This is some experimenting with Common Lisp following the excellent “On Lisp” by Paul Graham:
First let’s experiment with the Common Lisp macro member
(member "2" '("1" "2" "3"))
; => NILIf we define the function to use the equality on string test (instead of
the default eql):
(member "2" '("1" "2" "3") :test #'string=)
; => (2 3)If we need this special variant a lot, we can write a macro:
(defmacro string-member (e es)
`(member ,e ,es :test ,#'string=))
(string-member "2" '("1" "2" "3"))And we can use expansion to double check if things expand correctly:
(macroexpand '(string-member "x" '("x")))
; => (MEMBER "x" '("x") :TEST #<FUNCTION STRING=>)Naturally we can expand defmacro:
(macroexpand '(defmacro x (y) `(+ 1 y)))
; I am using SBCL here
;;=> (PROGN
;; (EVAL-WHEN (:COMPILE-TOPLEVEL)
;; (SB-C::%COMPILER-DEFMACRO :MACRO-FUNCTION 'X T))
;; (EVAL-WHEN (:COMPILE-TOPLEVEL :LOAD-TOPLEVEL :EXECUTE)
;; (SB-C::%DEFMACRO 'X
;; (SB-INT:NAMED-LAMBDA (MACRO-FUNCTION X)
;; (#:EXPR #:ENV)
;; (DECLARE (SB-C::LAMBDA-LIST (Y)))
;; (DECLARE (IGNORE #:ENV))
;; (SB-INT:NAMED-DS-BIND (:MACRO X . DEFMACRO)
;; (Y)
;; (CDR #:EXPR)
;; (BLOCK X `(+ 1 Y))))
;; (SB-C:SOURCE-LOCATION))))
;; TWe now define how a macro works:
(defmacro the-expander (name) `(get ,name 'expander))The expander is our way of storing macros definition. Given the name
of the macro, get will retrieve the =’expander= for the macro.
(defmacro the-defmacro (name params &body body)
(let ((g (gensym)))
`(progn
(setf (the-expander ',name)
#'(lambda (,g)
(block ,name
(destructuring-bind ,params (cdr ,g)
,@body))))
',name)))Something cool happens here: we setf the expander for the macro to
be a function. This takes a parameter with a random name (gensym)
and produces a block initialized with the future parameters. The
parameter will be our macro application!
(defun the-macroexpand-1 (expr)
(if (and (consp expr) (the-expander (car expr)))
(funcall (the-expander (car expr)) expr)
expr))At this point we can expand the macro by executing the function for
the given macro application. The if statement filters out the cases
where the expression is not a list or no macro expansion is defined
for the name.
Finally let’s use the-macroexpand-1 to define our previous
string-member macro:
(the-defmacro string-member1 (e es)
`(member ,e ,es :test ,#'string=))
(the-macroexpand-1 '(string-member1 "x" '("x" "2")))
;; => (MEMBER "x" '("x" "2") :TEST #<FUNCTION STRING=>)
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