slime's cool feature

in REPL,when I input "(" and a function name,and then,a space,something happens!slime will prompt the expected arguments in emacs's minibuffer,it's cool!and of course very useful.but,I found that when editing a common lisp source code file,slime does nothing when I do exactly as in REPL,it's strange.so I compared the two modes,at last it turned out that the slime function "slime-indent-and-complete-symbol" does the work I want.so I redefined the "TAB" key-binding like this:
(define-key slime-mode-map (kbd "TAB") 'slime-indent-and-complete-symbol)
everything is perfect now,oh,maybe.