Writing Emacs-function: how?

[Ulrich Scholz <d7@thispla.net>]

I try to write an Emacs function and bind it to a key.  My first try
is given below.  But I get the error
"Wrong type argument: commandp, my-replace"

I tried several versions but none worked.  Could you give me a correct
version. Thanks.

BTW, the function should replace all occurences of the string "<DF>"
by "ß" in the current buffer.

(defun my-replace nil "doc-string"
  (while (re-search-forward "<DF>" nil t)
    (replace-match "ß" nil nil)))

(global-set-key [f7] 'my-replace)