golint-sl

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interfaceconsistency

About 325 words

Ensures a package's components depend on each other through interfaces and get their dependencies injected.

Category

Testability

What It Checks

This analyzer reports two things:

  1. Concrete dependency fields - a struct field whose name contains a dependency word (Client, Service, Repository, Store, Provider, Handler, Resolver, Middleware) and whose type is a pointer to a concrete type declared in the same package. Each field is reported once, however many of the words its name contains.
  2. Dependencies created in place - a call to a New* function whose name contains one of those words, made from a function that is not itself a constructor.

It does not report:

  • Fields with a json tag; those are data (DTOs, CRDs, API types), not dependencies
  • Pointers to other packages' types, such as *http.Client or *sql.DB
  • Constructors (New* functions), whatever they return
  • Test files, main packages (the composition root) and NewTest* helpers

Why It Matters

A component that holds its collaborators as concrete types, or builds them itself, can't be tested without them. Depending on interfaces and receiving the implementations from the caller lets tests substitute fakes.

Constructors are a different matter: "accept interfaces, return structs" applies, so NewStore returning *store is fine even when the package declares a Store interface. See returninterface.

Examples

Bad: Concrete Dependency Field

type userRepository struct{ db *sql.DB }

type UserService struct {
    Repository *userRepository // reported
}

Good: Interface Field

type UserRepository interface {
    Get(id string) (*User, error)
}

type UserService struct {
    Repository UserRepository
}

Bad: Creating a Dependency in Place

func (s *Server) routes() {
    client := NewPaymentClient(s.cfg) // reported
    s.mux.Handle("/pay", payHandler(client))
}

Good: Injecting It

func NewServer(cfg Config, payments PaymentClient) *Server {
    return &Server{cfg: cfg, payments: payments}
}

Configuration

# .golint-sl.yaml
analyzers:
  interfaceconsistency: true  # enabled by default

When to Disable

  • Projects with minimal interface usage
analyzers:
  interfaceconsistency: false