eintr: message
1.8.5 The ‘message’ Function
----------------------------
Like ‘+’, the ‘message’ function takes a variable number of arguments.
It is used to send messages to the user and is so useful that we will
describe it here.
A message is printed in the echo area. For example, you can print a
message in your echo area by evaluating the following list:
(message "This message appears in the echo area!")
The whole string between double quotation marks is a single argument
and is printed in toto. (Note that in this example, the message itself
will appear in the echo area within double quotes; that is because you
see the value returned by the ‘message’ function. In most uses of
‘message’ in programs that you write, the text will be printed in the
echo area as a side-effect, without the quotes. 
‘multiply-by-seven’ in detail multiply-by-seven in detail, for an
example of this.)
However, if there is a ‘%s’ in the quoted string of characters, the
‘message’ function does not print the ‘%s’ as such, but looks to the
argument that follows the string. It evaluates the second argument and
prints the value at the location in the string where the ‘%s’ is.
You can see this by positioning the cursor after the following
expression and typing ‘C-x C-e’:
(message "The name of this buffer is: %s." (buffer-name))
In Info, ‘"The name of this buffer is: *info*."’ will appear in the echo
area. The function ‘buffer-name’ returns the name of the buffer as a
string, which the ‘message’ function inserts in place of ‘%s’.
To print a value as an integer, use ‘%d’ in the same way as ‘%s’.
For example, to print a message in the echo area that states the value
of the ‘fill-column’, evaluate the following:
(message "The value of fill-column is %d." fill-column)
On my system, when I evaluate this list, ‘"The value of fill-column is
72."’ appears in my echo area(1).
If there is more than one ‘%s’ in the quoted string, the value of the
first argument following the quoted string is printed at the location of
the first ‘%s’ and the value of the second argument is printed at the
location of the second ‘%s’, and so on.
For example, if you evaluate the following,
(message "There are %d %s in the office!"
(- fill-column 14) "pink elephants")
a rather whimsical message will appear in your echo area. On my system
it says, ‘"There are 58 pink elephants in the office!"’.
The expression ‘(- fill-column 14)’ is evaluated and the resulting
number is inserted in place of the ‘%d’; and the string in double
quotes, ‘"pink elephants"’, is treated as a single argument and inserted
in place of the ‘%s’. (That is to say, a string between double quotes
evaluates to itself, like a number.)
Finally, here is a somewhat complex example that not only illustrates
the computation of a number, but also shows how you can use an
expression within an expression to generate the text that is substituted
for ‘%s’:
(message "He saw %d %s"
(- fill-column 32)
(concat "red "
(substring
"The quick brown foxes jumped." 16 21)
" leaping."))
In this example, ‘message’ has three arguments: the string, ‘"He saw
%d %s"’, the expression, ‘(- fill-column 32)’, and the expression
beginning with the function ‘concat’. The value resulting from the
evaluation of ‘(- fill-column 32)’ is inserted in place of the ‘%d’; and
the value returned by the expression beginning with ‘concat’ is inserted
in place of the ‘%s’.
When your fill column is 70 and you evaluate the expression, the
message ‘"He saw 38 red foxes leaping."’ appears in your echo area.
---------- Footnotes ----------
(1) Actually, you can use ‘%s’ to print a number. It is
non-specific. ‘%d’ prints only the part of a number left of a decimal
point, and not anything that is not a number.
Info Catalog
eintr: Wrong Type of Argument
eintr: Arguments