Nathan Kamenchu (Dec 25 2024 at 23:30): Nathan Kamenchu (Dec 25 2024 at 23:30):I'm assuming it's not possible to handle implement Decode for tagged unions without some form of a macro system. Have there been any previous discussions of this? It does feel a bit incomplete for this to be missing
Richard Feldman (Dec 25 2024 at 23:32):it's possible, we just haven't implemented it
Richard Feldman (Dec 25 2024 at 23:32):it shouldn't need a macro system
Brendan Hansknecht (Dec 25 2024 at 23:32):We haven't implemented it? I thought it already worked
Brendan Hansknecht (Dec 25 2024 at 23:32):Just isn't implemented for the JSON decoder
Richard Feldman (Dec 25 2024 at 23:33):I don't think we have it for Decoding
Richard Feldman (Dec 25 2024 at 23:33):but I could be misremembering :big_smile:
Brendan Hansknecht (Dec 25 2024 at 23:33):Oh... intriguing. I don't recall at this point. That said, we still plan to revamp the whole system.
Brendan Hansknecht (Dec 25 2024 at 23:34):But yeah, no macro system needed.
Brendan Hansknecht (Dec 25 2024 at 23:35):You have autoderive or a manual implementation. Either is fine.
Nathan Kamenchu (Dec 25 2024 at 23:36):It's also missing from the Ability definition
Brendan Hansknecht (Dec 25 2024 at 23:38):Ok
Brendan Hansknecht (Dec 25 2024 at 23:39):Last I checked with the work to revamp decode, it was stuck on lambdaset bugs.
Brendan Hansknecht (Dec 25 2024 at 23:39):But we should be able to add tag decoding directly to the old decode
Nathan Kamenchu (Dec 25 2024 at 23:43):The compiler panics with
thread '<unnamed>' panicked at crates/compiler/mono/src/ir.rs:6257:10:
Ability specialization is unknown. Tip: check out <https://roc.zulipchat.com/#na
rrow/stream/231634-beginners/topic/Non-Functions.20in.20Abilities/near/456068617
>: DeriveError(Underivable)
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
I was trying out roc and decided to implement lamdera in roc by writing a platform but I've gotten stuck on sending messages over the wire.
Brendan Hansknecht (Dec 25 2024 at 23:50):This is with a tag?
Nathan Kamenchu (Dec 25 2024 at 23:50):Yes that is what happens with a tag
There's a different but similar message for Opaque types too
Brendan Hansknecht (Dec 25 2024 at 23:51):If so, we need to add tag decoding to the ability. Then we need to add autoderive for it in the compiler.
Brendan Hansknecht (Dec 25 2024 at 23:51):Would be very similar to the autoderrive for encoding a tag
Nathan Kamenchu (Dec 25 2024 at 23:56):Trying to decode with an opaque type errors with
An internal compiler expectation was broken.
This is definitely a compiler bug.
Please file an issue here: <https://github.com/roc-lang/roc/issues/new/choose>
expected to know a specialization for `17.IdentId(2)`#`13.IdentId(4)`, but it wasn't found
Location: crates/compiler/solve/src/specialize.rs:749:29
But my assumption is that this is intentional
Nathan Kamenchu (Dec 26 2024 at 00:00):Brendan Hansknecht said:
Would be very similar to the autoderrive for encoding a tag
Should I create an issue for it? You mentioned that there is work to revamp the code
I will take a look at the compiler and try to implement an autoderive.
Nathan Kamenchu (Dec 27 2024 at 12:44):What is reasonable API for a tag method in the DeocderFormatting ability?
Brendan Hansknecht (Dec 27 2024 at 16:22):With the current decode it should be very similar to a tuple. It should just decode a string tag name before decoding the tuple fields
Nathan Kamenchu (Dec 27 2024 at 16:32):I assume something similar to record and tuple like
tuple : state, (state, Str -> [Select (Decoder state fmt), Next]), (state -> Result val DecodeError) -> Decoder val fmt where fmt implements DecoderFormatting
but the nested decoding requirements where each variant would then need to decode a variable number of fields does feel like it may require more information like the index of the current tuple field
Nathan Kamenchu (Dec 27 2024 at 16:43):The alternative would just be push all of that to the state
Last updated: Jul 26 2025 at 12:14 UTC