Crystal interpreter: REPL and debugger like Ruby's Pry

To see it in action, Ary Borenszweig (the creator of Crystal) presented his prototype at the Crystal 1.0 Conference:

Full talk: https://www.youtube.com/watch?v=een_W1YEICw

REPL (starts at 1:06): https://youtu.be/een_W1YEICw?t=66 Pry-like debugger (starts at 9:36): https://youtu.be/een_W1YEICw?t=576