When a function is passed an argument of the wrong type, the Lisp
interpreter produces an error message. For example, the +
function expects the values of its arguments to be numbers. As an
experiment we can pass it the quoted symbol hello instead of a
number. Position the cursor after the following expression and type
C-x C-e:
(+ 2 'hello)
When you do this you will generate an error message. What has happened
is that + has tried to add the 2 to the value returned by
'hello, but the value returned by 'hello is the symbol
hello, not a number. Only numbers can be added. So +
could not carry out its addition.
You will create and enter a *Backtrace* buffer that says:
---------- Buffer: *Backtrace* ----------
Debugger entered--Lisp error:
(wrong-type-argument number-or-marker-p hello)
+(2 hello)
eval((+ 2 'hello) nil)
elisp--eval-last-sexp(t)
eval-last-sexp(nil)
funcall-interactively(eval-print-last-sexp nil)
call-interactively(eval-print-last-sexp nil nil)
command-execute(eval-print-last-sexp)
---------- Buffer: *Backtrace* ----------
As usual, the error message tries to be helpful and makes sense after you learn how to read it.4
The first part of the error message is straightforward; it says
‘wrong type argument’. Next comes the mysterious jargon word
‘number-or-marker-p’. This word is trying to tell you what
kind of argument the + expected.
The symbol number-or-marker-p says that the Lisp interpreter is
trying to determine whether the information presented it (the value of
the argument) is a number or a marker (a special object representing a
buffer position). What it does is test to see whether the + is
being given numbers to add. It also tests to see whether the
argument is something called a marker, which is a specific feature of
Emacs Lisp. (In Emacs, locations in a buffer are recorded as markers.
When the mark is set with the C-@ or C-SPC command,
its position is kept as a marker. The mark can be considered a
number—the number of characters the location is from the beginning
of the buffer.) In Emacs Lisp, + can be used to add the
numeric value of marker positions as numbers.
The ‘p’ of number-or-marker-p is the embodiment of a
practice started in the early days of Lisp programming. The ‘p’
stands for predicate. In the jargon used by the early Lisp
researchers, a predicate refers to a function to determine whether some
property is true or false. So the ‘p’ tells us that
number-or-marker-p is the name of a function that determines
whether it is true or false that the argument supplied is a number or
a marker. Other Lisp symbols that end in ‘p’ include zerop,
a function that tests whether its argument has the value of zero, and
listp, a function that tests whether its argument is a list.
Finally, the last part of the error message is the symbol hello.
This is the value of the argument that was passed to +. If the
addition had been passed the correct type of object, the value passed
would have been a number, such as 37, rather than a symbol like
hello. But then you would not have got the error message.