Domain Primitives in Go: One Type Per Real-World Concept

The provided Go code defines a Money type that encapsulates both the amount in cents and its currency, ensuring that operations like addition only proceed if the currencies match. This approach prevents common pitfalls such as accidentally adding amounts in different currencies or losing precision due to floating-point arithmetic.

Here's an overview of the key components:

1. Currency Definition:

   • Currency is a string type representing supported currencies (e.g., USD, EUR).
   • The ParseCurrency function validates and returns the currency as a Currency type if it matches one of the predefined values (USD, EUR, or GBP). If not, an error is returned.

2. Money Type:

   • Money struct contains two fields: cents (an integer representing the amount in cents) and currency (a string representing the currency).
   • The NewMoney function initializes a new Money instance with given cents and currency, ensuring that both values are valid.

3. Operations:

   • Add: Adds two Money instances if they have the same currency.
   • IsNegative: Returns true if the amount is negative (i.e., less than zero).

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