CLHS: Section 3.4.6

3.4.6 Boa Lambda Lists

A boa lambda list is a lambda list that is syntactically like an ordinary lambda list, but that is processed in ``by order of argument'' style.

A boa lambda list is used only in a defstruct form, when explicitly specifying the lambda list of a constructor function (sometimes called a ``boa constructor'').

The &optional, &rest, &aux, &key, and &allow-other-keys lambda list keywords are recognized in a boa lambda list. The way these lambda list keywords differ from their use in an ordinary lambda list follows.

Consider this example, which describes how destruct processes its :constructor option.

 (:constructor create-foo
         (a &optional b (c 'sea) &rest d &aux e (f 'eff)))

This defines create-foo to be a constructor of one or more arguments. The first argument is used to initialize the a slot. The second argument is used to initialize the b slot. If there isn't any second argument, then the default value given in the body of the defstruct (if given) is used instead. The third argument is used to initialize the c slot. If there isn't any third argument, then the symbol sea is used instead. Any arguments following the third argument are collected into a list and used to initialize the d slot. If there are three or fewer arguments, then nil is placed in the d slot. The e slot is not initialized; its initial value is implementation-defined. Finally, the f slot is initialized to contain the symbol eff. &key and &allow-other-keys arguments default in a manner similar to that of &optional arguments: if no default is supplied in the lambda list then the default value given in the body of the defstruct (if given) is used instead. For example:

 (defstruct (foo (:constructor CREATE-FOO (a &optional b (c 'sea)
                                             &key (d 2)
                                             &aux e (f 'eff))))
   (a 1) (b 2) (c 3) (d 4) (e 5) (f 6))
 
 (create-foo 10) =>  #S(FOO A 10 B 2 C SEA D 2 E implemention-dependent F EFF)
 (create-foo 10 'bee 'see :d 'dee) 
=>  #S(FOO A 10 B BEE C SEE D DEE E implemention-dependent F EFF)

If keyword arguments of the form ((key var) [default [svar]]) are specified, the slot name is matched with var (not key).

The actions taken in the b and e cases were carefully chosen to allow the user to specify all possible behaviors. The &aux variables can be used to completely override the default initializations given in the body.

If no default value is supplied for an aux variable variable, the consequences are undefined if an attempt is later made to read the corresponding slot's value before a value is explicitly assigned. If such a slot has a :type option specified, this suppressed initialization does not imply a type mismatch situation; the declared type is only required to apply when the slot is finally assigned.

With this definition, the following can be written:

 (create-foo 1 2)

instead of

 (make-foo :a 1 :b 2)

and create-foo provides defaulting different from that of make-foo.

Additional arguments that do not correspond to slot names but are merely present to supply values used in subsequent initialization computations are allowed. For example, in the definition

 (defstruct (frob (:constructor create-frob
                  (a &key (b 3 have-b) (c-token 'c) 
                          (c (list c-token (if have-b 7 2))))))
         a b c)

the c-token argument is used merely to supply a value used in the initialization of the c slot. The supplied-p parameters associated with optional parameters and keyword parameters might also be used this way.

The following X3J13 cleanup issues, not part of the specification, apply to this section: