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Programming in Emacs Lisp

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8.1.1 The interactive Expression

The interactive expression in the zap-to-char command looks like this:

 
(interactive "*p\ncZap to char: ")

The part within quotation marks, "*p\ncZap to char: ", specifies three different things. First, and most simply, the asterisk, `*', causes an error to be signalled if the buffer is read-only. This means that if you try zap-to-char in a read-only buffer you will not be able to remove text, and you will receive a message that says "Buffer is read-only"; your terminal may beep at you as well.

The version 21 implementation does not have the asterisk, `*'. The function works the same as in version 19: in both cases, it cannot remove text from a read-only buffer but the function does copy the text that would have been removed to the kill ring. Also, in both cases, you see an error message.

However, the version 19 implementation copies text from a read-only buffer only because of a mistake in the implementation of interactive. According to the documentation for interactive, the asterisk, `*', should prevent the zap-to-char function from doing anything at all when the buffer is read only. The function should not copy the text to the kill ring. It is a bug that it does.

In version 21, interactive is implemented correctly. So the asterisk, `*', had to be removed from the interactive specification. If you insert an `*' and evaluate the function definition, then the next time you run the zap-to-char function on a read-only buffer, you will not copy any text.

That change aside, and a change to the documentation, the two versions of the zap-to-char function are identical.

Let us continue with the interactive specification.

The second part of "*p\ncZap to char: " is the `p'. This part is separated from the next part by a newline, `\n'. The `p' means that the first argument to the function will be passed the value of a `processed prefix'. The prefix argument is passed by typing C-u and a number, or M- and a number. If the function is called interactively without a prefix, 1 is passed to this argument.

The third part of "*p\ncZap to char: " is `cZap to char: '. In this part, the lower case `c' indicates that interactive expects a prompt and that the argument will be a character. The prompt follows the `c' and is the string `Zap to char: ' (with a space after the colon to make it look good).

What all this does is prepare the arguments to zap-to-char so they are of the right type, and give the user a prompt.


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