Handle circular lists in display procedure.

Fixes GitLab issue #78 "Cycles detection in REPL".
* DisplayFormat.java (schemeDisplayFormat): Set checkSharing.
(schemeDisplaySimpleFormat): New static.
* circlist.scm: Test display of circular list.

gnu/kawa/functions/ChangeLog

+2020-11-14  Per Bothner  <per@bothner.com>
+
+	* DisplayFormat.java (schemeDisplayFormat): Set checkSharing.
+	Fixes GitLab issue #78 "Cycles detection in REPL".
+	(schemeDisplaySimpleFormat): New static.
+
 2020-03-24  Per Bothner  <per@bothner.com>
 
 	* CompileArith.java (getReturnKind): Fix for DIVIDE_GENERIC.

gnu/kawa/functions/DisplayFormat.java

@@ -96,6 +96,8 @@ public class DisplayFormat extends GenericFormat
   public static final ThreadLocation outRadix
     = new ThreadLocation("out-radix");
 
+    public static final DisplayFormat schemeDisplaySimpleFormat
+        = new DisplayFormat(false, 'S');
     public static final DisplayFormat schemeDisplayFormat
         = new DisplayFormat(false, 'S');
 
@@ -106,6 +108,7 @@ public class DisplayFormat extends GenericFormat
     public static final DisplayFormat schemeWriteSharedFormat
         = new DisplayFormat(true, 'S');
     static {
+        schemeDisplayFormat.checkSharing = 0;
         schemeWriteFormat.checkSharing = 0;
         schemeWriteSharedFormat.checkSharing = 1;
     }

testsuite/ChangeLog

+2020-11-14  Per Bothner  <per@bothner.com>
+
+	* circlist.scm: Test display of circular list.
+
 2020-06-30  Duncan Mak <duncanmak@gmail.com>
 
 	* match1.scm: Test for match for nested literal.

testsuite/circlist.scm

@@ -8,3 +8,12 @@
 (write (circular-list 3 1 4 1 5))
 (newline)
 ;; Output: #0=(3 1 4 1 5 . #0#)
+
+;; GitLab issue #78
+(let ((x (list 1 (list "2") 3)))
+  (set-cdr! (cadr x) (cdr x))
+  (set-cdr! (cddr x) x)
+  (write x) (newline)
+  (display x) (newline))
+;; Output: #0=(1 . #1=(("2" . #1#) 3 . #0#))
+;; Output: #0=(1 . #1=((2 . #1#) 3 . #0#))