Gringotts: A complete payment library for Elixir and Phoenix Framework

I had a quick look at the library. I think this is an awesome project and something very needed in the community. I have some some remarks, though.

Why are floats used for money? This is a complete no-go for me since floating point arithmetic is inherently inacurate. I haven’t looked in depth if this is actually the case, but that’s what is presented in the examples.

Why is the project using GenServers in the middle? It should be perfectly enough to have “just” modules and functions. The worker process does not use the state in any way, so the only thing it does, is creating a bottleneck in the system.

Yup - decimals or integers.

Also worth checking out this post.

There are a few libs that provide Money functions. money, ex_money and monetized (the first 3 on hex.pm) each seem to represent Money as a struct with a currency which is an ISO 4217 code and amount which is a Decimal.

It would be useful if Gringotts could support an abstraction for money types so that integration would be straight forward given than there’s a reasonable likelihood that someone using Gringotts is probably also using a Money lib. Maybe a Money protocol?

(disclaimer: I’m the author or ex_money)

I’ll just “upvote” this, because if this library is going to use floats internally it will be unreliable. The simplest way is to just use integers and have everything in “cents”, not “dollars” (that is if you don’t need to support currencies like the old pounds divisions in UK http://projectbritain.com/moneyold.htm).

What parts of the payment gateways are you targeting? Is it just plain payments or are you attempting to support Hosted Payment Pages, subscriptions, refunds, betting? Do you have a list of gateways your looking to support or some kind of roadmap?

Just noting even “cents” needs care since an implied decimal place of fixed size (cents == 100) is also problematic for the 52 currencies that have no decimal places, the 6 that have 3 decimal places and the one that has 4 decimal places.

I had no idea such currencies exist

3 decimal places: Libyan dinar, Bahraini Dinar and Omani rial
4 decimal places: Chilean unit of account (UF) which is a legal tender (Unidad de Fomento - Wikipedia)

Money stuff is fun!

And all those digital or cryptocurrencies? Don’t they have 6 and more decimals?

ISO 4217 will probably take quite a while to catch up with those

Either way, assumptions about money need care - including the assumption of how many decimal places a given currency may have. Even ISO 4217 allows the creation of private-use codes and they can have whatever digits and rounding strategy you want.