Replace floating point numbers with a Money lib about gringotts HOT 5 CLOSED

We could work on @kipcole9's suggestion of implementing a protocol and accept any Money lib that implements it. But how do we go about doing that?
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Let's use ex_money or a protocol and track it's integration into Gringotts here.

You could do both. Use a protocol for the values sent to your public interface functions and convert those to money structs used internally.

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I think its important not to tie Gringotts to a single lib in any way. That would limit choices and potentially stifle innovation.

I support @LostKobrakai's suggestion. Protocol support at the public api level where money is being submitted to the gateway. For a return value from the gateway use your own Gingotts.Money.t type defined however you need as long as it has a currency code and a decimal amount. And make sure your Gingotts.Money.t type supports the Protocol :-)

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Context

• In #71, the protocol defines just 2 methods: value and currency.
• All money libs provide a to_string method that returns a string including the currency symbol/code.

Additions to the protocol

• float_string and integer_string (or similarly named) methods because most gateways accept a pair of "amount-as-string" and "currency code"; and though it's easy to extract and format the currency, converting the Decimal amount into a "precision aware string" is non trivial (rounding issues).
  • Interested libs could perhaps provide this by super-charging their to_string:

    # Notice the automatic rounding
    price = Money.new(4.1234, :USD)
    Money.to_string(price, currency: false) #=> "4.12"

    Such a to_string will make implementation of float_string trivially simple.
  • integer_string is useful for gateways that require amount as integer (like cents instead of dollars)
    Examples:

    Gringotts.Money.float_string(price) #=> "4.12"
    Gringotts.Money.integer_string(price) #=> "412"
    Money.new(4.1234, :BHD)
    |> Gringotts.Money.integer_string #=> "4123"
    # the Bahraini dinar is divided into 1000 fils unlike the dollar
    # which is divided in 100 cents

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Looks good. I believe that the integer output option should be a separate function. Following this discussion I have implemented, for ex_money two functions, to_integer_exp/1 and from_integer/2

to_integer/1

# current ex_money version
iex> m = Money.new(:USD, "200.012356")
#Money<:USD, 200.012356>

# Returns {iso4217, integer, exponent, remainder}
iex> Money.to_integer_exp(m)
{:USD, 20001, -2, Money.new(:USD, 0.002356)}

The reason that this should be separate from to_string/1 and should also be part of the protocol is because the money libs should take care of the relevant rounding - this is not as straight forward as it seems since different currencies have different rounding rules. My current version also returns a remainder since after rounding a remainder my exists. That is not Gringotts concern however so I propose the protocol function be to_integer(Money.t) :: {ISO4217 :: String.t, amount :: integer, exponent, integer}. I think the exponent should be included because again, I don't think Gringotts should need to have domain knowledge to calculate that but I think its important as an integrity check on the interface.

from_integer/1

iex> Money.from_integer(20000, :USD)
#Money<:USD, 200.00>

iex> Money.from_integer(200, :JPY)
#Money<:JPY, 200>

I think that could be the same API for Gringotts, being from_integer(amount :: integer, currency :: String.t). Note that is the money libraries responsibility to determine the exponent for the currency type.

Its possible that the payment gateway always assumes that the exponent is -2 - it would be good to know if that is the case. If it is, then the exponent would have to be passed in from Gringotts as a third parameter.

As soon as you settle on a draft protocol I'll implement it in ex_money master so you can try it out.

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Moving further progress to #85
A separate integer output function does seem better. We are not adding a from_integer right now.

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