I do try to be as clear as possible in documentation about the purpose and intent of ex_money. Always happy to accept a PR that makes it clearer! (I’m the author).

Introduction to Money

Money implements a set of functions to store, retrieve, convert and perform arithmetic
on a %Money{} type that is composed of an ISO 4217 currency code and a currency amount.

Money is opinionated in the interests of serving as a dependable library that can underpin accounting and financial applications.

How is this opinion expressed?

1. Money must always have both a amount and a currency code.

2. The currency code must always be a valid ISO4217 code. Current and historical currency codes can be used. See the ISO Currency for more information. You can also identify the relevant codes by:

   • Money.known_currencies/0 returns all the currency codes known to Money
   • Money.known_current_currencies/0 returns the currency codes currently in use
   • Money.known_historic_currencies/0 returns the list of historic currency codes
   • Money.known_tender_currencies/0 returns the list of currencies known to be legal tender

3. Money arithmetic can only be performed when both operands are of the same currency.

4. Money amounts are represented as a Decimal.

5. Money can be serialised to the database as a composite Postgres type that includes both the amount and the currency. For MySQL, money is serialized into a json column with the amount converted to a string to preserve precision since json does not have a decimal type. Serialization is entirely optional.

6. All arithmetic functions work on a Decimal. No rounding occurs automatically (unless expressly called out for a function, as is the case for Money.split/2).

7. Explicit rounding obeys the rounding rules for a given currency. The rounding rules are defined by the Unicode consortium in its CLDR repository as implemented by the hex package ex_cldr. These rules define the number of fractional digits for a currency and the rounding increment where appropriate.

8. Money output string formatting output using the hex package ex_cldr that correctly rounds to the appropriate number of fractional digits and to the correct rounding increment for currencies that have minimum cash increments (like the Swiss Franc and Australian Dollar)

My take is that the :money library is mostly providing a datatype, while :ex_money is a toolkit of dealing with money, especially in an internationalized fashion.

Some of those factors:

• Available currencies
  In :money the list of currencies is static/hardcoded and you can only add additional ones. :ex_money depends on :cldr for currencies instead of maintaining a list on it’s own and also has means for adding additional ones.
• Rounding rules (a.k.a. business rules of currencies)
  With :cldr in the back :ex_cldr does not only know the number of decimal places a currency uses, but also about things like alternate rounding rules, like when cash is supposed to round differently than e.g. accounting.
• Formatting
  :money does only support a single set of default formatting or customized per API call. :ex_money again uses :cldr to adjust formatting to settings matching a certain locale (like the users’).

So all in all it’s :ex_money for me if only for the fact of harnessing the knowledge of a unicode maintained database of those mentioned things as opposed to a library maintainer having a (likely worse) maintained copy of it.

I think the only thing lacking is for each library to make some reference to the other.

For example…

:money could have this in its README

Looking for something more comprehensive and globally applicable? Look at :ex_money

and :ex_money could have this in its README

Looking for something simple and lighter without any :cldr deps? Look at :money

something like that?