Reconcile DecCoin/s with Coin/s

[alexanderbez]

Summary

Reconcile the DecCoin and DecCoins APIs with the Coin and Coins APIs for consistency.

e.g.

• Not allowing negative amounts
• Not returning zero amounts in plus
• Consistent constructors and naming conventions
• Improve godocs & test coverage
• etc...

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[rigelrozanski]

What did you have in mind? like testing through an interface? golang is kinda inhibiting for actually combining these two types unfortunately

[alexanderbez]

Yeah I wish we could keep it DRY as possible, but I mean just making sure the APIs are the same (e.g. plus doesn't make sure zero coins are removed).

[alessio]

How about defining these common interfaces?

type Coin interface {
	SameDenomAs(Coin) bool
	Zero() bool
	GTE(Coin) bool
	LT(Coin) bool
	Equal(Coin) bool
	Plus(Coin) Coin
	Minus(Coin) Coin
	Positive() bool
	Negative() bool
	String() string
}

type CoinSet interface {
	Valid() bool
	Plus(CoinSet) Coin
	Minus(CoinSet) Coin
	SafeMinus(CoinSet) Coin
	AllGT(CoinSet) bool
	AllGTE(CoinSet) bool
	AllLTE(CoinSet) bool
	Zero() bool
	Equal(CoinSet) bool
	Empty() bool
	AllPositive() bool
	AllNotNegative() bool
        AnyNegative() bool
	Sort() CoinSet
	String() string
}

Note that I have removed the Is prefix from all types - IMHO it does not add any value to the methods names.

[alessio]

We might need an Amount interface to allow Int/Dec comparisons