits on top of
Agus Zubiaga (Jun 05 2024 at 14:11):I don't know if there's a reason you'd want to truly update the model in the middle of your handler. One of the benefits of not doing so is that you'd get an "automatic rollback" if the task fails.
Agus Zubiaga (Jun 05 2024 at 14:15):and if you still need to update the Model when the task fails, you can use Task.onErr to do so
Brendan Hansknecht (Jun 05 2024 at 14:49):General side question: Why do we have requires for this? Can't it just be a type variable in the platform?
Oskar Hahn (Jun 05 2024 at 14:57):I also thought about it. One part of the idea was, to unify handleReadRequest and handleWriteRequest. It is not possible for a read request to update the model. In the case of my Task example, the updateModel-Task would return an error on a GET-Request. Task (Model, Response) _ would only be possible on a handleWriteRequest.
Another part of the idea is, that the host knows, if the model has changed or not. For example on a POST request, on a unknown path, the server should return a 404 and will (probably) not change the model. The host can not know, if the model has not changed. So the host has to do all is "model-save-stuff". So the type should be more like Task ([NotChanged, Changed Model], Response) _. A Task would be nicer.
But I think that beside of my use case there might be a need in other cases as well. It seems strange, that you have functions in your platform/main.roc, that you can not put into another file/module. Maybe there is another platform, that does more stuff on the roc side. Why should this platform has to have to have all of its code in platform/main.roc. This is even more relevant, that it is currently not possible to export a function from main.roc. As far as I know, an application can only import functions from modules in platform, an not from the main.roc.
Oskar Hahn (Jun 05 2024 at 14:59):Brendan Hansknecht said:
General side question: Why do we have requires for this? Can't it just be a type variable in the platform?
What is a type variable in the platform? I imitated, what the wasm4 platform did. How would this type variable look in the wasm4 platform?
Oskar Hahn (Sep 05 2024 at 08:28):I read in another thread, that we have module parameters now. I am really exited to here this.
Will it be possible now, to give the Model to the effect Module?
I can not find any example of the new module parameters. Can someone tell me, how the syntax looks like?
Luke Boswell (Sep 05 2024 at 09:25):Here's the tests which show the syntax https://github.com/roc-lang/roc/tree/main/crates/cli/tests/module_params
Oskar Hahn (Sep 05 2024 at 12:40):Is it correct, that the hosted modules still use the old module header?
hosted Effect
exposes [...]
imports []
Is it possible to use module params on them? If not, is there a workaround or will this be possible in the future?
Agus Zubiaga (Sep 05 2024 at 13:02):Module params are only for values and functions, not types. They are not a solution to this problem.
The main motivations for them are to allow packages to perform effects without depending on a specific platform and for effects to be simulated in tests.
By explicitly providing effects to a module, you can also easily tell everything it might do, and you can sandbox/adapt them by writing some code in the middle.
Agus Zubiaga (Sep 05 2024 at 13:08):I believe #compiler development > sending unsized values to the host is the most recent discussion on a solution to what you’re describing
Oskar Hahn (Sep 05 2024 at 15:12):I read through the topic about sending unsized values to the host, but I don't get, how this could help me.
For me, it does not matter, if I send Model or Box Model. My problem is, that there is currently no syntax in roc, to define an Task with a generic argument. At least, as far as I can see.
Is this something, that will be possible in the future? I though I had to wait for module params to solve this. Is there another thing, I have to wait for? For example the effect interpreter, because with it, Tasks do not have to be defined inside an "hosted" module, but can be defined in the main.roc file?
Agus Zubiaga (Sep 05 2024 at 15:35):I haven’t had time to read through the whole thing, but I think the idea is that Model would just become a type variable you can send to the host (as long as its Boxed, so its type is statically known)
Oskar Hahn (Sep 05 2024 at 15:46):I am not so familiar with the term type variable. What term would you use, that Model currently is?
If Model would get a type variable, what would be the syntax to get it inside the hosted module? Would it then be possible to use as a module parameter?
Agus Zubiaga (Sep 05 2024 at 15:52):Oh, it just means a generic parameter (lowercase name) in an annotation, like:
updateModel : model -> Task model []
model is a type variable.
Agus Zubiaga (Sep 05 2024 at 15:54):You probably don’t need to return it back, but you get the point
Agus Zubiaga (Sep 05 2024 at 15:56):Like I said before, I’m not up to date with this discussion, so I’m not sure what your use case is and whether it’ll be supported. I recommend bring it up in that topic.
Oskar Hahn (Sep 05 2024 at 15:56):Thank you. I will to this
Oskar Hahn (Sep 05 2024 at 20:54):I still not fully get it. The idea from @Brendan Hansknecht sounds good, but I don't think it fully solves my case.
I want to make another example: Take the basic-webserver. The example in the README says, that you can use the server.init function, to set up a database connection. But what if this database connection has a timeout? Then you need a way to create a new database connection. For this, it would be nice, if basic-webserver would expose a Task like updateModel : Model-> Task {} {}. When the connection expires, the application can create a new connection (like it did with server.init) and then send the new connection to the host, so it is used on future calls to server.respond.
If basic-webserver would get a feature request like this. How would you implement this updateModel Task?
I don't think it is possible to do it with
Funcs model: {
init: {} -> Task model []
update : model -> Task model []
}
Becaues there also is the server.respond function with this definition: respond : Http.Request, Model -> Task Http.Response [ServerErr Str]_. I don't see, how you could tell roc, that the Model in respond has to have the same type as the model in Funcs.
Is there a way to solve this in current roc or in a planed future of roc?
Brendan Hansknecht (Sep 05 2024 at 21:03):Ah, I see what you are pointing out. I think that is dealt with by effect interpretters
Brendan Hansknecht (Sep 05 2024 at 21:04):In the effect interpretter world, all effects are actually tags that unify together
Brendan Hansknecht (Sep 05 2024 at 21:06):So updateModel : Model-> Task {} {} would really be [UpdateModel (Box model) (Box (Result {} {} -> ...))]. It would be one part of a greater tag that all effect build up together.
Brendan Hansknecht (Sep 05 2024 at 21:07):With roc today, that can not be done safely.
Brendan Hansknecht (Sep 05 2024 at 21:07):As such, it must be dealt with by the core platform api and not tasks
Brendan Hansknecht (Sep 05 2024 at 21:08):So in basic webserver, either special concrete type effects would be needed outside of model all together, or respond would need to return an updated model
Oskar Hahn (Sep 05 2024 at 21:20):In the effect interpretter world, where can you define this Effect-Tag? Because with today's module syntax, the symbol Model only exists in platform/main.roc. To define your example of UpdateModel, it would also have to stay in the main.roc file. With today's syntax, it would be impossible, to define it somewhere else.
Is this the plan or should the module syntax change so it is possible to expose Model to other modules?
If so, then do I really have to wait for the effect interpreter, or only for the update on the module syntax? Because if Model could be imported from today's hosted module, it would be possible to create the updateModel Task without the effect interpreter.
Brendan Hansknecht (Sep 05 2024 at 21:22):Oh sure, today with some import fixes and a concrete Model I think it should theoretically work.
Brendan Hansknecht (Sep 05 2024 at 21:22):I think in the future a concrete Model wouldn't be allowed unless the platform defined the Model type. It would have to be a type variable model.
Brendan Hansknecht (Sep 05 2024 at 21:23):I'm not fully sure what our plan will be around defining the effect tag. But I know this is a case we will want to support.
Oskar Hahn (Sep 06 2024 at 06:22):I am not sure, what you mean with concrete Model. Is that a Model where the type is known at compile?
What is not possible is, that basic-webserver's respond would return an updated model. Since respond is called in parallel. If you call respond N-times, it would return N different Models and therefore open N new connections to the database.
I can accept, that roc will not allow a Task, that accept a Model in the foreseeable future. But I still think, it would be a useful feature.
Luke Boswell (Sep 06 2024 at 06:42):Platforms host it built before the app, so it cannot know the size or shape of the Model as that is defined in the app.
Luke Boswell (Sep 06 2024 at 06:42):So to the platform, the Model is abstract and not concrete.
Last updated: Jul 26 2025 at 12:14 UTC