See the following posts for more details:

http://www.jacoboakes.com/2015/09/crystal-vs-ruby-part-1-compilation-and.html

http://www.jacoboakes.com/2015/09/crystal-vs-ruby-part-2-typing-classes.html

http://www.jacoboakes.com/2015/09/crystal-vs-ruby-part-3-knapsack-problem.html