Rolf Kreibaum (Jan 01 2024 at 23:04): Rolf Kreibaum (Jan 01 2024 at 23:04):Hi, the website throws an error at me that I don't understand when I try to use the embedded repl:
Is this a known thing? Is my input even valid Roc?
Notification Bot (Jan 01 2024 at 23:05):A message was moved here from #beginners > Issues related to type annotations by Rolf Kreibaum.
Brendan Hansknecht (Jan 01 2024 at 23:09):Looks like a bug in the dev wasm backend
Rolf Kreibaum (Jan 01 2024 at 23:17):Where would I go to report that? https://github.com/roc-lang/roc/issues or some other place?
Rolf Kreibaum (Jan 01 2024 at 23:21):I found similar reports in the tracker, so I'll report it there.
Luke Boswell (Jan 01 2024 at 23:21):Thank you
Rolf Kreibaum (Jan 01 2024 at 23:26):Moved to https://github.com/roc-lang/roc/issues/6334 and the issue also links back to this chat. Thanks for the quick and friendly replies :-)
Notification Bot (Jan 01 2024 at 23:26):Rolf Kreibaum has marked this topic as resolved.
Folkert de Vries (Jan 02 2024 at 12:37):the problem is not specific to the web repl actually
The rockin' roc repl
────────────────────────
Enter an expression, or :help, or :q to quit.
» List.map [0.1,0.2,0.3] (\x -> x*x)
…
thread 'main' panicked at 'not yet implemented: NumMulWrap: layout, Builtin(Decimal)', crates/compiler/gen_dev/src/generic64/mod.rs:1500:18
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
@Brendan Hansknecht what should "mulwrap" for decimals do? just disregard overflow and whatever happens happens?
Folkert de Vries (Jan 02 2024 at 12:39):in practice that would mean that mulWrap is the same as mulSaturated. @Richard Feldman may also have an opinion here: what is the behavior we expect multiplication to have for decimals ?
Richard Feldman (Jan 02 2024 at 13:08):mulWrap and the like only accept integers
Richard Feldman (Jan 02 2024 at 13:08):so the behavior I expect is a type mismatch :big_smile:
Folkert de Vries (Jan 02 2024 at 13:51):no? floats implement mulwrap too
Folkert de Vries (Jan 02 2024 at 13:52):or another way to say this is: what would normal mul do for floats then when they overflow? is that just ignored then?
Richard Feldman (Jan 02 2024 at 13:57):floats overflow to infinity
Richard Feldman (Jan 02 2024 at 13:58):I guess there's an argument for extending it to work with any number type
Richard Feldman (Jan 02 2024 at 13:59):for the sake of Dec performance
Richard Feldman (Jan 02 2024 at 13:59):so I guess the answer for Dec would be that we would do whatever's fastest and gets the correct answer if there's no overflow
Richard Feldman (Jan 02 2024 at 13:59):and then if there is overflow, it does something that's probably undesirable but not UB haha
Richard Feldman (Jan 02 2024 at 14:00):so you'd reach for it for Dec when you want a but more perf and you're really confident it won't overflow
Richard Feldman (Jan 02 2024 at 14:01):and if it does overflow you're ok with it silently getting the wrong answer if it means more perf
Folkert de Vries (Jan 02 2024 at 14:47):for dec, I think saturated is equivalent in performance to any alternative
Folkert de Vries (Jan 02 2024 at 15:00):in any case, NumWrap should not even be used here. That was a shortcut in the dev backend. I've implemented it properly here https://github.com/roc-lang/roc/pull/6337.
Folkert de Vries (Jan 02 2024 at 15:01):and also fixed a bug that made roc_panic not work (crash worked before, I just messed up an argument type somewhere)
Folkert de Vries (Jan 02 2024 at 15:02):btw, I assume @Brendan Hansknecht is there a reason not to use zig's builtin https://ziglang.org/documentation/master/#toc-mulWithOverflow ?
Brendan Hansknecht (Jan 02 2024 at 16:05):mulWithOverflow doesn't work cause we don't know if it is an overflow until the Dec has been divided by 10^18.
Brendan Hansknecht (Jan 02 2024 at 16:06):To multiple 2 decs, we multiply into a i256, then divide by 10^18.
Brendan Hansknecht (Jan 02 2024 at 16:07):So overflow would be if that final result doesn't fit back into an i128
Last updated: Jul 26 2025 at 12:14 UTC