refactor: replace string-based emitter with type-safe generic event system (#28)

## Description

This PR refactors the event emitter system from a string-based message passing approach to a type-safe generic implementation using typed events.
The previous system relied on string message names which were error-prone and lacked compile-time safety.
This refactoring introduces a generic `BaseEmitter[E comparable]` that provides type safety while consolidating the various tracker packages into a unified plugins architecture.

Key changes:
- Replace `Emitter` with generic `BaseEmitter[E comparable]` for type-safe event handling.
- Add `Event` type enumeration with `StartEvent`, `StepEvent`, and `StopEvent` constants.
- Create `Listener[E]` interface with `BaseListener` implementation for better abstraction.
- Consolidate `explanation`, `performance`, and `statistics` packages into unified `internal/plugins` package.
- Simplify CLI initialization by using plugin constructors that handle their own event subscriptions.
- Add `Items()` iterator method to `Set` for idiomatic Go 1.23+ range loops over sets.

### Decisions

Use generics for type-safe event handling.
This provides compile-time guarantees that event types match their handlers while maintaining flexibility for future event types.

Consolidate trackers into plugins architecture.
Previously separate packages (`explanation`, `performance`, `statistics`) now live under `internal/plugins`, making the plugin pattern explicit and easier to extend.

Plugin constructors self-register with engine.
Each plugin's `New*` constructor now handles its own event subscriptions, reducing boilerplate in the main CLI.

## Benefits

Type safety prevents runtime errors from typos in event names.
The compiler now catches mismatched event types at compile time rather than failing silently at runtime.

Cleaner plugin architecture makes adding new features easier.
New plugins follow a consistent pattern and live in a single location.

Reduced boilerplate in main CLI.
Plugin initialization is now a single function call rather than manual event registration.

Better testability through interface-based design.
The `Listener[E]` interface allows for easier mocking and testing of event handlers.

## Checklist

- [x] Code follows conventional commit format.
- [x] Branch follows naming convention (`<type>/<description>`).
- [x] Tests pass (if applicable).
- [x] Documentation updated (if applicable).

Reviewed-on: #28
Co-authored-by: M.V. Hutz <git@maximhutz.me>
Co-committed-by: M.V. Hutz <git@maximhutz.me>

cmd/lambda/lambda.go

@@ -6,9 +6,7 @@ import (
 	"git.maximhutz.com/max/lambda/internal/cli"
 	"git.maximhutz.com/max/lambda/internal/config"
 	"git.maximhutz.com/max/lambda/internal/engine"
-	"git.maximhutz.com/max/lambda/internal/explanation"
-	"git.maximhutz.com/max/lambda/internal/performance"
-	"git.maximhutz.com/max/lambda/internal/statistics"
+	"git.maximhutz.com/max/lambda/internal/plugins"
 	"git.maximhutz.com/max/lambda/pkg/convert"
 	"git.maximhutz.com/max/lambda/pkg/lambda"
 	"git.maximhutz.com/max/lambda/pkg/saccharine"
@@ -42,30 +40,23 @@ func main() {
 	// If the user selected to track CPU performance, attach a profiler to the
 	// process.
 	if options.Profile != "" {
-		profiler := performance.Track(options.Profile)
-		process.On("start", profiler.Start)
-		process.On("end", profiler.End)
+		plugins.NewPerformance(options.Profile, process)
 	}
 
 	// If the user selected to produce a step-by-step explanation, attach an
 	// observer here.
 	if options.Explanation {
-		explanation.Track(process)
+		plugins.NewExplanation(process)
 	}
 
 	// If the user opted to track statistics, attach a tracker here, too.
 	if options.Statistics {
-		statistics := statistics.Track()
-		process.On("start", statistics.Start)
-		process.On("step", statistics.Step)
-		process.On("end", statistics.End)
+		plugins.NewStatistics(process)
 	}
 
 	// If the user selected for verbose debug logs, attach a reduction tracker.
 	if options.Verbose {
-		process.On("step", func() {
-			logger.Info("reduction", "tree", lambda.Stringify(compiled))
-		})
+		plugins.NewLogs(logger, process)
 	}
 
 	process.Run()

internal/engine/engine.go

@@ -12,21 +12,25 @@ import (
 type Engine struct {
 	Config     *config.Config
 	Expression *lambda.Expression
-	emitter.Emitter
+	emitter.BaseEmitter[Event]
 }
 
 // Create a new engine, given an unreduced λ-expression.
 func New(config *config.Config, expression *lambda.Expression) *Engine {
-	return &Engine{Config: config, Expression: expression}
+	return &Engine{
+		Config:      config,
+		Expression:  expression,
+		BaseEmitter: *emitter.New[Event](),
+	}
 }
 
 // Begin the reduction process.
 func (e Engine) Run() {
-	e.Emit("start")
+	e.Emit(StartEvent)
 
 	lambda.ReduceAll(e.Expression, func() {
-		e.Emit("step")
+		e.Emit(StepEvent)
 	})
 
-	e.Emit("end")
+	e.Emit(StopEvent)
 }

internal/engine/events.go

@@ -0,0 +1,9 @@
+package engine
+
+type Event int
+
+const (
+	StartEvent Event = iota
+	StepEvent
+	StopEvent
+)

internal/plugins/debug.go

@@ -0,0 +1,25 @@
+package plugins
+
+import (
+	"log/slog"
+
+	"git.maximhutz.com/max/lambda/internal/engine"
+	"git.maximhutz.com/max/lambda/pkg/lambda"
+)
+
+type Logs struct {
+	logger  *slog.Logger
+	process *engine.Engine
+}
+
+func NewLogs(logger *slog.Logger, process *engine.Engine) *Logs {
+	plugin := &Logs{logger, process}
+	process.On(engine.StopEvent, plugin.Step)
+
+	return plugin
+}
+
+func (t *Logs) Step() {
+	stringified := lambda.Stringify(*t.process.Expression)
+	t.logger.Info("reduction", "tree", stringified)
+}

internal/plugins/explanation.go

@@ -1,6 +1,6 @@
 // Package "explanation" provides a observer to gather the reasoning during the
 // reduction, and present a thorough explanation to the user for each step.
-package explanation
+package plugins
 
 import (
 	"fmt"
@@ -10,23 +10,23 @@ import (
 )
 
 // Track the reductions made by a reduction proess.
-type Tracker struct {
+type Explanation struct {
 	process *engine.Engine
 }
 
 // Attaches a new explanation tracker to a process.
-func Track(process *engine.Engine) *Tracker {
-	tracker := &Tracker{process: process}
-	process.On("start", tracker.Start)
-	process.On("step", tracker.Step)
+func NewExplanation(process *engine.Engine) *Explanation {
+	plugin := &Explanation{process: process}
+	process.On(engine.StartEvent, plugin.Start)
+	process.On(engine.StepEvent, plugin.Step)
 
-	return tracker
+	return plugin
 }
 
-func (t *Tracker) Start() {
+func (t *Explanation) Start() {
 	fmt.Println(lambda.Stringify(*t.process.Expression))
 }
 
-func (t *Tracker) Step() {
+func (t *Explanation) Step() {
 	fmt.Println("    =", lambda.Stringify(*t.process.Expression))
 }

internal/plugins/performance.go

@@ -1,28 +1,34 @@
 // Package "performance" provides a tracker to observer CPU performance during
 // execution.
-package performance
+package plugins
 
 import (
 	"os"
 	"path/filepath"
 	"runtime/pprof"
+
+	"git.maximhutz.com/max/lambda/internal/engine"
 )
 
 // Observes a reduction process, and publishes a CPU performance profile on
 // completion.
-type Tracker struct {
+type Performance struct {
 	File        string
 	filePointer *os.File
 	Error       error
 }
 
 // Create a performance tracker that outputs a profile to "file".
-func Track(file string) *Tracker {
-	return &Tracker{File: file}
+func NewPerformance(file string, process *engine.Engine) *Performance {
+	plugin := &Performance{File: file}
+	process.On(engine.StartEvent, plugin.Start)
+	process.On(engine.StopEvent, plugin.Stop)
+
+	return plugin
 }
 
 // Begin profiling.
-func (t *Tracker) Start() {
+func (t *Performance) Start() {
 	var absPath string
 
 	absPath, t.Error = filepath.Abs(t.File)
@@ -47,7 +53,7 @@ func (t *Tracker) Start() {
 }
 
 // Stop profiling.
-func (t *Tracker) End() {
+func (t *Performance) Stop() {
 	pprof.StopCPUProfile()
 	t.filePointer.Close()
 }

internal/plugins/statistics.go

@@ -0,0 +1,44 @@
+package plugins
+
+import (
+	"fmt"
+	"os"
+	"time"
+
+	"git.maximhutz.com/max/lambda/internal/engine"
+	"git.maximhutz.com/max/lambda/internal/statistics"
+)
+
+// An observer, to track reduction performance.
+type Statistics struct {
+	start time.Time
+	steps uint64
+}
+
+// Create a new reduction performance Statistics.
+func NewStatistics(process *engine.Engine) *Statistics {
+	plugin := &Statistics{}
+	process.On(engine.StartEvent, plugin.Start)
+	process.On(engine.StepEvent, plugin.Step)
+	process.On(engine.StopEvent, plugin.Step)
+
+	return plugin
+}
+
+func (t *Statistics) Start() {
+	t.start = time.Now()
+	t.steps = 0
+}
+
+func (t *Statistics) Step() {
+	t.steps++
+}
+
+func (t *Statistics) Stop() {
+	results := statistics.Results{
+		StepsTaken:  t.steps,
+		TimeElapsed: uint64(time.Since(t.start).Milliseconds()),
+	}
+
+	fmt.Fprint(os.Stderr, results.String())
+}

internal/statistics/tracker.go

@@ -1,36 +0,0 @@
-package statistics
-
-import (
-	"fmt"
-	"os"
-	"time"
-)
-
-// An observer, to track reduction performance.
-type Tracker struct {
-	start time.Time
-	steps uint64
-}
-
-// Create a new reduction performance tracker.
-func Track() *Tracker {
-	return &Tracker{}
-}
-
-func (t *Tracker) Start() {
-	t.start = time.Now()
-	t.steps = 0
-}
-
-func (t *Tracker) Step() {
-	t.steps++
-}
-
-func (t *Tracker) End() {
-	results := Results{
-		StepsTaken:  t.steps,
-		TimeElapsed: uint64(time.Since(t.start).Milliseconds()),
-	}
-
-	fmt.Fprint(os.Stderr, results.String())
-}

pkg/emitter/emitter.go

@@ -2,53 +2,38 @@ package emitter
 
 import "git.maximhutz.com/max/lambda/pkg/set"
 
-type Observer struct {
-	fn      func()
-	message string
-	emitter *Emitter
+type Emitter[E comparable] interface {
+	On(string, func()) Listener[E]
+	Off(Listener[E])
+	Emit(E)
 }
 
-type Emitter struct {
-	listeners map[string]*set.Set[*Observer]
+type BaseEmitter[E comparable] struct {
+	listeners map[E]*set.Set[Listener[E]]
 }
 
-func Ignore[T any](fn func()) func(T) {
-	return func(T) { fn() }
+func (e *BaseEmitter[E]) On(kind E, fn func()) Listener[E] {
+	if e.listeners[kind] == nil {
+		e.listeners[kind] = set.New[Listener[E]]()
 	}
 
-func (e *Emitter) On(message string, fn func()) *Observer {
-	observer := &Observer{
-		fn:      fn,
-		message: message,
-		emitter: e,
+	listener := &BaseListener[E]{kind, fn}
+	e.listeners[kind].Add(listener)
+	return listener
 }
 
-	if e.listeners == nil {
-		e.listeners = map[string]*set.Set[*Observer]{}
+func (e *BaseEmitter[E]) Emit(event E) {
+	if e.listeners[event] == nil {
+		e.listeners[event] = set.New[Listener[E]]()
 	}
 
-	if e.listeners[message] == nil {
-		e.listeners[message] = set.New[*Observer]()
+	for listener := range e.listeners[event].Items() {
+		listener.Run()
+	}
 }
 
-	e.listeners[message].Add(observer)
-	return observer
-}
-
-func (o *Observer) Off() {
-	if o.emitter.listeners[o.message] == nil {
-		return
-	}
-
-	o.emitter.listeners[o.message].Remove(o)
-}
-
-func (e *Emitter) Emit(message string) {
-	if e.listeners[message] == nil {
-		return
-	}
-
-	for listener := range *e.listeners[message] {
-		listener.fn()
+func New[E comparable]() *BaseEmitter[E] {
+	return &BaseEmitter[E]{
+		listeners: map[E]*set.Set[Listener[E]]{},
 	}
 }

pkg/emitter/listener.go

@@ -0,0 +1,19 @@
+package emitter
+
+type Listener[E comparable] interface {
+	Kind() E
+	Run()
+}
+
+type BaseListener[E comparable] struct {
+	kind E
+	fn   func()
+}
+
+func (l BaseListener[E]) Kind() E {
+	return l.kind
+}
+
+func (l BaseListener[E]) Run() {
+	l.fn()
+}

pkg/set/set.go

@@ -1,5 +1,7 @@
 package set
 
+import "iter"
+
 type Set[T comparable] map[T]bool
 
 func (s *Set[T]) Add(items ...T) {
@@ -34,6 +36,16 @@ func (s Set[T]) ToList() []T {
 	return list
 }
 
+func (s Set[T]) Items() iter.Seq[T] {
+	return func(yield func(T) bool) {
+		for item := range s {
+			if !yield(item) {
+				return
+			}
+		}
+	}
+}
+
 func New[T comparable](items ...T) *Set[T] {
 	result := &Set[T]{}