cl: Macros
5 Macros
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This package implements the various Common Lisp features of ‘defmacro’,
such as destructuring, ‘&environment’, and ‘&body’. Top-level ‘&whole’
is not implemented for ‘defmacro’ due to technical difficulties. 
Argument Lists.
Destructuring is made available to the user by way of the following
macro:
-- Macro: cl-destructuring-bind arglist expr forms...
This macro expands to code that executes FORMS, with the variables
in ARGLIST bound to the list of values returned by EXPR. The
ARGLIST can include all the features allowed for ‘cl-defmacro’
argument lists, including destructuring. (The ‘&environment’
keyword is not allowed.) The macro expansion will signal an error
if EXPR returns a list of the wrong number of arguments or with
incorrect keyword arguments.
This package also includes the Common Lisp ‘define-compiler-macro’
facility, which allows you to define compile-time expansions and
optimizations for your functions.
-- Macro: cl-define-compiler-macro name arglist forms...
This form is similar to ‘defmacro’, except that it only expands
calls to NAME at compile-time; calls processed by the Lisp
interpreter are not expanded, nor are they expanded by the
‘macroexpand’ function.
The argument list may begin with a ‘&whole’ keyword and a variable.
This variable is bound to the macro-call form itself, i.e., to a
list of the form ‘(NAME ARGS...)’. If the macro expander returns
this form unchanged, then the compiler treats it as a normal
function call. This allows compiler macros to work as optimizers
for special cases of a function, leaving complicated cases alone.
For example, here is a simplified version of a definition that
appears as a standard part of this package:
(cl-define-compiler-macro cl-member (&whole form a list &rest keys)
(if (and (null keys)
(eq (car-safe a) 'quote)
(not (floatp (cadr a))))
(list 'memq a list)
form))
This definition causes ‘(cl-member A LIST)’ to change to a call to
the faster ‘memq’ in the common case where A is a
non-floating-point constant; if A is anything else, or if there are
any keyword arguments in the call, then the original ‘cl-member’
call is left intact. (The actual compiler macro for ‘cl-member’
optimizes a number of other cases, including common ‘:test’
predicates.)
-- Function: cl-compiler-macroexpand form
This function is analogous to ‘macroexpand’, except that it expands
compiler macros rather than regular macros. It returns FORM
unchanged if it is not a call to a function for which a compiler
macro has been defined, or if that compiler macro decided to punt
by returning its ‘&whole’ argument. Like ‘macroexpand’, it expands
repeatedly until it reaches a form for which no further expansion
is possible.
Macro Bindings, for descriptions of the ‘cl-macrolet’ and
‘cl-symbol-macrolet’ forms for making “local” macro definitions.
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