scheme.rb

def _eval(exp)
  if not list?(exp)
    if immediate_val?(exp)
      exp
    else
      lookup_primitive_fun(exp)
    end
  else
    fun = _eval(car(exp))
    args = eval_list(cdr(exp))
    apply(fun, args)
  end
end

def list?(exp)
  exp.is_a? Array
end

def lookup_primitive_fun(exp)
  $primitive_fun_env[exp]
end

$primitive_fun_env = {
  :+ => [:prim, lambda{|x,y| x + y }],
  :- => [:prim, lambda{|x,y| x - y }],
  :* => [:prim, lambda{|x,y| x * y }],
}

def car(list)
  list[0]
end

def cdr(list)
  list[1..-1]
end

def eval_list(exp)
  exp.map {|e| _eval(e)}
end

def immediate_val?(exp)
  num?(exp)
end

def num?(exp)
  exp.is_a? Numeric
end

def apply(fun, args)
  apply_primitive_fun(fun, args)
end

def apply_primitive_fun(fun, args)
  fun_val = fun[1]
  fun_val.call(*args)
end

# puts _eval([:+, 1, 2])
puts _eval([:+, [:+, 1, 2], 3])