defvar2

Provide an implementation for a global variable

http://awwx.ws/defvar1.patch:

--- a/ac.scm	2009-12-05 15:37:50.000000000 -0500
+++ b/ac.scm	2009-12-05 15:51:53.000000000 -0500
@@ -227,10 +227,13 @@
                  acc
                  keepsep?))))
 
+(defarc (ac-defined-var? s)
+  #f)
+
 (define (ac-var-ref s env)
-  (if (lex? s env)
-      s
-      (ac-global-name s)))
+  (cond ((lex? s env)        s)
+        ((ac-defined-var? s) (list (ac-global-name s)))
+        (#t                  (ac-global-name s))))
 
 ; quasiquote
 
@@ -414,6 +417,7 @@
                (cond ((eqv? a 'nil) (err "Can't rebind nil"))
                      ((eqv? a 't) (err "Can't rebind t"))
                      ((lex? a env) `(set! ,a zz))
+                     ((ac-defined-var? a) `(,(ac-global-name a) zz))
                      (#t `(namespace-set-variable-value! ',(ac-global-name a) 
                                                          zz)))
                'zz))

http://awwx.ws/defvar2.arc:

(= defined-variables* (table))

(def ac-defined-var? (name)
  (if defined-variables*.name scheme-t scheme-f))

(mac defvar (name impl)
  `(do (ac-set-global ',name ,impl)
       (set (defined-variables* ',name))
       nil))

(mac defvar-impl (name)
  (let gname (ac-global-name name)
    `(scheme ,gname)))

(mac undefvar (name)
  `(do (wipe (defined-variables* ',name))
       (ac-set-global ',name nil)))

This hack provides for a way to provide your own implementation for a global variable, to say what happens when the variable is used or set:

(defvar v f)

causes the function f to provide the implementation for the variable v: when v is used in an expression, f is called with no arguments to provide the value that v evaluates to; when v is set, f is called with one argument, the value v is being set to.

arc> (defvar a
       (let x 5
         (fn args
           (if args
               (do (prn "Being set to " (car args) ".")
                   (= x (car args)))
               (do (prn "Returning " x ".")
                   x)))))
nil

arc> (++ a)
Returning 5.
Being set to 6.
6

Get this hack

Using the hackinator:

$ hack \
    ycombinator.com/arc/arc3.1.tar \
    awwx.ws/defarc0.patch \
    awwx.ws/defarc-ac0.patch \
    awwx.ws/extend0.arc \
    awwx.ws/scheme0.arc \
    awwx.ws/defvar1.patch \
    awwx.ws/defvar2.arc

License

Same as Arc.

Contact me