On Mon, Jan 23, 2017 at 4:12 PM, Jim Nasby <jim.na...@bluetreble.com> wrote:

> On 1/23/17 3:45 PM, Peter van Hardenberg wrote:
>> A new currency type would be nice, and if kept small in scope, might be
>> manageable.
> I'd be rather nervous about this. My impression of community consensus on
> this is a currency type that doesn't somehow support conversion between
> different currencies is pretty useless, and supporting conversions opens a
> 55 gallon drum of worms. I could certainly be mistaken in my impression,
> but I think there'd need to be some kind of consensus on what a currency
> type should do before putting that up for GSoC.

There's a relatively simple solution to the currency conversion problem
which avoids running afoul of the various mistakes some previous
implementations have made. Track currencies separately and always ask for a
conversion chart at operation time.

Let the user specify the values they want at conversion time. That looks
like this:

=> select '1 CAD'::currency + '1 USD'::currency + '1 CHF'::currency
'1.00CAD 1.00USD 1.00CHF'

=> select convert('10.00CAD'::new_currency, ('USD, '1.25', 'CHF',
'1.50')::array, 'USD')

The basic concept is that the value of a currency type is that it would
allow you to operate in multiple currencies without accidentally adding
them. You'd flatten them to a single type if when and how you wanted for
any given operation but could work without fear of losing information.

I have no opinion about the most pleasing notation for the currency
conversion chart, but I imagine it would be reasonable to let users provide
a default set of conversion values somewhere.

There are interesting and worthwhile conversations to have about
non-decimal currencies, but I think it would be totally reasonable not to
support them at all in a first release. As for currency precision, I would
probably consider leaning on numeric under the hood for the actual currency
values themselves but IANAA (though I have done quite a lot of work on
billing systems).

If it would be helpful, I could provide a detailed proposal on the wiki for
others to critique?

Peter van Hardenberg
San Francisco, California
"Everything was beautiful, and nothing hurt."—Kurt Vonnegut

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