ddd-ts-kit/ports-implements-never-extends

Architecture id: ports/implements-never-extends. Default severity: error in recommended, error in strict.

Applies to: every linted file.

Why

An adapter honours a contract; it does not inherit from it. implements lets the compiler check the adapter against the port and keeps the port free of behaviour, so that a fake, an in-memory class and a remote client are interchangeable. extends turns the port into a base class and ties every adapter to it.

Reported

• a class carrying @Repository(Port, ..), @Gateway(Port, ..) or @Adapter(Port, ..) that does not write implements Port, or that extends Port
• the class given to bind(Port).toClass(Class, ..) or rebind(Port).toClass(Class, ..) that does not implement Port, or that extends it
• under infrastructure/ of a context: a class that extends a class imported from a port file

Incorrect:

@Repository(InvoiceRepository)
export class KnexInvoiceRepository extends InvoiceRepository {
  constructor(private readonly database: Database) {}
}

builder.bind(PaymentGateway).toClass(StripePaymentGateway, [PaymentSettings]);
// where: export class StripePaymentGateway {}

Correct:

@Repository(InvoiceRepository)
export class KnexInvoiceRepository implements InvoiceRepository {
  constructor(private readonly database: Database) {}
}

export class StripePaymentGateway implements PaymentGateway {}

Details

The class given to toClass is looked up in the linted file, then in the file it is imported from by a relative path or the import alias: that file is read from disk and parsed without types. When it cannot be found (a class of a package, a barrel, a renaming export), nothing is reported. A class bound to itself (bind(Registry).toClass(Registry)) is no port and is not judged. Port files are domain/repositories/*.repository.ts and application/ports/*.port.ts.

Options

None.

rules."ports/implements-never-extends" in ddd-ts-kit.config.json set to off silences the rule, and // ddd-ts-kit-disable-next-line ports/implements-never-extends -- <reason> silences one line, exactly as for ddd-ts-kit check.

Limits

• implements is compared by name: implements kernel.Clock for a port named Clock is not recognised; import the port by name.
• An adapter that extends its port through an intermediate class of another file is not seen: the decorators refuse it when the file loads.

With the CLI checker

ddd-ts-kit check reports the same id from the text of the files. This rule is the authority for the code.