From 588f4cd521a4f6ca283befc09ac3867db91bb55b Mon Sep 17 00:00:00 2001
From: "M.V. Hutz" <git@maximhutz.me>
Date: Mon, 12 Jan 2026 19:39:48 -0500
Subject: [PATCH] feat: added swap to iterator

---
 pkg/lambda/iterator.go | 35 ++++++++++++++++++++++-------------
 pkg/lambda/reduce.go   | 36 +++---------------------------------
 pkg/lifo/lifo.go       | 35 -----------------------------------
 3 files changed, 25 insertions(+), 81 deletions(-)
 delete mode 100644 pkg/lifo/lifo.go

diff --git a/pkg/lambda/iterator.go b/pkg/lambda/iterator.go
index 76b711a..31f82f9 100644
--- a/pkg/lambda/iterator.go
+++ b/pkg/lambda/iterator.go
@@ -5,13 +5,15 @@ type Iterator struct {
 }
 
 func NewIterator(expr *Expression) *Iterator {
-	return &Iterator{
-		trace: []*Expression{expr},
-	}
+	return &Iterator{[]*Expression{expr}}
+}
+
+func (i *Iterator) Done() bool {
+	return len(i.trace) == 0
 }
 
 func (i *Iterator) Current() *Expression {
-	if len(i.trace) < 1 {
+	if i.Done() {
 		return nil
 	}
 
@@ -26,6 +28,22 @@ func (i *Iterator) Parent() *Expression {
 	return i.trace[len(i.trace)-2]
 }
 
+func (i *Iterator) Swap(with Expression) {
+	current := i.Current()
+	if current != nil {
+		*current = with
+	}
+}
+
+func (i *Iterator) Back() bool {
+	if i.Done() {
+		return false
+	}
+
+	i.trace = i.trace[:len(i.trace)-1]
+	return true
+}
+
 func (i *Iterator) Next() {
 	switch typed := (*i.Current()).(type) {
 	case *Abstraction:
@@ -48,12 +66,3 @@ func (i *Iterator) Next() {
 		i.trace = []*Expression{}
 	}
 }
-
-func (i *Iterator) Back() bool {
-	if len(i.trace) == 0 {
-		return false
-	}
-
-	i.trace = i.trace[:len(i.trace)-1]
-	return true
-}
diff --git a/pkg/lambda/reduce.go b/pkg/lambda/reduce.go
index 8e56bd0..7bcf50c 100644
--- a/pkg/lambda/reduce.go
+++ b/pkg/lambda/reduce.go
@@ -1,32 +1,5 @@
 package lambda
 
-import (
-	"git.maximhutz.com/max/lambda/pkg/lifo"
-)
-
-func ReduceOnce(e *Expression) bool {
-	stack := lifo.New(e)
-
-	for !stack.Empty() {
-		top := stack.MustPop()
-
-		switch typed := (*top).(type) {
-		case *Abstraction:
-			stack.Push(&typed.body)
-		case *Application:
-			if fn, fnOk := typed.abstraction.(*Abstraction); fnOk {
-				*top = Substitute(fn.body, fn.parameter, typed.argument)
-				return true
-			}
-
-			stack.Push(&typed.argument)
-			stack.Push(&typed.abstraction)
-		}
-	}
-
-	return false
-}
-
 func IsViable(e *Expression) (*Abstraction, Expression, bool) {
 	if e == nil {
 		return nil, nil, false
@@ -42,18 +15,15 @@ func IsViable(e *Expression) (*Abstraction, Expression, bool) {
 func ReduceAll(e *Expression, step func()) {
 	it := NewIterator(e)
 
-	for it.Current() != nil {
-		current := it.Current()
-
-		if fn, arg, ok := IsViable(current); !ok {
+	for !it.Done() {
+		if fn, arg, ok := IsViable(it.Current()); !ok {
 			it.Next()
 		} else {
-			*current = Substitute(fn.body, fn.parameter, arg)
+			it.Swap(Substitute(fn.body, fn.parameter, arg))
 			step()
 
 			if _, _, ok := IsViable(it.Parent()); ok {
 				it.Back()
-			} else {
 			}
 		}
 	}
diff --git a/pkg/lifo/lifo.go b/pkg/lifo/lifo.go
deleted file mode 100644
index 6cc51d1..0000000
--- a/pkg/lifo/lifo.go
+++ /dev/null
@@ -1,35 +0,0 @@
-package lifo
-
-import "fmt"
-
-type LIFO[T any] []T
-
-func New[T any](items ...T) *LIFO[T] {
-	l := LIFO[T](items)
-	return &l
-}
-
-func (l *LIFO[T]) Push(item T) {
-	*l = append(*l, item)
-}
-
-func (l *LIFO[T]) Empty() bool {
-	return len(*l) == 0
-}
-
-func (l *LIFO[T]) MustPop() T {
-	var item T
-
-	*l, item = (*l)[:len(*l)-1], (*l)[len(*l)-1]
-	return item
-}
-
-func (l *LIFO[T]) Pop() (T, error) {
-	var item T
-
-	if l.Empty() {
-		return item, fmt.Errorf("stack is exhausted")
-	}
-
-	return l.MustPop(), nil
-}