When constructing an Amount, should it be possible to specify the number of fraction/significant digits or should that functionality exist only in the `.with` method?

[jessealama]

I certainly agree that currency and unit are needed, but I wonder if the others could be pushed to with and toString? In other words, one could imagine constructing an Amount based on a digit string and a currency, and then applying .with to specify a number of fraction/significant digits.

Originally posted by @jessealama in #29 (comment)

[eemeli]

Why? What benefit is there to needing to do

new Amount(n).with({ fractionDigits: 2 })

instead of

new Amount(n, { fractionDigits: 2 })

?

[jessealama]

The benefit -- which may be a small one, I admit -- is that the constructor wouldn't have to do any rounding. Rounding would then be something that exists only in with and toString/toLocaleString.

[eemeli]

Another way of achieving the same would be for the full precision of the MV to be retained until something like toString is called. Then we would have:

let a = new Amount('12.345', { significantDigits: 3 });
a.toString(); // '12.3'
a.with({ significantDigits: 6 }).toString(); // '12.3450'

This way the precision would be carried around much the same as unit or currency, as a sort of attribute of the MV, and only applied when we need the MV to be cast into some other data type.

[jessealama]

I like that idea. I think I was also getting something wrong when I made the issue. I think it's OK to specify fractionDigits and significantDigits in the constructor. I do still think that we don't need roundingMode in the constructor. AFAICS the only place where that's needed is toString/toLocaleString.

[eemeli]

We should still support setting roundingMode in the constructor, so that it can be set at the same time as fractionDigits and significantDigits is set.