currency

README

Currency

Defining what a "value" is in this application has made me overthink this more than I should have. Normally it should be simple: I expect to spend x amount in my local currency for this entry, and I create expenses for that in the local currency which I add up to compare with my original expectation. Simple enough.

But even if we approach the problem like this alone, the primary instinct would be to just store the values as floats, which is actually not recommended. Many sources like this one and this one advice against that. TLDR; the operations can be innacurate and you're not making use of all the available decimals. I started the app by storing the smallest unit of my country's RON currency (i.e. a value of 1 RON is stored as 100 in the database).

This was fine at start, but I quickly realised that I was also being paid in euros at that time, and some transactions were in euros. Again, the first instinct is to convert in my country's currency. But then I got all philosophical into thinking things like "what is a value, in fact" and "what if I have to use something whose minimum unit might change", and shit like that. I essentially wanted to generalize the meaning of value -- what if I wanted to make a payment in oil at some point? 😒...

I didn't really want to study currencies and their minor units to know which numbers to put in my database. I also didn't really want to store huge numbers in my database (for example, the smallest Bitcoin unit is 0.00000001 of a BTC) so I came up with the following system: I would store

• the currency (can be RON, USD, Bitcoin etc.)
• the value
• the exponent (such that value * exponent represents the value in the unit of that currency)

For example, by storing a value of 150 with currency RON and exponent -2, I'm actually storing 1.5 RON. This gives me enough flexibility and essentially allows me to not care at all about the minor unit, or whether if it unexpectedly changes.

The main purpose of this package is to provide a Value struct that encapsulates such an object defined previously, and provides a set of constructors in order to always instantiate only valid values of this object. The application will create values using these constructors, and the database will convert these values (which are always correct) to its internal representation of the value.

There are some things that I had considered (e.g. the unit of the currency changing, such as the revaluation of my country's currency) or storing the exchange rates at the day of the exchange to be able to accurately, reproduce and understand the history of my expenses, but I decided to quit pursuing these thoughts for the sake of my mental health. The real use case of this application is in the present and short-term past and future, in which case things like the exact exchange rate at the moment of the transaction doesn't matter.

Documentation

Index

• Constants
• func ValidateCurrency(currency string) error
• type ErrInvalidCurrency
  • func NewErrInvalidCurrency(attemptedCurrency string) ErrInvalidCurrency
  • func (e ErrInvalidCurrency) Error() string
• type Value
  • func NewEURValue(amount int) Value
  • func NewRONValue(amount int) Value
  • func NewUSDValue(amount int) Value
  • func NewValue(amount int, currency string, exp int) (Value, error)
  • func NewValueBasedOnMinorCurrency(amount int, currency string, exponent *int) (Value, error)

Constants

const (
	EUR = "EUR"
	RON = "RON"
	USD = "USD"
	BTC = "BTC"
)

ISO 4217 compliant currency codes

Functions

func ValidateCurrency

func ValidateCurrency(currency string) error

ValidateCurrency provides a way to test whether or not a string can represent a valid currency.

Types

type ErrInvalidCurrency

type ErrInvalidCurrency struct {
	// contains filtered or unexported fields
}

func NewErrInvalidCurrency

func NewErrInvalidCurrency(attemptedCurrency string) ErrInvalidCurrency

func (ErrInvalidCurrency) Error

func (e ErrInvalidCurrency) Error() string

type Value

type Value struct {
	Currency string
	Amount   int
	Exponent int
}

Value represents a currency value in a certain currency. The actual value shall be multiplied by "10eExp" to obtain the value in the base unit of the currency.

Value should only be created via the provided constructors, to prevent invalid "values".

E.g. a Value of 100 with Exp = -2 and USD currency is the equivalent of 1$.

func NewEURValue

func NewEURValue(amount int) Value

func NewRONValue

func NewRONValue(amount int) Value

func NewUSDValue

func NewUSDValue(amount int) Value

func NewValue

func NewValue(amount int, currency string, exp int) (Value, error)

func NewValueBasedOnMinorCurrency

func NewValueBasedOnMinorCurrency(amount int, currency string, exponent *int) (Value, error)