From 7212f3d9f6c1e3af3b6b6dd0c628d4dd97afadef Mon Sep 17 00:00:00 2001
From: David Ashby <delta.mu.alpha@gmail.com>
Date: Sun, 14 Feb 2021 11:58:43 -0500
Subject: [PATCH] add return stack

---
 builtins.go | 37 +++++++++++++++++++++++++++++++++++++
 eval.go     |  1 +
 main.go     |  5 +++++
 3 files changed, 43 insertions(+)

diff --git a/builtins.go b/builtins.go
index 050a12b..5bf339c 100644
--- a/builtins.go
+++ b/builtins.go
@@ -184,6 +184,43 @@ func (b *Builtins) Emit(s *Stack) func() error {
 	}
 }
 
+// ToR pops from the stack to the return stack
+func (b *Builtins) ToR(s *Stack, r *Stack) func() error {
+	return func() error {
+		r1, err := s.Pop()
+		if err != nil {
+			return err
+		}
+		r.Push(r1)
+		return nil
+	}
+}
+
+// RFrom pops from the return stack to the stack
+func (b *Builtins) RFrom(s *Stack, r *Stack) func() error {
+	return func() error {
+		r1, err := r.Pop()
+		if err != nil {
+			return err
+		}
+		s.Push(r1)
+		return nil
+	}
+}
+
+// RFetch copies from the return stack to the stack
+func (b *Builtins) RFetch(s *Stack, r *Stack) func() error {
+	return func() error {
+		r1, err := r.Pop()
+		if err != nil {
+			return err
+		}
+		r.Push(r1)
+		s.Push(r1)
+		return nil
+	}
+}
+
 // CR prints a newline to standard out
 func (b *Builtins) CR() func() error {
 	return func() error {
diff --git a/eval.go b/eval.go
index a1d1ad7..915a9f9 100644
--- a/eval.go
+++ b/eval.go
@@ -10,6 +10,7 @@ import (
 type Context struct {
 	Dictionary Dictionary
 	Stack      *Stack
+	RStack     *Stack
 }
 
 // Eval evaulates a given line, recursively descending into given words as needed
diff --git a/main.go b/main.go
index ee97b85..42b5eeb 100644
--- a/main.go
+++ b/main.go
@@ -10,6 +10,7 @@ import (
 
 func main() {
 	stack := Stack{values: []int{}}
+	rstack := Stack{values: []int{}}
 
 	dict := Dictionary{}
 	b := &Builtins{}
@@ -26,12 +27,16 @@ func main() {
 	dict.AddWord("WORDS", b.Words(dict), "")
 	dict.AddWord(".S", b.Debug(&stack), "")
 	dict.AddWord("EMIT", b.Emit(&stack), "")
+	dict.AddWord("R>", b.RFrom(&stack, &rstack), "")
+	dict.AddWord(">R", b.ToR(&stack, &rstack), "")
+	dict.AddWord("R@", b.RFetch(&stack, &rstack), "")
 	dict.AddWord("CR", b.CR(), "")
 	dict.AddWord("QUIT", b.Quit(), "")
 
 	c := Context{
 		Dictionary: dict,
 		Stack:      &stack,
+		RStack:     &rstack,
 	}
 
 	reader := bufio.NewReader(os.Stdin)