Makefile

@@ -21,7 +21,7 @@ build:
 	chmod +x ${BINARY_NAME}
 
 run: build
-	./${BINARY_NAME} -f ./samples/$(TEST).txt -o program.out
+	./${BINARY_NAME} -s -f ./samples/$(TEST).txt -o program.out
 
 profile: build
 	./${BINARY_NAME} -p profile/cpu.prof -f ./samples/$(TEST).txt -o program.out

internal/engine/engine.go

@@ -24,9 +24,9 @@ func New(config *config.Config, expression *lambda.Expression) *Engine {
 func (e Engine) Run() {
 	e.Emit("start")
 
-	for lambda.ReduceOnce(e.Expression) {
+	lambda.ReduceAll(e.Expression, func() {
 		e.Emit("step")
-	}
+	})
 
 	e.Emit("end")
 }

pkg/fifo/fifo.go

@@ -1,35 +0,0 @@
-package fifo
-
-import "fmt"
-
-type FIFO[T any] []T
-
-func New[T any](items ...T) *FIFO[T] {
-	f := FIFO[T](items)
-	return &f
-}
-
-func (f *FIFO[T]) Push(item T) {
-	*f = append(*f, item)
-}
-
-func (f *FIFO[T]) Empty() bool {
-	return len(*f) == 0
-}
-
-func (f *FIFO[T]) MustPop() T {
-	var item T
-
-	*f, item = (*f)[:len(*f)-1], (*f)[len(*f)-1]
-	return item
-}
-
-func (f *FIFO[T]) Pop() (T, error) {
-	var item T
-
-	if f.Empty() {
-		return item, fmt.Errorf("stack is exhausted")
-	}
-
-	return f.MustPop(), nil
-}

pkg/lambda/reduce.go

@@ -1,9 +1,11 @@
 package lambda
 
-import "git.maximhutz.com/max/lambda/pkg/fifo"
+import (
+	"git.maximhutz.com/max/lambda/pkg/lifo"
+)
 
 func ReduceOnce(e *Expression) bool {
-	stack := fifo.New(e)
+	stack := lifo.New(e)
 
 	for !stack.Empty() {
 		top := stack.MustPop()
@@ -13,8 +15,7 @@ func ReduceOnce(e *Expression) bool {
 			stack.Push(&typed.body)
 		case *Application:
 			if fn, fnOk := typed.abstraction.(*Abstraction); fnOk {
-				reduced := Substitute(fn.body, fn.parameter, typed.argument)
+				*top = Substitute(fn.body, fn.parameter, typed.argument)
-				*top = reduced
 				return true
 			}
 
@@ -25,3 +26,35 @@ func ReduceOnce(e *Expression) bool {
 
 	return false
 }
+
+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.Current() != nil {
+		current := it.Current()
+
+		if fn, arg, ok := IsViable(current); !ok {
+			it.Next()
+		} else {
+			*current = Substitute(fn.body, fn.parameter, arg)
+			step()
+
+			if _, _, ok := IsViable(it.Parent()); ok {
+				it.Back()
+			} else {
+			}
+		}
+	}
+}

pkg/lifo/lifo.go

@@ -0,0 +1,35 @@
+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
+}

samples/church.txt

@@ -2,6 +2,6 @@
 inc n := \f x.(f (n f x))
 exp n m := (m n)
 
-N := (inc (inc (inc (inc (inc 0)))))
+N := (inc (inc (inc (inc (inc (inc 0))))))
 
 (exp N N)