implement PICK on stacks to make the implementation of I less silly

builtins.go

@@ -237,16 +237,10 @@ func (b *Builtins) Swap(s *Stack) func() error {
 // Over duplicates NOS to TOS, resulting in NOS TOS NOS
 func (b *Builtins) Over(s *Stack) func() error {
 	return func() error {
-		tos, err := s.Pop()
+		nos, err := s.Pick(1)
 		if err != nil {
 			return err
 		}
-		nos, err := s.Pop()
-		if err != nil {
-			return err
-		}
-		s.Push(nos)
-		s.Push(tos)
 		s.Push(nos)
 		return nil
 	}
@@ -286,7 +280,7 @@ func (b *Builtins) Rot(s *Stack) func() error {
 func (b *Builtins) Words(d Dictionary) func() error {
 	return func() error {
 		for n := range d {
-			fmt.Printf("%s %s\n", n, d[n].Source)
+			fmt.Printf("%s ", n)
 		}
 		return nil
 	}
@@ -423,22 +417,11 @@ func (b *Builtins) Loop(s *Stack, r *Stack) func() error {
 // I puts the current value of the loop counter on the top of the stack
 func (b *Builtins) I(s *Stack, r *Stack) func() error {
 	return func() error {
-		tors, err := r.Pop()
-		if err != nil {
-			return err
-		}
-		nors, err := r.Pop()
-		if err != nil {
-			return err
-		}
-		counter, err := r.Pop()
+		counter, err := r.Pick(2)
 		if err != nil {
 			return err
 		}
 		s.Push(counter)
-		r.Push(counter)
-		r.Push(nors)
-		r.Push(tors)
 		return nil
 	}
 }

stack.go

@@ -21,3 +21,11 @@ func (s *Stack) Pop() (int, error) {
 func (s *Stack) Push(i int) {
 	s.values = append([]int{i}, s.values...)
 }
+
+// Pick grabs a value from the given depth in the stack, non-destructively.
+func (s *Stack) Pick(i int) (int, error) {
+	if len(s.values) < i {
+		return 0, fmt.Errorf("cannot pick value from beyond stack depth")
+	}
+	return s.values[i], nil
+}