Eval: Numbers & Symbols

The evaluator is the core. Take an AST node, return its value.

The evaluation rules:

• Numbers: return themselves.
• Symbols: look up in environment.
• Lists (calls): first eval the operator (a function), then args, then call.

Special forms (don't eval all args):

• if: only evaluate the chosen branch
• define: bind a symbol; don't evaluate the symbol itself
• quote: return the unevaluated expression
• lambda: create a function

Most operators are JUST functions in the environment. +, *, <, print — all pre-defined functions. The interpreter dispatches automatically.