EnDeRBeaT (Aug 22 2025 at 13:36): EnDeRBeaT (Aug 22 2025 at 13:36):Hello, I have started learning roc for fun, and I have read static dispatch proposal. I am a bit confused about substituting abilities with it.
Rust allows to impl trait A for type B if either trait or type came from the module the impl block is.
While proposal talks about how you can easily substitute abilities with static dispatch by having a function in the module, there is absolutely nothing that talks about how can I extend existing types when I want to create my ability.
Can you guys tell me how it should go?
Anton (Aug 22 2025 at 14:44):Hi @EnDeRBeaT,
Allowing extension of existing types is not planned. From :
In general, methods only get defined in the module of the custom type
Feel free to share a specific problem that you would solve with extension of an existing type and we can discuss how to tackle it instead.
EnDeRBeaT (Aug 22 2025 at 15:30):Hi!
I do not have a concrete example on hand, I just wanted to point out that it doesn't seem like static dispatch can't fully replace abilities.
For example, you would not be able to define a new "ability" (https://www.roc-lang.org/abilities#defining-a-new-ability), because you can't extend other types.
Anton (Aug 22 2025 at 16:00):With the example provided there under "Advanced Topic: Defining a new Ability", you need to control the type definition of Color to add CustomInspect { .... This will work similarly with static dispatch except you need to make a module (file) Color.roc. I believe current abilities do not support the extension of existing types. You can only "extend" existing types if you wrap them in a new type, which will also be possible with static dispatch.
EnDeRBeaT (Aug 22 2025 at 16:02):Anton said:
if you wrap them in a new type
Okay, I think that it might be easier to do in Roc than in Rust, hence there is not a lot of need. Thank you
Notification Bot (Aug 22 2025 at 16:02):EnDeRBeaT has marked this topic as resolved.
Last updated: Sep 09 2025 at 12:16 UTC