From aeffe6480486f1bc0efc417eeb294f0386250dbd Mon Sep 17 00:00:00 2001
From: "M.V. Hutz" <git@maximhutz.me>
Date: Fri, 16 Jan 2026 18:56:03 -0500
Subject: [PATCH] fix: update main and tests for new Reducer API

Update call sites to match the new Reducer interface where the expression
is passed to the constructor and Reduce() takes no arguments.
---
 cmd/lambda/lambda.go      |  6 +++---
 cmd/lambda/lambda_test.go |  4 ++--
 pkg/lambda/reducer.go     | 37 +++++++++++++++++--------------------
 pkg/reducer/reducer.go    |  2 +-
 4 files changed, 23 insertions(+), 26 deletions(-)

diff --git a/cmd/lambda/lambda.go b/cmd/lambda/lambda.go
index 3d8570f..999e722 100644
--- a/cmd/lambda/lambda.go
+++ b/cmd/lambda/lambda.go
@@ -33,8 +33,8 @@ func main() {
 	compiled := convert.SaccharineToLambda(ast)
 	logger.Info("compiled λ expression", "tree", compiled.String())
 
-	// Create reducer.
-	reducer := lambda.NewNormalOrderReducer()
+	// Create reducer with the compiled expression.
+	reducer := lambda.NewNormalOrderReducer(compiled)
 
 	// If the user selected to track CPU performance, attach a profiler.
 	if options.Profile != "" {
@@ -58,7 +58,7 @@ func main() {
 	}
 
 	// Run reduction.
-	reducer.Reduce(compiled)
+	reducer.Reduce()
 
 	// Return the final reduced result.
 	result := reducer.Expression().String()
diff --git a/cmd/lambda/lambda_test.go b/cmd/lambda/lambda_test.go
index 3855fc5..73bdb85 100644
--- a/cmd/lambda/lambda_test.go
+++ b/cmd/lambda/lambda_test.go
@@ -30,8 +30,8 @@ func runSample(samplePath string) (string, error) {
 	compiled := convert.SaccharineToLambda(ast)
 
 	// Create and run the reducer.
-	reducer := lambda.NewNormalOrderReducer()
-	reducer.Reduce(compiled)
+	reducer := lambda.NewNormalOrderReducer(compiled)
+	reducer.Reduce()
 
 	return reducer.Expression().String() + "\n", nil
 }
diff --git a/pkg/lambda/reducer.go b/pkg/lambda/reducer.go
index 2fe0d80..4c5ea19 100644
--- a/pkg/lambda/reducer.go
+++ b/pkg/lambda/reducer.go
@@ -6,20 +6,18 @@ import (
 	"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 {
 	emitter.BaseEmitter[reducer.Event]
-	expression expr.Expression
+	expression Expression
 }
 
 // NewNormalOrderReducer creates a new normal order reducer.
-func NewNormalOrderReducer() *NormalOrderReducer {
+func NewNormalOrderReducer(expression Expression) *NormalOrderReducer {
 	return &NormalOrderReducer{
 		BaseEmitter: *emitter.New[reducer.Event](),
+		expression:  expression,
 	}
 }
 
@@ -30,23 +28,22 @@ func (r *NormalOrderReducer) Expression() expr.Expression {
 
 // 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) expr.Expression {
-	r.expression = e
+func (r *NormalOrderReducer) Reduce() {
+	r.Emit(reducer.StartEvent)
+	it := NewIterator(&r.expression)
 
-	lambdaExpr, ok := e.(Expression)
-	if !ok {
-		return e
+	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()
+			}
+		}
 	}
 
-	r.Emit(reducer.StartEvent)
-
-	ReduceAll(&lambdaExpr, func() {
-		r.expression = lambdaExpr
-		r.Emit(reducer.StepEvent)
-	})
-
-	r.expression = lambdaExpr
 	r.Emit(reducer.StopEvent)
-
-	return lambdaExpr
 }
diff --git a/pkg/reducer/reducer.go b/pkg/reducer/reducer.go
index ec2d02a..114691c 100644
--- a/pkg/reducer/reducer.go
+++ b/pkg/reducer/reducer.go
@@ -20,7 +20,7 @@ type Reducer interface {
 	// Emits StartEvent before reduction, StepEvent after each step, and
 	// StopEvent after completion.
 	// Returns the final reduced expression.
-	Reduce(e expr.Expression) expr.Expression
+	Reduce()
 
 	// Expression returns the current expression state.
 	Expression() expr.Expression