Question

I am reading through An Introduction to Programming in Emacs Lisp, and I see the following:

This code works:

(message "The name of this buffer is: %s." (buffer-name))

while this one fails:

(message "The name of this buffer is: %s." buffer-name)

However, this code works:

(message "The value of fill-column is %d." fill-column)

while this one fails:

(message "The value of fill-column is %d." (fill-column))

My question is why? What is the difference between buffer-name and fill-column? How do I know when to use parentheses?

Answer 1

Simply put – buffer-name is a function (that returns a string) and fill-column is a variable (that evaluates to an integer).

Functions calls in all Lisp dialects should be surrounded by parentheses.

To see details about a function in Emacs press C-h f function-name RET.
To see details about a variable in Emacs press C-h v variable-name RET.