Common Lisp the Language, 2nd Edition
This function is occasionally useful as an argument to other functions that require functions as arguments. (Got that?)
[Function]
identity object
The object is returned as the value of identity.
The identity function is the default value for the :key
argument to many sequence functions (see chapter 14).
Table 12-1 illustrates the behavior in the complex plane of the identity function regarded as a function of a complex numerical argument.
Many other constructs in Common Lisp have the behavior of identity when given a single argument. For example, one might well use values in place of identity. However, writing values of a single argument conventionally indicates that the argument form might deliver multiple values and that the intent is to pass on only the first of those values.