Constructing an Amount from an Amount?

[ben-allen]

In the comments for #49 the question of whether we should be able to construct an Amount from an Amount arose, which seems worth asking outside of the context of that issue.

So: bracketing off the questions related to using with to increase the number of significant digits that were raised in that issue, should we treat:

let a = new Amount(1.234, { significantDigits: 4 });
let b = new Amount(a, { significantDigits: 2 });

as equivalent to

let b = new Amount(1.234, {significantDigits: 4}).with({ significantDigits: 2 })

and, more generally, should we allow Amounts as arguments to the Amount constructor?

[ljharb]

I don't see why not - in theory, though, the toString output would cover all options?

[gibson042]

For recent precedent, the Temporal constructors require each field to be specified in its own numeric argument, and therefore do not accept other instances as valid input—but each of them also has a static from method that does accept a single item which may be a Temporal instance from which to create a new one, similar to the much older Array/TypedArray API pattern and partially echoed by %Iterator% (whose constructor accepts no input and from method returns already-iterator arguments unmodified). There is precedent the other way, with e.g. Date and RegExp playing argument hockey, but that's been implicitly if not explicitly abandoned (cf. Promise, DataView, ArrayBuffer, etc.).

IMO, the way to construct an Amount from a value that might already be an Amount should be an Amount.from method that would also accept an untyped property bag.