Tell about DDD (Domain-Driven Design) and the functional paradigm — what have you read, how have you implemented?

Domain-Driven Design (DDD) is an approach to software development that focuses on a deep understanding of the subject area and building a model that reflects business logic. In DDD, concepts such as entities, aggregates, repositories, domain services, and domain events are distinguished. The goal is to make the code as close as possible to real business processes.

The functional paradigm in programming is based on the use of pure functions, immutability of data, and the absence of side effects. This helps create more predictable and testable code.

In Golang, although the language is not inherently functional, elements of functional style can be applied: using first-class functions, avoiding mutations, and applying function composition.

Implementing DDD with a functional approach, I tried to:

• Clearly separate layers (domain, infrastructure, application).
• Use immutable structures and pure functions for business logic in the domain.
• Manage state using aggregates with controlled changes.

An example of a simple aggregate in Go with a functional style:

 type Order struct {
     ID     string
     Status string
 }

 func (o Order) Pay() Order {
     if o.Status == "Pending" {
         return Order{ID: o.ID, Status: "Paid"}
     }
     return o
 }

Here, the Pay method does not mutate the original object but returns a new one with an updated state, which aligns with the functional approach.