#### 3.4.4.4 Lambda-list-directed Destructuring by Lambda Lists ¶

An extension of data-directed destructuring of trees is lambda-list-directed destructuring. This derives from the analogy between the three-element destructuring pattern

`(first second third)`

and the three-argument lambda list

`(first second third)`

Lambda-list-directed destructuring is identical to data-directed destructuring if no lambda list keywords appear in the pattern. Any list in the pattern (whether a sub-list or the whole pattern itself) that contains a lambda list keyword is interpreted specially. Elements of the list to the left of the first lambda list keyword are treated as destructuring patterns, as usual, but the remaining elements of the list are treated like a function’s lambda list except that where a variable would normally be required, an arbitrary destructuring pattern is allowed. Note that in case of ambiguity, lambda list syntax is preferred over destructuring syntax. Thus, after &optional a list of elements is a list of a destructuring pattern and a default value form.

The detailed behavior of each lambda list keyword in a lambda-list-directed destructuring pattern is as follows:

&optional

  Each following element is a variable or a list of a destructuring pattern, a default value form, and a supplied-p variable. The default value and the supplied-p variable can be omitted. If the list being destructured ends early, so that it does not have an element to match against this destructuring (sub)-pattern, the default form is evaluated and destructured instead. The supplied-p variable receives the value nil if the default form is used, t otherwise.

&rest, &body

  The next element is a destructuring pattern that matches the rest of the list. &body is identical to &rest but declares that what is being matched is a list of forms that constitutes the body of form. This next element must be the last unless a lambda list keyword follows it.

&aux

  The remaining elements are not destructuring patterns at all, but are auxiliary variable bindings.

&whole

  The next element is a destructuring pattern that matches the entire form in a macro, or the entire subexpression at inner levels.

&key

  Each following element is one of

  ``

    a variable,

  or

    a list of a variable, an optional initialization form, and an optional supplied-p variable.

  or

    a list of a list of a keyword and a destructuring pattern, an optional initialization form, and an optional supplied-p variable.

  The rest of the list being destructured is taken to be alternating keywords and values and is taken apart appropriately.

&allow-other-keys

  Stands by itself.