Common Lisp the Language, 2nd Edition
In this section, we introduce changes to the terminology defined in section 1.2.4.
A condition is an interesting situation in a program that has been detected and announced. Later we allow this term also to refer to objects that programs use to represent such situations.
An error is a condition in which normal program execution may not continue without some form of intervention (either interactively by the user or under some sort of program control, as described below).
The process by which a condition is formally announced by a program is called signaling. The function signal is the primitive mechanism by which such announcement is done. Other abstractions, such as error and cerror, are built using signal.
The first edition is ambiguous about the reason why a particular program action ``is an error.'' There are two principal reasons why an action may be an error without being required to signal an error:
For example, (+ nil 3) is an error. It is likely that the designers of Common Lisp believed this would be an error in all implementations but felt it might be excessively expensive to detect the problem in compiled code on stock hardware, so they did not require that it signal an error.
For example, (loop for x from 1 to 3 do (print x)) is an error because loop is not defined to take atoms in its body. In fact, however, some implementations offer an extension that makes this well-defined. In order to leave room for such extensions, the first edition used the ``is an error'' terminology to keep implementors from being forced to signal an error in the extended implementations.
[This example was written well before the vote by X3J13 in January 1989 to add exactly this extension to the forthcoming draft standard (see chapter 26).-GLS]
In this chapter, we use the following terminology. [Compare this to the terminology presented in section 28.1.1.-GLS]