elisp macro to write a function?

I have written a few nearly identical functions, except for their names. For example:

; x is name, such as function/paragraph/line/etc.
(defun my-x-function
 (interactive)
 (mark-x) (do-more-stuff) (modify-x))
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Is there a way to automatically generate such functions? I have a feeling this is what macros do, but I am not sure how to use them. Any help, maybe including a small example would be great.

Thanks!

Yep, that's exactly what macros do. Here's a straightforward macro that builds functions according to the pattern you specified:

(defmacro make-my-function (name)
  (list 'defun (intern (format "my-%s-function" name)) ()
        (list 'interactive)
        (list (intern (format "mark-%s" name)))
        (list 'do-more-stuff)
        (list (intern (format "modify-%s" name)))))

You can copy this macro to a *scratch* buffer in Emacs and evaluate it, and then check that it works like this:

(make-my-function x) ; type control-J here
my-x-function ; <-- Emacs's output
(symbol-function 'my-x-function) ; type control-J here
(lambda nil (interactive) (mark-x) (do-more-stuff) (modify-x)) ; <-- Emacs's output

More commonly one would use the backquote facility to write macros more concisely, but all macros essentially work in the same manner as the above example.

Macros can do that, but there are lots of template modules for emacs to do similar work. I use a thing called yasnippet.el to do quick code-generation things. For example, in a C-source file, if I type for<TAB>, I get a for loop template; it allows me to fill in the template, setting the variable name, limits, and internal loop contents.

looks like this:

You can set up templates for anything you like. Function definitions, if statements, switch statements, whatever. Set up different templates for different modes. The template for a for loop in C is different than the template for a for loop in C#, and so on. Very handy.