Macro Bindings (Common Lisp Extensions)

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5.3.3 Macro Bindings

These forms create local macros and “symbol macros”.

Macro: cl-macrolet (bindings…) forms… ¶

This form is analogous to cl-flet, but for macros instead of functions. Each binding is a list of the same form as the arguments to cl-defmacro (i.e., a macro name, argument list, and macro-expander forms). The macro is defined accordingly for use within the body of the cl-macrolet.

Because of the nature of macros, cl-macrolet is always lexically scoped. The cl-macrolet binding will affect only calls that appear physically within the body forms, possibly after expansion of other macros in the body. Calls of cl-macrolet bound macros are expanded in the global environment.

Macro: cl-symbol-macrolet (bindings…) forms… ¶

This form creates symbol macros, which are macros that look like variable references rather than function calls. Each binding is a list ‘(var expansion)’; any reference to var within the body forms is replaced by expansion.

(setq bar '(5 . 9))
(cl-symbol-macrolet ((foo (car bar)))
 (cl-incf foo))
bar
 ⇒ (6 . 9)

A setq of a symbol macro is treated the same as a setf. I.e., (setq foo 4) in the above would be equivalent to (setf foo 4), which in turn expands to (setf (car bar) 4). A let (or let*, lambda, ...) binding of the same symbol will locally shadow the symbol macro as is the case in Common Lisp.

There is no analogue of defmacro for symbol macros; all symbol macros are local. A typical use of cl-symbol-macrolet is in the expansion of another macro:

(cl-defmacro my-dolist ((x list) &rest body)
 (let ((var (cl-gensym)))
 (list 'cl-loop 'for var 'on list 'do
 (cl-list* 'cl-symbol-macrolet
 (list (list x (list 'car var)))
 body))))
(setq mylist '(1 2 3 4))
(my-dolist (x mylist) (cl-incf x))
mylist
 ⇒ (2 3 4 5)

In this example, the my-dolist macro is similar to dolist (see Iteration) except that the variable x becomes a true reference onto the elements of the list. The my-dolist call shown here expands to

(cl-loop for G1234 on mylist do
 (cl-symbol-macrolet ((x (car G1234)))
 (cl-incf x)))

which in turn expands to

(cl-loop for G1234 on mylist do (cl-incf (car G1234)))

See Loop Facility, for a description of the cl-loop macro. This package defines a nonstandard in-ref loop clause that works much like my-dolist.

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