I wrote a function, and I want to call it via M-x, and bind it to a key. This is my function:
(defun my-function ()
(message "This is a great function"))
If I try to call it with M-x my-function, I get the error: [no match] in the mini-buffer.
If I try to bind it to a key (or mouse click):
(global-set-key (kbd "C-c a") 'my-function)
It appears to work, but when I try to call it with C-c a, I get the error
Wrong type argument: commandp, my-function
Why can't I use my function?
The core point is that there is a difference between a function and a command.
In Emacs lisp, functions are not interactively callable by default. That means you can't access them via M-x or bind them to a key or mouse click. If you want to do that, you need to explicitly declare the function to be interactive, which you do by adding an (interactive) form as the first line in the body (following the documentation string). An interactive function is called a command This is explained in the manual: (info "(elisp) Using Interactive") (online version).
The error message you see, Wrong type argument: commandp, my-function, is indicating that you're trying to call a function interactively, but that function isn't a command.
To explain the actual error, the letter p is often used in lisp to indicate a predicate or test. In this case, Emacs is testing my-function to see if it is a command using the test commandp. It isn't, which leads to the error. Similar errors pop up whenever you use an object of the wrong type: if Emacs expects a string and you pass a symbol, you might see a reference to stringp, for example.
To answer the question as asked, you need to add the (interactive) line to the definition:
(defun my-function ()
(interactive)
(message "This is a great function"))
There are a lot of options for the interactive form, supporting all kinds of ways of passing information to your function. Check the manual for all the details.
Keyboard macros are a special case in this context. A keyboard macro is a sequence of input events, represented as a string. Keyboard macros behave like commands, so you can bind them to keys without worrying about adding an interactive declaration. For example, in the following:
(global-set-key (kbd "C-c l") "λ")
"λ" is a keyboard macro, so we can bind it to C-c l without a problem. If we try to do the same thing with a function we have to be sure to define the function as interactive:
(global-set-key (kbd "C-c k")
(lambda () (insert "λ"))
;; C-c k is undefined! We tried to bind it to a function
(global-set-key (kbd "C-c m")
(lambda () (interactive) (insert "λ"))
;; C-c m is bound to a command that inserts λ
M-x my-function in your example. Also as an emacs beginner I'm really not 100% clear yet on what exactly C-x C-e does or when you need to run it, but it appears that uh... when you run it, it parses the buffer and overwrites my-function in memory because if I don't run C-x C-e M-x runs the function from the last time I ran C-x C-e
C-x C-e evaluates the buffer, it seems that the result of evaluating a buffer containing a defun is registering the function, though this article seems to make me think that it should just run the function, and yet the only thing shown in the minibuffer is my-function (implying it returns the function?), not This is a great function. I must be missing something here.
interactive- e.g., sometimes a command disappears from a new version of a library. The error can be raised in any context where Emacs expects a command.commandp, to try to find other Qs that can be closed as duplicates of this one. Be careful to read the Q&A, though, as some are different. In some cases, the answer (and the Q context) might be worth repeating here. In other cases the question is unrelated and should be left as is (not closed).