Macro Lambda Lists (ANSI and GNU Common Lisp Document)

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#### 3.4.4 Macro Lambda Lists

A macro lambda list is used in describing macros defined by the operators in Figure 3–17.

      define-compiler-macro  defmacro  macrolet
      define-setf-expander

      Figure 3–17: Operators that use Macro Lambda Lists

With the additional restriction that an environment parameter may appear only once (at any of the positions indicated), a macro lambda list has the following syntax:

reqvars ::={var | !pattern}*

optvars ::=`[`&optional {var | ({var | !pattern} [init-form [supplied-p-parameter]])}*`]`

restvar ::=`[`{`&rest` | &body} {var | !pattern}`]`

keyvars ::=[&key {var | ({var | (keyword-name {var | !pattern})} [init-form [supplied-p-parameter]])}* [`&allow-other-keys`]]

auxvars ::=`[`&aux {var | (var [init-form])}*`]`

envvar ::=`[`&environment var`]`

wholevar ::=`[`&whole var`]`

lambda-list ::=(!wholevar !envvar !reqvars !envvar !optvars !envvar !restvar !envvar !keyvars !envvar !auxvars !envvar) | (!wholevar !envvar !reqvars !envvar !optvars !envvar `.` var)

pattern ::=(!wholevar !reqvars !optvars !restvar !keyvars !auxvars) | (!wholevar !reqvars !optvars `.` var)

A macro lambda list can contain the lambda list keywords shown in Figure 3–18.

      &allow-other-keys  &environment  &rest
      &aux               &key          &whole
      &body              &optional

      Figure 3–18: Lambda List Keywords used by Macro Lambda Lists

Optional parameters (introduced by &optional) and keyword parameters (introduced by &key) can be supplied in a macro lambda list, just as in an ordinary lambda list. Both may contain default initialization forms and supplied-p parameters.

&body

is identical in function to &rest, but it can be used to inform certain output-formatting and editing functions that the remainder of the form is treated as a body, and should be indented accordingly. Only one of &body or &rest can be used at any particular level; see Destructuring by Lambda Lists.

&body can appear at any level of a macro lambda list; for details, see Destructuring by Lambda Lists.

&whole

is followed by a single variable that is bound to the entire macro-call form; this is the value that the macro function receives as its first argument.

If &whole and a following variable appear, they must appear first in lambda-list,

before any other parameter or lambda list keyword.

&whole can appear at any level of a macro lambda list. At inner levels, the &whole variable is bound to the corresponding part of the argument, as with &rest, but unlike &rest, other arguments are also allowed. The use of &whole does not affect the pattern of arguments specified.

&environment

is followed by a single variable that is bound to an environment representing the lexical environment in which the macro call is to be interpreted. This environment should be used with

macro-function,

get-setf-expansion,

compiler-macro-function,

and macroexpand (for example) in computing the expansion of the macro, to ensure that any lexical bindings or definitions established in the compilation environment are taken into account.

&environment can only appear at the top level of a macro lambda list, and can only appear once, but can appear anywhere in that list;

the &environment parameter is bound along with &whole before any other variables in the lambda list, regardless of where &environment appears in the lambda list.

The object that is bound to the environment parameter has dynamic extent.

Destructuring allows a macro lambda list to express the structure of a macro call syntax. If no lambda list keywords appear, then the macro lambda list is a tree containing parameter names at the leaves. The pattern and the macro form must have compatible tree structure; that is, their tree structure must be equivalent, or it must differ only in that some leaves of the pattern match non-atomic objects of the macro form.

For information about error detection in this situation, see Destructuring Mismatch.

A destructuring lambda list (whether at top level or embedded) can be dotted, ending in a parameter name. This situation is treated exactly as if the parameter name that ends the list had appeared preceded by &rest.

It is permissible for a macro form (or a subexpression of a macro form) to be a dotted list only when `(... &rest var)` or `(... . var)` is used to match it. It is the responsibility of the macro to recognize and deal with such situations.

[Editorial Note by KMP: Apparently the dotted-macro-forms cleanup doesn’t allow for the macro to ‘manually’ notice dotted forms and fix them as well. It shouldn’t be required that this be done only by &REST or a dotted pattern; it should only matter that ultimately the non-macro result of a full-macro expansion not contain dots. Anyway, I plan to address this editorially unless someone raises an objection.]

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