A lisp in Crystal.

Cues taken from:

• Lispy tutorial, building a lisp in Python
• Crisp, another lisp in Crystal

$ crystal src/lisp.cr -- run
Lisp REPL (cntl-d to quit):
>

• Learn Crystal ✓
• Implement a dynamic language ✓
• Do something with lisp for once ✓

• Implement a full-featured, useful lisp ✗

• Parse strings
• Build expressions
• Evaluate expressions in bindings
• Lazy eval (if)
• Modify current scope (define)
• Lists (quote)
• REPL
• Custom functions (lambda)
• Implement all those cool lisp-y list functions
• Follow Lispy2