Common Lisp the Language, 2nd Edition

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25.5. Identity Function

This function is occasionally useful as an argument to other functions that require functions as arguments. (Got that?)

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.

Compatibility note: In Maclisp, progn was a function of any number of arguments that returned its last argument, so progn could be used as an identity function. In Common Lisp, progn is a special form and therefore cannot be used for that purpose.