Emacs : Redefining command in Haskell-mode (haskell-mode-hook)

What you want to do is run the function that C-x C-s runs followed by running the function C-c C-l does. You can find out what function is run by some key binding via C-h k. That is, first type C-h k then the key command you're interested in.

This gives us (save-buffer &optional ARGS) for C-x C-s and (inferior-haskell-load-file &optional RELOAD) for C-c C-l. The &optional means exactly what you think it does--that argument is optional, so we don't care about it.

Now write the function that does both of them:

(defun my-haskell-mode-save-buffer ()
  (interactive)
  (save-buffer)
  (inferior-haskell-load-file)) 

Now you can bind this function to C-x C-s in haskell mode exactly how you've been doing it:

(add-hook 'haskell-mode-hook (lambda () 
                                (local-set-key (kbd "C-x C-s") 'my-haskell-mode-save-buffer)))

EDIT: It seems C-c C-l saves your file by default before loading it. This means you can just write

(add-hook 'haskell-mode-hook (lambda ()
                            (local-set-key (kbd "C-x C-s") 'inferior-haskell-load-file)))

and have exactly the same effect without writing your own function. However, I think writing it my way is a good learning exercise :P. That approach works whenever you want to combine multiple different key bindings into one.