lisp by ruby

gistfile1.rb

Global_env =
{
    '+'       => :+,
    '-'       => :-,
    '*'       => :*,
    '/'       => :/,
    'not'     => :not,
    '>'       => :>,
    '<'       => :<,
    '>='      => :>=,
    '<='      => :<=,
    '='       => :==,
    'equal?'  => :==,
    'eq?'     => :instance_of?,
    'length'  => :length,
    'cons'    => Proc.new { |x, xs| [x] + xs },
    'car'     => Proc.new { |xs| xs[0] },
    'cdr'     => Proc.new { |xs| xs[1..-1] },
    'append'  => :+,
    'list'    => Proc.new { |*xs| xs },
    'list?'   => Proc.new { |xs| xs.instance_of? Array },
    'null?'   => :empty?,
    'symbol?' => Proc.new { |x| x.is_a? String }
}

def eval(x, env = Global_env)
  if x.instance_of? String
    env[x]
  elsif not x.instance_of? Array
    x
  elsif x[0] == 'quote'
    _, exp = x
    exp
  elsif x[0] == 'if'
    _, test, conseq, alt = x
    eval(if eval(test, env) then conseq else alt end, env)
  elsif x[0] == 'set!'
    _, var, exp = x
    raise "undefined variable" unless env.key?(var)
    env[var] = eval(exp, env)
  elsif x[0] == 'define'
    _, var, exp = x
    env[var] = eval(exp, env)
  elsif x[0] == 'lambda'
    _, vars, exp = x
    Proc.new { |*args| eval(exp, Hash[vars.zip(args)].merge(env)) }
  elsif x[0] == 'begin'
    result = nil
    x[1..-1].each { |exp| result = eval(exp, env) }
    result
  else
    exps = x.map{ |exp| eval(exp, env) }
    fn = exps.shift
    if fn.instance_of? Proc
      fn.call(*exps)
    else
      op1 = exps.shift
      op1.method(fn).call(*exps)
    end
  end
end

def read(s)
  read_from tokenize(s)
end

def tokenize(s)
  s.gsub('(', ' ( ').gsub(')', ' ) ').split
end

def read_from(tokens)
  raise 'unexpected EOF while reading' if tokens.empty?
  token = tokens.shift
  if token == '('
    l = []
    while tokens[0] != ')'
      l.push read_from(tokens)
    end
    tokens.shift
    l
  elsif token == ')'
    raise 'unexpected'
  else
    atom(token)
  end
end

def atom(token)
  begin
    Integer(token)
  rescue
    begin
      Float(token)
    rescue
      token
    end
  end
end

class Array
  def to_s
    '(' + join(' ') + ')'
  end
end

def repl(prompt = 'lis.rb> ')
  while true
    print prompt
    val    = read(STDIN.gets)
    result = eval(val)
    print "#{result}\n"
  end
end

repl if __FILE__ == $0