M-x haskell-mode-stylish-buffer to call stylish-haskell on the current buffer.
To call it for every save, make sure you have C-x C-s rebound to haskell-mode-save-buffer.
To enable Stylish Haskell, pop the following somewhere in your Emacs configuration:
(setq haskell-stylish-on-save t)
Or in your custom set variables:
(custom-set-variables '(haskell-stylish-on-save t))
Or you can
M-x customize-group haskell and pretend none of this exists!