Go Interface Design

Extension Contract (4 parts)

1. Interface (small, 1-4 methods)

type ChatModel interface {
    Generate(ctx context.Context, msgs []schema.Message, opts ...GenerateOption) (*schema.AIMessage, error)
    Stream(ctx context.Context, msgs []schema.Message, opts ...GenerateOption) iter.Seq2[schema.StreamChunk, error]
    BindTools(tools []tool.Tool) ChatModel
    ModelID() string
}

2. Hooks (all fields optional, nil = skip)

type Hooks struct {
    BeforeGenerate func(ctx context.Context, msgs []schema.Message) (context.Context, []schema.Message, error)
    AfterGenerate  func(ctx context.Context, resp *schema.AIMessage, err error) (*schema.AIMessage, error)
    OnError        func(ctx context.Context, err error) error
}
func ComposeHooks(hooks ...Hooks) Hooks { /* chain: each receives output of previous */ }

3. Middleware: func(T) T

type Middleware func(ChatModel) ChatModel
func ApplyMiddleware(model ChatModel, mws ...Middleware) ChatModel {
    for i := len(mws) - 1; i >= 0; i-- { model = mws[i](model) }
    return model
}

4. Registry

See go-framework skill.

Rules

• Accept interfaces, return structs.
• If interface > 4 methods, split it.
• Optional capabilities via type assertion: if br, ok := r.(BatchRetriever); ok { ... }
• Embed for composition: type MyAgent struct { agent.BaseAgent; ... }
• Hook naming: Before<Action> (modify input), After<Action> (modify output), On<Event> (observe/modify).
• Middleware applies first (outermost), hooks fire within execution.