Auto-Generation of Sub-Menu Items (Keys) from a list of lists

[Nordlöw]

I am trying to automatically create a Sub-menu to "C++"-menu from the
list c++-iostream-objects in the following way:

(defvar c++-iostream-objects
  '(
    ("cin" [cin] "Standard input stream." 'object)
    ("cout" [cout] "Standard output stream." 'object)
    ("cerr" [cerr] "Standard output stream for errors." 'object)
    ("clog" [clog] "Standard output stream for logging." 'object)
    )
  "List of C++ IOstream Objects.")

(defun generate-keymap-menu (keymap-menu-name insert-prefix alist)
  (let ((m (make-sparse-keymap keymap-menu-name)))
    (dolist (elm (reverse alist))
      (let* ((str (concat insert-prefix (elt elm 0)))
             (key (elt elm 1))
             (doc (elt elm 2))
             )
        (define-key m key `(menu-item ,str '(lambda () (interactive)
(insert ,str)) :help ,doc))))
    m))

(defvar c++-iostream-objects-menu)
(setq c++-iostream-objects-menu
      (generate-keymap-menu "IOstream Objects" "std::" c++-iostream-
objects))

(add-hook 'c++-mode-hook
          '(lambda () (define-key-after c-c++-menu [iostream-objects]
                        (cons "IOstream Objects" c++-iostream-objects-
menu) t))
          t)

The value of c++-iostream-objects-menu after evaluating the above is:

(keymap
 (cin menu-item "std::cin"
      '(lambda nil
         (interactive)
         (insert "std::cin"))
      :help "Standard input stream.")
 (cout menu-item "std::cout"
       '(lambda nil
          (interactive)
          (insert "std::cout"))
       :help "Standard output stream.")
 (cerr menu-item "std::cerr"
       '(lambda nil
          (interactive)
          (insert "std::cerr"))
       :help "Standard output stream for errors.")
 (clog menu-item "std::clog"
       '(lambda nil
          (interactive)
          (insert "std::clog"))
       :help "Standard output stream for logging.")
 "IOstream Objects")

and the sub-menu elements gets created but when I press on one of them
I get the error:

Debugger entered--Lisp error: (wrong-type-argument commandp (quote
(lambda nil (interactive) (insert "std::cin"))))
  call-interactively((quote (lambda nil (interactive) (insert
"std::cin"))) nil nil)

This is really strange since the evaluation of

(call-interactively (quote (lambda nil (interactive) (insert
"std::cin"))))

works correctly and

(commandp (quote (lambda nil (interactive) (insert "std::cin"))))

evaluates to t.

What on earth have I missed? This nut is really hard to crack!

Many thanks in advance,
Nordlöw