1. Refined criteria for gradual typing;Siek,2015
2. Gradual typing for objects;Siek,2007
3. Static typing where possible dynamic typing when needed: The end of the cold war between programming languages;Meijer,2004
4. J. Strachan, Groovy 2.0 release notes, http://groovy-lang.org/releasenotes/groovy-2.0.html (2014).
5. PHP Objects Patterns and Practice;Zandstra,2016