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Programming in Emacs Lisp

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1.8.4 Using the Wrong Type Object as an Argument

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.

In GNU Emacs version 21, 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 (quote hello)))
---------- Buffer: *Backtrace* ----------

As usual, the error message tries to be helpful and makes sense after you learn how to read it.

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.

In GNU Emacs version 20 and before, the echo area displays an error message that says:

Wrong type argument: number-or-marker-p, hello

This says, in different words, the same as the top line of the `*Backtrace*' buffer.

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