25.5. Identity Function

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.

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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.

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