Emacs provides a mechanism for advising functions, which is useful in situations when you need to override the original function with your own one, transform the arguments before passing them down to the advised function, execute some code before, or after the function being advised is called, etc.

For more information about the possible ways you can compose the original function and an advise, please make sure to check the Emacs documentation.

In this post we will see how to create an advising function for hyperspec-lookup - the function, which takes care for looking up documentation in the Common Lisp HyperSpec.

When you invoke hyperspec-lookup (bound to C-c C-d h when using Slime) the function will open up your browser with the URL to the CLHS symbol you are looking up. The browser that will be used is defined by the the browse-url-browser-function variable, which by default is set to browse-url-default-browser.

Here is how we can create an Emacs advising function for hyperspec-lookup, which will use the w3m text-based browser. This is useful when you want to stay within your Emacs session and simply have a new buffer open up with the documentation, instead of having to switch to an external browser.

(defun hyperspec-lookup--hyperspec-lookup-w3m (orig-fun &rest args)
  (let ((browse-url-browser-function 'w3m-browse-url))
    (apply orig-fun args)))

Next, we need to add the advice.

(advice-add 'hyperspec-lookup :around #'hyperspec-lookup--hyperspec-lookup-w3m)

You can put above code in your ~/.emacs file. Try M-x hyperspec-lookup or C-c C-d h and it should open up a tab in your w3m buffer.