Stream: beginners

Topic: mono/src/ir Option::unwrap() on None

Zenna (Dec 05 2024 at 09:57):

hey y'all running into a compiler panic when trying to work with dicts. This is the error I'm getting

thread '<unnamed>' panicked at crates/compiler/mono/src/ir.rs:6166:56:
called `Option::unwrap()` on a `None` value
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

and this is the code causing it

partOne = \(rules, updates) ->
    rulesDict = List.walk rules (Dict.empty {}) \dict, (k, r) ->
        Dict.update dict k \res -> when res is
            Err Missing -> Ok [r]
            Ok rs -> Ok (List.append rs r)
    rulesDict

was hoping someone could point me to a workaround

Luke Boswell (Dec 05 2024 at 10:03):

Does it help if you add a type annotation for rulesDict or the function?

Luke Boswell (Dec 05 2024 at 10:04):

I suspect the issue here might be that the compiler is having trouble inferring the type for the Dict

Zenna (Dec 05 2024 at 10:12):

it does! thank you :)

Luke Boswell (Dec 05 2024 at 10:13):

Ideally the compiler shouldn't crash, and at least give a nice error message.

Luke Boswell (Dec 05 2024 at 10:16):

I've had a brief look at it, but mono is a bit of black magic to me and I'm not sure how we might thread an error back through the compiler from there.

Sam Mohr (Dec 05 2024 at 10:21):

Two things about mono errors:

• It's likely that whatever's causing this error is upstream from mono, so we probably have a bug somewhere else
• You're right that it's tricky to thread errors back up from mono, which is why we should just keep a list of mono errors and report them if any are found in mono. We might already be doing this, to be fair...

Last updated: Jul 26 2025 at 12:14 UTC