--- step3_env -----------------------------------
import types, reader, printer, env

READ(str): return reader.read_str(str)

eval_ast(ast,env):
  switch type(ast):
    symbol:      return env.get(ast)
    list,vector: return ast.map((x) -> EVAL(x,env))
    hash:        return ast.map((k,v) -> list(k, EVAL(v,env)))
    _default_:   return ast

EVAL(ast,env):
    if not list?(ast): return eval_ast(ast, env)
    if empty?(ast): return ast
    switch ast[0]:
      'def!:        return env.set(ast[1], EVAL(ast[2], env))
      'let*:        let_env = ...; return EVAL(ast[2], let_env)
      _default_:    f, args = eval_ast(ast, env)
                    return apply(f, args)

PRINT(exp): return printer.pr_str(exp)

repl_env = new Env()
rep(str): return PRINT(EVAL(READ(str),repl_env))

repl_env.set('+, add_fn)
    ...

main loop:
  try:      println(rep(readline("user> ")))
  catch e:  println("Error: ", e)

--- env module ----------------------------------
class Env (outer=null)
    data = hash_map()
  set(k,v): return data.set(k,v)
  find(k): return data.has(k) ? this : (if outer ? find(outer) : null)
  get(k): return data.find(k).get(k) OR raise "'" + k + "' not found"