Add fromArray method to Buffer

[xlaywan]

Useful to use in postupgrade in combination with buffer.toArray() in preupgrade

[nomeata]

Thanks! Why not simply

Suggested change

	/// Construct a buffer from an array.
	public func fromArray<T>(arr: [T]): Buffer<T> {
	let count = arr.size();
	let buffer = Buffer<T>(count);
	var i = 0;
	label l loop {
	if (i >= count) break l;
	buffer.add(arr[i]);
	i += 1;
	};
	buffer
	};
	/// Construct a buffer from an array.
	public func fromArray<T>(arr: [T]): Buffer<T> {
	return { count = arr.size(); elems = Array.thaw(arr) }
	}

[xlaywan]

Looks definitely better!
But can it create an instance of the Buffer class? Motoko Playground shows error:

It was intended to be a static method like let buf = Buffer.fromArray<Nat>([1, 2, 3]);
Probably it will work as an instance method like:

let buf = Buffer.Buffer<Nat>(0);
buf.fromArray([1, 2, 3]);

Not sure we can leave the name fromArray in this case, because in other files in base lib from* methods are static.

Link to the playground https://m7sm4-2iaaa-aaaab-qabra-cai.raw.ic0.app/?tag=2337373062

[nomeata]

Ah, right, Motoko classes have only one constructor usually…

Maybe first add buffer.addArray (which can be implemented efficiently using Array.thraw, especially if the buffer is empty before), and then the fromArray function.

[crusso]

This looks closely related to #387. Perhaps it can be closed now? @matthewhammer ?

[matthewhammer]

The discussion here seems stalled on the most efficient way to do the operation.

In a closely related PR, we are proposing the same method functionality and name, but with the naive implementation. Maybe we can start there and refine?