Stream: beginners

Topic: porting data model from Scala

Jim Balhoff (Mar 17 2023 at 15:25):

I'm just getting started with Roc and trying to get used to its tags and types, etc. Reading the tutorial it sounds great, but when I try to implement my data model I find I'm not sure how best to do it. I'm trying to implement the Concept types from this file. I created a tag union like this:

Role : { id : Str }

Concept : [
    AtomicConcept { id : Str },
    Conjunction { left : Concept, right : Concept },
    ExistentialRestriction { role : Role, concept : Concept }
]

But sometimes I need to have List containing just one of those types, (like List Conjunction). In Scala all of these are also first class types. What's the recommended approach for this in Roc? I tried to make a type for the structure, like this:

ConjunctionRecord : {
    left: Concept,
    right: Concept
}

Concept : [
    AtomicConcept { id : Str },
    Conjunction ConjunctionRecord,
    ExistentialRestriction { role : Role, concept : Concept }
]

But roc check gives an error: fatal runtime error: stack overflow

Brendan Hansknecht (Mar 17 2023 at 15:34):

could you file an issue with repro on the stack overflow?

Brendan Hansknecht (Mar 17 2023 at 15:35):

Oh, I think what you want to do is currently bugged in roc such that you can't clean write it in roc.

Brendan Hansknecht (Mar 17 2023 at 15:35):

Fundamentally, your second example should work, but we have a recursive type issue currently.

Brendan Hansknecht (Mar 17 2023 at 15:36):

It makes it so that the ConjunctionRecord need to be inlined into the Concept definition because it recursively uses Concept

Brendan Hansknecht (Mar 17 2023 at 15:38):

What you can do currently:

# Define the limited type to avoid the bug
Concept : [
    AtomicConcept { id : Str },
    Conjunction { left : Concept, right : Concept },
    ExistentialRestriction { role : Role, concept : Concept }
]

# When you need to use just the `ConjunctionRecord`, don't type, or type manually
usesConjunctionsOnly : List {left: Concept, right: Concept}
usesConjunctionsOnly = \list -> ...

Jim Balhoff (Mar 17 2023 at 15:41):

Thanks! Would you say that my second approach would be the recommended style if not for the bug?

Brendan Hansknecht (Mar 17 2023 at 15:42):

I think so.

Jim Balhoff (Mar 17 2023 at 15:49):

In Scala I also define a trait Entity which groups things that have an id property. So AtomicConcept and Role are both entities. I would create a function like this:

signature : Concept -> Set Entity

I guess for that I would define another tag union, but I feel like I would repeat some of the info I've put in my Concept definition.

Brendan Hansknecht (Mar 17 2023 at 15:51):

four options:

1. another tag union
2. if they have the same field in a record, just make a functions \{id} -> id it will extract from any record with an id field.
3. use a higher order function an pass around a lambda to extract the field to functions that need it.
4. Abilities which are very much like Traits

Brendan Hansknecht (Mar 17 2023 at 15:52):

Oh, though you want to put them all in a Set. I assume a Set Entity would have a mix of types in it? Not just one Entity type at a time?

Jim Balhoff (Mar 17 2023 at 15:53):

Brendan Hansknecht said:

Oh, though you want to put them all in a Set. I assume a Set Entity would have a mix of types in it? Not just one Entity type at a time?

That's right.

Brendan Hansknecht (Mar 17 2023 at 15:53):

If so, the tag approach is the only option. The tag is what enables unifying them in memory representation to put into a Set.

Jim Balhoff (Mar 17 2023 at 15:55):

Great, thanks for all the info. I'll keep experimenting and hopefully get used to Roc's approach.

Jim Balhoff (Mar 17 2023 at 18:30):

This looks like it could be the same stack overflow error: https://github.com/roc-lang/roc/issues/4709

Brendan Hansknecht (Mar 17 2023 at 18:35):

Yeah, I think it is the same

Jim Balhoff (Mar 20 2023 at 14:37):

A similar? issue that I haven't found a workaround for: https://github.com/roc-lang/roc/issues/5162

Jim Balhoff (Mar 26 2023 at 17:50):

@Brendan Hansknecht I'm still trying to get my project to build—I opened a couple more issues on GitHub. Should I keep posting those as I encounter them? Don't want to become annoying. :-)

Brendan Hansknecht (Mar 26 2023 at 18:13):

Generally better to post multiple bugs if you aren't sure they are duplicates and let us merge them.

Last updated: Nov 09 2025 at 12:14 UTC