Fix slice next elem func

internal/util.go

@@ -28,20 +28,35 @@ func isLower(s string) bool {
 	return true
 }
 
-func SliceNextElem(v reflect.Value) reflect.Value {
-	if v.Len() < v.Cap() {
-		v.Set(v.Slice(0, v.Len()+1))
-		return v.Index(v.Len() - 1)
-	}
-
+func MakeSliceNextElemFunc(v reflect.Value) func() reflect.Value {
 	elemType := v.Type().Elem()
 
 	if elemType.Kind() == reflect.Ptr {
-		elem := reflect.New(elemType.Elem())
-		v.Set(reflect.Append(v, elem))
-		return elem.Elem()
+		elemType = elemType.Elem()
+		return func() reflect.Value {
+			if v.Len() < v.Cap() {
+				v.Set(v.Slice(0, v.Len()+1))
+				elem := v.Index(v.Len() - 1)
+				if elem.IsNil() {
+					elem.Set(reflect.New(elemType))
+				}
+				return elem.Elem()
+			}
+
+			elem := reflect.New(elemType)
+			v.Set(reflect.Append(v, elem))
+			return elem.Elem()
+		}
 	}
 
-	v.Set(reflect.Append(v, reflect.Zero(elemType)))
-	return v.Index(v.Len() - 1)
+	zero := reflect.Zero(elemType)
+	return func() reflect.Value {
+		if v.Len() < v.Cap() {
+			v.Set(v.Slice(0, v.Len()+1))
+			return v.Index(v.Len() - 1)
+		}
+
+		v.Set(reflect.Append(v, zero))
+		return v.Index(v.Len() - 1)
+	}
 }