Emacs hooks explanation

Amazing tutorial here at 1:13:40 - https://www.youtube.com/watch?v=gfZDwYeBlO4&t=2637s All of this is done in scratch buffer.

First, lets find what kind of function we want to add our code to. Let's say org-clock-in. When I do org-clock-in command, I want my piece of code to run. Okay. Let's find the "org-clock-in" source code. Press C-h k - this will ask us to input a command. We input the command we are curious about. It is "org-clock-in" command, which is C-c C-x C-i. As we write that command and press enter, we get the following result:

C-c C-x TAB runs the command org-clock-in (found in org-mode-map), which is an interactive autoloaded compiled Lisp function in ‘org-clock.el’.

Aha! Function in 'org-clock.el'. Okay, we click on that and it leads us to the source code of "org-clock-in". We look for a hook function in there. We do C-s to find work, find the "org-clock-in-hook" function and now we are ready to write our little piece of code, since we know a hook for it exists.

And that would be the little piece of code. We are adding a hook, and after lambda inputting our function. Great.

(add-hook 'org-clock-in-hook '(lambda () (message "this works")))

We then evaluate that function by C-c C-e with our cursor at the last ) and then we can go to an .org file and call "org-clock-in" function and watch our message bar. The message will show up there.