contract: avoid leaving observable in a bad state after update contract error

The original implementation applied val-set to the result of applying
update-proc to proc, which was too late. Instead, pass a proc' to
update-proc that applies val-set to the updated value, ensuring that
val-set is called before the box is mutated.

Author: Bogdanp

gui-easy-lib/gui/easy/contract.rkt

@@ -48,10 +48,9 @@
             [(obs? o)
              (check-init-value (obs-peek o) neg-party)
              (impersonate-obs o
-                              #:set
-                              (lambda (_ v)
-                                (check-updated-value v neg-party)
-                                v))]
+                              #:set (lambda (_ v)
+                                      (begin0 v
+                                        (check-updated-value v neg-party))))]
             [else
              (raise-blame-error
               #:missing-party neg-party
@@ -83,6 +82,7 @@
      #rx"expected: \\(>=/c 5\\)"
      (λ ()
        (obs-update! o sub1)))
+    (check-equal? (obs-peek o) 5)
     (check-exn
      #rx"promised: \\(>=/c 5\\)"
      (λ ()

gui-easy-lib/gui/easy/private/observable.rkt

@@ -100,13 +100,13 @@
                       #f ; witness
                       obs-update-value-box!
                       (and val-set
-                           (λ (o update-proc) ;; noqa
+                           (λ (o update-proc) ; update-proc: (a -> b) -> b ;; noqa
                              (do-impersonate-procedure
-                              update-proc
-                              (λ (proc)
-                                (values
-                                 (λ (v) (val-set o v))
-                                 proc)))))
+                              update-proc ; update-proc': (a -> b) -> c
+                              (λ (proc) ; proc: a -> b
+                                (λ (v) ; proc': a -> c
+                                  ; val-set: b -> c
+                                  (val-set o (proc v)))))))
                       set-obs-update-value-box!!
                       #f))
 

gui-easy-lib/info.rkt

@@ -1,7 +1,7 @@
 #lang info
 
 (define license 'BSD-3-Clause)
-(define version "0.19")
+(define version "0.19.1")
 (define collection "racket")
 (define deps
   '("base"