The Register source has this comment:
// Registers can't be a record type because the values need to be truncated to 8 bits when writing, so setters are needed
// This is for the web renderer as Fable transpiles uint8 to Number (more than 8 bits) in JS and doesn't apply any truncation
// Known non-standard behaviour in Fable (https://fable.io/docs/javascript/compatibility.html#numeric-types)
Oof, thanks for pointing that out, I hadn't noticed and I've only ever used F# on .NET.
That's terrible on Fable's part, the least they could do is truncate. I wasn't aware Fable's translation is so naive.
I haven't used Fable much, but apparently it maps .NET arrays to js TypedArray. Presumably you could keep the registers in 8-element array and fable will properly produce a Uint8Array. I'd like to benchmark that.
Fable is great but it has a surprising number of these hidden behaviour changes that are really hard to detect when writing code against it.
It’s really hard to please everyone all of the time on this front.
This kind of thing is why Roc compiles to WASM but not JS.