Add record type printer for srfi-45.

* module/srfi/srfi-45.scm: Add record type printer for promises.
  (promise-visit): New helper for visiting lazy promises.

module/srfi/srfi-45.scm

@@ -39,7 +39,8 @@
              eager
              promise?)
   #:replace (delay force promise?)
-  #:use-module (srfi srfi-9))
+  #:use-module (srfi srfi-9)
+  #:use-module (srfi srfi-9 gnu))
 
 (cond-expand-provide (current-module) '(srfi-45))
 
@@ -76,3 +77,17 @@
 ;; (*) These two lines re-fetch and check the original promise in case
 ;;     the first line of the let* caused it to be forced.  For an example
 ;;     where this happens, see reentrancy test 3 below.
+
+(define* (promise-visit promise #:key on-eager on-lazy)
+  (define content (promise-val promise))
+  (case (value-tag content)
+    ((eager) (on-eager (value-proc content)))
+    ((lazy)  (on-lazy (value-proc content)))))
+
+(set-record-type-printer! promise
+  (lambda (promise port)
+    (promise-visit promise
+      #:on-eager (lambda (value)
+                   (format port "#<promise = ~s>" value))
+      #:on-lazy  (lambda (proc)
+                   (format port "#<promise => ~s>" proc)))))