observer-app.rkt

#lang racket
;; > (require "observer-app.rkt")
;; > (hash-set! (observers-namespace) '+ `(,(lambda (a . b) (printf "adding ~a to ~a~n" a b))))
;; > (+ 1 2 3)
;; adding 1 to (2 3)
;; 6

(provide (rename-out [observer-app #%app])
	       observers-namespace)

(define observers-namespace (make-parameter (make-hash)))
(define (lookup-observers x) (hash-ref (observers-namespace) x list))

(define-syntax (observer-app stx)
  (syntax-case stx ()
    [(_ f args ...)
     (let ([sym (syntax-e #'f)])
       #`((lambda (proc . args2 )
	   (for ([o (lookup-observers '#,sym)])
	     (apply o args2))
	   (apply proc args2 )) f args ...))]))