chickadee » miscmacros » modify-location

(modify-location LOC PROC)syntax

Expands into a call to PROC with two arguments: a zero-argument procedure for retrieving the value from the settable location LOC and a one argument procedure for setting the value of LOC. Care is taken to evaluate any subforms in LOC only once.

modify-location is intended to create modification macros for generalized locations (as in SRFI-17.

 (define-syntax-rule (increment! loc)
     (lambda (ref upd) (upd (add1 (ref)))) ) )
 (define x (vector 123))
 (increment! (vector-ref x (print 0)))   ; sets x to #(124) and prints "0" only once