Download Programming in Emacs Lisp : an introduction by Robert J Chassell; Free Software Foundation (Cambridge, PDF

By Robert J Chassell; Free Software Foundation (Cambridge, Mass.)

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In Lisp, any value that is not nil—is not the empty list—is considered true. This means that if an evaluation returns something that is not an empty list, an if expression will test true. For example, if a number is put in the slot for the test, it will be evaluated and will return itself, since that is what numbers do when evaluated. In this case, the if expression will test true. The expression tests false only when nil, an empty list, is returned by evaluating the expression. You can see this by evaluating the two expressions in the following examples.

This feature is very helpful. In addition to recording the values of point and mark, save-excursion keeps track of the current buffer, and restores it, too. This means you can write code that will change the buffer and have save-excursion switch you back to the original buffer. This is how save-excursion is used in append-to-buffer. ) The body of the function is one or more expressions that will be evaluated in sequence by the Lisp interpreter. If there is more than one expression in the body, the value of the last one will be returned as the value of the save-excursion function.

You can see the consequences of doing this if you evaluate the following version of the type-of-animal function definition to install it and then evaluate the two subsequent expressions to pass different arguments to the function. (defun type-of-animal (characteristic) ; Second version. "Print message in echo area depending on CHARACTERISTIC. ". " would be printed; and it would be misleading! 9 Truth and Falsehood in Lisp There is an important aspect to the truth test in an if expression. So far, we have spoken of ‘true’ and ‘false’ as values of predicates as if they were new kinds of Lisp objects.

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