[debugnik]

Cool to see F# here! Emulators are a great way to learn a language. On first sight you chose well between more or less idiomatic F# for each job.

Some low hanging fruit to reduce allocations: the discriminated unions in Instructions.fs could be [<Struct>], reusing field names to reuse internal fields.

Also, minor nitpick but I'm confused about some of the registers. They are already of type byte, the setters with `a &&& 0xFFuy` don't add anything over `member val A = 0uy with get, set`. I'm guessing this changed over time.

[ibejoeb]

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)

So, I think, it's just conservatively cleaning the data due to Fable's widening via js Number on the web target.

[debugnik]

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.

[ibejoeb]

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.

[omcnoe]

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.

[rienbdj]

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.

[keithnz]

It's actually discussed in the article in the part where he ports it to fable (he also tried blazor)

[debugnik]

I admit I skimmed from there on because I don't find web dev exciting, but you're right, it is. That's a terribly naive translation on Fable's part.