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debugnik • yesterday at 6:13 PM • 2 replies • view on HN

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.

Replies

ibejoeb • yesterday at 8:42 PM

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.
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keithnz • yesterday at 11:02 PM

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