[thnk 4 the previous answers !] Good [morning, afternoon, night], I try to better understand some things... maybe you can help me. Id' like to know what are the pros and cons of (not) having static typing. Same question for (direct support of) side effects. To help you to find answers, here I quote this page : http://paulgraham.com/lispfaq1.html [Most hackers I know have been disappointed by the ML family. Languages with static typing would be more suitable if programs were something you thought of in advance, and then merely translated into code. But that's not how programs get written. The inability to have lists of mixed types is a particularly crippling restriction. It gets in the way of exploratory programming (it's convenient early on to represent everything as lists), and it means you can't have real macros.] Thanks a lot, minh thu