From 3f4cbcbfd851b238d02c05d28395af1f4dbb18c1 Mon Sep 17 00:00:00 2001
From: Robby Findler <robby@racket-lang.org>
Date: Wed, 2 May 2018 18:53:48 -0500
Subject: [PATCH] memoize the creation of the name in the blame object

---
 racket/collects/racket/contract/private/blame.rkt | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/racket/collects/racket/contract/private/blame.rkt b/racket/collects/racket/contract/private/blame.rkt
index f745f7c0eb..4b6032e45d 100644
--- a/racket/collects/racket/contract/private/blame.rkt
+++ b/racket/collects/racket/contract/private/blame.rkt
@@ -76,10 +76,17 @@
            (unless positive
              (raise-type-error 'make-blame "(not/c #f)" 3
                                source value build-name positive negative original?))
+           (define build/memo-name
+             (let* ([uniq (box #f)]
+                    [ans uniq])
+               (λ ()
+                 (when (eq? uniq ans)
+                   (set! ans (build-name)))
+                 ans)))
            (make-blame
             source
             value
-            build-name
+            build/memo-name
             (list positive)
             (and negative (list negative))
             original?