diff --git a/builtins.go b/builtins.go
index 6af28d4..fac4261 100644
--- a/builtins.go
+++ b/builtins.go
@@ -321,20 +321,26 @@ func (b *Builtins) Rot(s *Stack) func() error {
 }
 
 // Words outputs a list of all known words in the dictionary
-func (b *Builtins) Words(d Dictionary) func() error {
+func (b *Builtins) Words(out io.Writer, d Dictionary) func() error {
+	if out == nil {
+		out = os.Stdout
+	}
 	return func() error {
 		for n := range d {
-			fmt.Printf("%s ", n)
+			fmt.Fprintf(out, "%s ", n)
 		}
 		return nil
 	}
 }
 
 // Flags outputs a list of all flags
-func (b *Builtins) Flags(c Context) func() error {
+func (b *Builtins) Flags(out io.Writer, c Context) func() error {
+	if out == nil {
+		out = os.Stdout
+	}
 	return func() error {
 		for n := range c.Flags {
-			fmt.Printf("%s %v\n", n, c.Flags.GetFlag(n))
+			fmt.Fprintf(out, "%s %v\n", n, c.Flags.GetFlag(n))
 		}
 		return nil
 	}
@@ -529,9 +535,12 @@ func (b *Builtins) Quit() func() error {
 }
 
 // Debug prints the stack without modifying it
-func (b *Builtins) Debug(s *Stack) func() error {
+func (b *Builtins) Debug(out io.Writer, s *Stack) func() error {
+	if out == nil {
+		out = os.Stdout
+	}
 	return func() error {
-		fmt.Print(s.values, " ")
+		fmt.Fprint(out, s.values, " ")
 		return nil
 	}
 }
diff --git a/main.go b/main.go
index 3ebea2b..ecec366 100644
--- a/main.go
+++ b/main.go
@@ -60,11 +60,11 @@ func main() {
 	dict.AddWord("DROP", Word{Name: "DROP", Impl: b.Drop(&stack)})
 	dict.AddWord("ROT", Word{Name: "ROT", Impl: b.Rot(&stack)})
 	// debugging
-	dict.AddWord("WORDS", Word{Name: "WORDS", Impl: b.Words(dict)})
-	dict.AddWord("FLAGS", Word{Name: "FLAGS", Impl: b.Flags(c)})
-	dict.AddWord(".S", Word{Name: ".S", Impl: b.Debug(&stack)})
-	dict.AddWord(".R", Word{Name: ".R", Impl: b.Debug(&rstack)})
-	dict.AddWord(".I", Word{Name: ".I", Impl: b.Debug(&ifstack)})
+	dict.AddWord("WORDS", Word{Name: "WORDS", Impl: b.Words(os.Stdout, dict)})
+	dict.AddWord("FLAGS", Word{Name: "FLAGS", Impl: b.Flags(os.Stdout, c)})
+	dict.AddWord(".S", Word{Name: ".S", Impl: b.Debug(os.Stdout, &stack)})
+	dict.AddWord(".R", Word{Name: ".R", Impl: b.Debug(os.Stdout, &rstack)})
+	dict.AddWord(".I", Word{Name: ".I", Impl: b.Debug(os.Stdout, &ifstack)})
 	dict.AddWord("R>", Word{Name: "R>", Impl: b.RFrom(&stack, &rstack)})
 	dict.AddWord(">R", Word{Name: ">R", Impl: b.ToR(&stack, &rstack)})
 	dict.AddWord("R@", Word{Name: "R@", Impl: b.RFetch(&stack, &rstack)})