function :: Parser Expr
function = do
  reserved "def"
  name <- identifier
  args <- parens $ many identifier
  body <- expr
  return $ Function name args body
function :: Parser Expr
function = do
  reserved "def"
  name <- identifier
  args <- parens $ many identifier
  body <- expr
  return $ Function name args body
function = do
  reserved "def"
  name <- identifier
  args <- parens $ many identifier
  body <- expr
  return $ Function name args body