refactor: move event system to reducer, remove engine package (#32)

## Description

This PR completes the MVC-inspired refactoring by moving the event system from the engine into the reducer.
The engine package is now removed entirely, as the reducer handles both reduction logic and lifecycle events.

- Add `pkg/reducer/events.go` with `StartEvent`, `StepEvent`, and `StopEvent`.
- Extend `Reducer` interface to embed `Emitter[Event]` and add `Expression()` method.
- Update `NormalOrderReducer` to embed `BaseEmitter` and emit lifecycle events during reduction.
- Update all plugins to attach to `Reducer` instead of `Engine`.
- Remove `internal/engine` package entirely.
- Add `Off()` method to `BaseEmitter` to complete the `Emitter` interface.
- Fix `Emitter.On` signature to use generic type `E` instead of `string`.

### Decisions

- The `Reducer` interface now combines reduction logic with event emission, making it the single orchestration point.
- Plugins attach directly to the reducer, simplifying the architecture.
- The `Expression()` method on `Reducer` provides access to current state for plugins.

## Benefits

- Simpler architecture with one fewer abstraction layer.
- Plugins are now mode-agnostic - they work with any `Reducer` implementation.
- Cleaner separation: reducers handle reduction, plugins observe via events.
- Easier to add new evaluation modes - just implement `Reducer` with embedded emitter.

## Checklist

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

Reviewed-on: #32
Co-authored-by: M.V. Hutz <git@maximhutz.me>
Co-committed-by: M.V. Hutz <git@maximhutz.me>

pkg/emitter/emitter.go

@@ -3,7 +3,7 @@ package emitter
 import "git.maximhutz.com/max/lambda/pkg/set"
 
 type Emitter[E comparable] interface {
-	On(string, func()) Listener[E]
+	On(E, func()) Listener[E]
 	Off(Listener[E])
 	Emit(E)
 }
@@ -22,6 +22,13 @@ func (e *BaseEmitter[E]) On(kind E, fn func()) Listener[E] {
 	return listener
 }
 
+func (e *BaseEmitter[E]) Off(listener Listener[E]) {
+	kind := listener.Kind()
+	if e.listeners[kind] != nil {
+		e.listeners[kind].Remove(listener)
+	}
+}
+
 func (e *BaseEmitter[E]) Emit(event E) {
 	if e.listeners[event] == nil {
 		e.listeners[event] = set.New[Listener[E]]()

pkg/lambda/reduce.go

@@ -1,30 +0,0 @@
-package lambda
-
-func IsViable(e *Expression) (*Abstraction, Expression, bool) {
-	if e == nil {
-		return nil, nil, false
-	} else if app, appOk := (*e).(*Application); !appOk {
-		return nil, nil, false
-	} else if fn, fnOk := app.abstraction.(*Abstraction); !fnOk {
-		return nil, nil, false
-	} else {
-		return fn, app.argument, true
-	}
-}
-
-func ReduceAll(e *Expression, step func()) {
-	it := NewIterator(e)
-
-	for !it.Done() {
-		if fn, arg, ok := IsViable(it.Current()); !ok {
-			it.Next()
-		} else {
-			it.Swap(Substitute(fn.body, fn.parameter, arg))
-			step()
-
-			if _, _, ok := IsViable(it.Parent()); ok {
-				it.Back()
-			}
-		}
-	}
-}

pkg/lambda/reducer.go

@@ -1,33 +1,61 @@
 package lambda
 
 import (
+	"git.maximhutz.com/max/lambda/pkg/emitter"
 	"git.maximhutz.com/max/lambda/pkg/expr"
 	"git.maximhutz.com/max/lambda/pkg/reducer"
 )
 
-// Ensure NormalOrderReducer implements reducer.Reducer.
-var _ reducer.Reducer = (*NormalOrderReducer)(nil)
-
 // NormalOrderReducer implements normal order (leftmost-outermost) reduction
 // for lambda calculus expressions.
-type NormalOrderReducer struct{}
+type NormalOrderReducer struct {
+	emitter.BaseEmitter[reducer.Event]
+	expression *Expression
+}
 
 // NewNormalOrderReducer creates a new normal order reducer.
-func NewNormalOrderReducer() *NormalOrderReducer {
-	return &NormalOrderReducer{}
+func NewNormalOrderReducer(expression *Expression) *NormalOrderReducer {
+	return &NormalOrderReducer{
+		BaseEmitter: *emitter.New[reducer.Event](),
+		expression:  expression,
+	}
+}
+
+// Expression returns the current expression state.
+func (r *NormalOrderReducer) Expression() expr.Expression {
+	return *r.expression
+}
+
+func isViable(e *Expression) (*Abstraction, Expression, bool) {
+	if e == nil {
+		return nil, nil, false
+	} else if app, appOk := (*e).(*Application); !appOk {
+		return nil, nil, false
+	} else if fn, fnOk := app.abstraction.(*Abstraction); !fnOk {
+		return nil, nil, false
+	} else {
+		return fn, app.argument, true
+	}
 }
 
 // Reduce performs normal order reduction on a lambda expression.
 // The expression must be a lambda.Expression; other types are returned unchanged.
-func (r *NormalOrderReducer) Reduce(e expr.Expression, onStep func(expr.Expression)) expr.Expression {
-	lambdaExpr, ok := e.(Expression)
-	if !ok {
-		return e
+func (r *NormalOrderReducer) Reduce() {
+	r.Emit(reducer.StartEvent)
+	it := NewIterator(r.expression)
+
+	for !it.Done() {
+		if fn, arg, ok := isViable(it.Current()); !ok {
+			it.Next()
+		} else {
+			it.Swap(Substitute(fn.body, fn.parameter, arg))
+			r.Emit(reducer.StepEvent)
+
+			if _, _, ok := isViable(it.Parent()); ok {
+				it.Back()
+			}
+		}
 	}
 
-	ReduceAll(&lambdaExpr, func() {
-		onStep(lambdaExpr)
-	})
-
-	return lambdaExpr
+	r.Emit(reducer.StopEvent)
 }

pkg/reducer/events.go

@@ -0,0 +1,13 @@
+package reducer
+
+// Event represents lifecycle events during reduction.
+type Event int
+
+const (
+	// StartEvent is emitted before reduction begins.
+	StartEvent Event = iota
+	// StepEvent is emitted after each reduction step.
+	StepEvent
+	// StopEvent is emitted after reduction completes.
+	StopEvent
+)

pkg/reducer/reducer.go

@@ -2,14 +2,26 @@
 // reduction strategies.
 package reducer
 
-import "git.maximhutz.com/max/lambda/pkg/expr"
+import (
+	"git.maximhutz.com/max/lambda/pkg/emitter"
+	"git.maximhutz.com/max/lambda/pkg/expr"
+)
 
 // Reducer defines the interface for expression reduction strategies.
 // Different evaluation modes (normal order, applicative order, SKI combinators,
 // etc.) implement this interface with their own reduction logic.
+//
+// Reducers also implement the Emitter interface to allow plugins to observe
+// reduction lifecycle events (Start, Step, Stop).
 type Reducer interface {
-	// Reduce performs all reduction steps on the expression, calling onStep
-	// after each reduction.
+	emitter.Emitter[Event]
+
+	// Reduce performs all reduction steps on the expression.
+	// Emits StartEvent before reduction, StepEvent after each step, and
+	// StopEvent after completion.
 	// Returns the final reduced expression.
-	Reduce(e expr.Expression, onStep func(expr.Expression)) expr.Expression
+	Reduce()
+
+	// Expression returns the current expression state.
+	Expression() expr.Expression
 }