How interpret the Lisp way to include arguments in functions?

Question

I have some difficulties to understand how Lisp wanted to receive the arguments. When I look into the documentation about arguments in functions, I need to name the arguments exactly as in the source code, when calling the same function.

When I look into the documentation of Emacs function indent-region, I get the following:

= runs the command indent-region, which is an interactive compiled
Lisp function in `indent.el'.

It is bound to =, <menu-bar> <emacs-lisp> <indent-region>, C-M-\.

(indent-region START END &optional COLUMN)

Indent each nonblank line in the region.
A numeric prefix argument specifies a column: indent each line to that column.

Then I create a function, that pass the arguments to the function itself, as described in the documentation:

(defun foo (start end)
  (interactive)
  (indent-region start end)
  )

I see no difference when comparing with how another custom functions handles the functionality. When I visually selected a region, and call the function, I got the error that the arguments are of the wrong type.

So this led me to wonder why the start and end arguments are not accepted in this case?

Answer 1

This risks being closed as unclear. Show your code that provokes an error, as well as telling how you invoke it. Show a backtrace from setting debug-on-error to t.

Your function foo looks OK; bar does not, because indent-region needs two arguments and you are passing only one. What's more, that argument would be the result of evaluating the sexp (start end), which means applying function start to the value of variable end. If you wanted to pass the list (start end) as an argument then you would need to quote it: '(start end). But that's no good for indent-region, which expects separate arguments for the start and end positions.

A guess is that you tried to use M-x foo instead of M-: (foo (region-beginning) (region-end)).

If you want foo to be a command, so that you can bind it to a key or invoke it using M-x, then you need an interactive spec. Use C-h f interactive. That tells you that to pass the region start and end positions you can use (interactive "r") -- there is only one r there, but two arguments are passed to the function (e.g. foo).

I suggest doing C-h i, choosing Emacs Lisp Intro, and starting to read it. You can also get tips for learning Emacs Lisp on Emacs Wiki.

Answer 2

Experimentation can only go so far without fundamentals. The first two chapters of SICP will teach you all the LISP you need to know for Emacs. It takes about half a day to go through it.

After this comes the easy part: learn the names of standard functions at your own pace. Some of them you already know, others you can look up with apropos.