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.
As usual, the error message tries to be helpful and makes sense after you learn how to read it. What it says is this:
Wrong type argument: integer-or-marker-p, hello
The first part of the error message is straightforward; it says
`Wrong type argument'. Next comes the mysterious jargon word
`integer-or-marker-p'. This word is trying to tell you what
kind of argument the + expected.
The symbol integer-or-marker-p says that the Lisp interpreter is
trying to determine whether the information presented it (the value of
the argument) is an integer (that is, a whole number) or a marker (a
special object representing a buffer position). What it does is test to
see whether the + is being given whole 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 integer-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
integer-or-marker-p is the name of a function that determines
whether it is true or false that the argument supplied is an integer 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.
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