7.6 Gotchas

7.6.1 Using set! to Assign to Variables Provided via provide/contract

The contract library assumes that variables exported via provide/contract are not assigned to, but does not enforce it. Accordingly, if you try to set! those variables, you may be surprised. Consider the following example:

> (module server scheme       (define (inc-x!) (set! x (+ x 1)))       (define x 0)       (provide/contract [inc-x! (-> void?)]                         [x integer?]))

> (module client scheme       (require 'server)            (define (print-latest) (printf "x is ~s\n" x))            (print-latest)       (inc-x!)       (print-latest))

> (require 'client)

x is 0   x is 0

Both calls to print-latest print 0, even though the value of x has been incremented (and the change is visible inside the module x).

To work around this, export accessor functions, rather than exporting the variable directly, like this:

#lang scheme

(define (get-x) x)

(define (inc-x!) (set! x (+ x 1)))

(define x 0)

(provide/contract [inc-x! (-> void?)]

[get-x (-> integer?)])

This is a bug we hope to address in a future release.