mutator-macro.scm

; Modifies a variable by applying a function to said variable

(define-syntax ! (syntax-rules ()
  [(! x (fn arg ...))
    (set! x (fn x arg ...))]
  [(! x fn)
    (set! x (fn x))]))

; for example...

(define-syntax square! (syntax-rules ()
  [(square! x)
    (! x (* x))]))

(define a 6)
(square! a)
; a = 36


; This is safer, and allows (! x (compose square add1))

(define-syntax ! (syntax-rules ()
  [(! x fn arg ...)
    (set! x (fn x arg ...))]))