Define interfaces in the consumer package, narrow to what's used

byob-interfaces.1 interfaces

Problem: commands that depend on concrete types can't be tested without mocking deep internals, and they can't be extended to a second backend without editing the command.

Idea: define interfaces in the consumer package, narrow to what the consumer actually uses. Concrete implementations live elsewhere and satisfy the interface structurally. "Accept interfaces, return structs" — at every package seam that matters.

Common seams:

Store — commands use List, Get, Save; implementations are sqlite, postgres, in-memory.
Prompter — commands use Confirm, Select; implementations are live (stdin-reading) and fake (scripted replies).
Backend — commands use Create, Destroy; implementations are libvirt, proxmox, ...

Tradeoffs: slightly more types. Huge payoff the first time you add a second backend or write a test that needs a fake.

Design

// pkg/cmd/list/list.go
type listStore interface {
    ListItems(ctx context.Context) ([]Item, error)
}

type Options struct {
    IO    *iostreams.IOStreams
    Store func() (listStore, error) // narrow!
}

// pkg/cmd/list/list_test.go
type fakeStore struct{ items []Item }
func (f *fakeStore) ListItems(context.Context) ([]Item, error) {
    return f.items, nil
}

// The production sqlite.Store satisfies both listStore and createStore
// structurally, without importing either cmd package.