evaluate expression only once for eval:check

scribble-lib/scribble/eval.rkt

@@ -259,19 +259,19 @@
                        (map (current-print) v))
                      (close-output-port out)
                      in)))])))
-  (define (do-ev s)
-    (with-handlers ([(lambda (x) (not (exn:break? x)))
-                     (lambda (e)
-                       (cons ((scribble-exn->string) e)
-                             (get-outputs)))])
-      (cons (render-value (do-plain-eval ev s #t)) (get-outputs))))
   (define (do-ev/expect s expect)
+    (define-values (val render+output)
+      (with-handlers ([(lambda (x) (not (exn:break? x)))
+                       (lambda (e)
+                         (cons ((scribble-exn->string) e)
+                               (get-outputs)))])
+        (define val (do-plain-eval ev s #t))
+        (values val (cons (render-value val) (get-outputs)))))
     (when expect
-      (let ([r (do-plain-eval ev s #t)]
-            [expect (do-plain-eval ev (car expect) #t)])
-        (unless (equal? r expect)
+      (let ([expect (do-plain-eval ev (car expect) #t)])
+        (unless (equal? val expect)
           (raise-syntax-error 'eval "example result check failed" s))))
-    (do-ev s))
+    render+output)
   (lambda (str)
     (if (eval-results? str)
         (list (map formatted-result (eval-results-contents str))