add `.sliceToImmutable`?

[bakkot]

In many use cases you want to take a snapshot of some mutable data. The snapshot should be immutable, but the original data remains mutable.

Right now, that requires two steps: mutable.slice(0).transferToImmutable(). In practice I would guess that the intermediate mutable copy is quite cheap. But it may be worth checking this assumption with engines to verify this, because if the intermediate mutable copy is actually expensive, it could be worth adding another method to combine these steps into one.

[phoddie]

FWIW – sliceToImmutable() is included in @erights' slides in the Open Questions section.

sliceToImmutable(start?: number, end?: number) :ArrayBuffer

[bakkot]

Ah, nice, sorry I missed that. I'll leave this open as a place to track for now.

[erights]

In addition to @bakkot 's argument, an additional reason to add this is that there's otherwise(*) no way to make a zero-copy immutable slice of an immutable arrayBuffer. Without sliceToImmutable, the code (similar to @bakkot 's example) would be immutable.slice(s,e).transferToImmutable(), which would have a hard time avoiding an intermediate mutable copy.

(*) Under the normal implementation assumption that implementations do not attempt heroics like a copy-on-write implementation.

I currently favor adding this operation. If tc39 agrees, I will close with that decision.

[erights]

I currently favor adding this operation. If tc39 agrees, I will close with that decision.

At the December tc39 plenary, tc39 did agree. However, I'll leave this issue open until the spec text in this repo is revised accordingly.