From ecff9d969acef5a08d3963d16eac83a5210de86e Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Thu, 3 Mar 2022 13:07:50 +0100
Subject: [PATCH 1/2] printValue(): Don't show repeated values

Fixes #6157.
---
 src/libexpr/eval.cc  | 25 +++++++++++++------------
 src/libexpr/value.hh |  4 ++--
 src/nix/repl.cc      |  2 +-
 3 files changed, 16 insertions(+), 15 deletions(-)

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 193358161..d9d36fab2 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -86,15 +86,10 @@ RootValue allocRootValue(Value * v)
 }
 
 
-void printValue(std::ostream & str, std::set<const Value *> & active, const Value & v)
+void printValue(std::ostream & str, std::set<const Value *> & seen, const Value & v)
 {
     checkInterrupt();
 
-    if (!active.insert(&v).second) {
-        str << "<CYCLE>";
-        return;
-    }
-
     switch (v.internalType) {
     case tInt:
         str << v.integer;
@@ -120,10 +115,14 @@ void printValue(std::ostream & str, std::set<const Value *> & active, const Valu
         str << "null";
         break;
     case tAttrs: {
+        seen.insert(&v);
         str << "{ ";
         for (auto & i : v.attrs->lexicographicOrder()) {
             str << i->name << " = ";
-            printValue(str, active, *i->value);
+            if (seen.count(i->value))
+                str << "<REPEAT>";
+            else
+                printValue(str, seen, *i->value);
             str << "; ";
         }
         str << "}";
@@ -132,9 +131,13 @@ void printValue(std::ostream & str, std::set<const Value *> & active, const Valu
     case tList1:
     case tList2:
     case tListN:
+        seen.insert(&v);
         str << "[ ";
         for (auto v2 : v.listItems()) {
-            printValue(str, active, *v2);
+            if (seen.count(v2))
+                str << "<REPEAT>";
+            else
+                printValue(str, seen, *v2);
             str << " ";
         }
         str << "]";
@@ -161,15 +164,13 @@ void printValue(std::ostream & str, std::set<const Value *> & active, const Valu
     default:
         abort();
     }
-
-    active.erase(&v);
 }
 
 
 std::ostream & operator << (std::ostream & str, const Value & v)
 {
-    std::set<const Value *> active;
-    printValue(str, active, v);
+    std::set<const Value *> seen;
+    printValue(str, seen, v);
     return str;
 }
 
diff --git a/src/libexpr/value.hh b/src/libexpr/value.hh
index 3fdff71a5..84c4d09f8 100644
--- a/src/libexpr/value.hh
+++ b/src/libexpr/value.hh
@@ -114,8 +114,8 @@ struct Value
 private:
     InternalType internalType;
 
-friend std::string showType(const Value & v);
-friend void printValue(std::ostream & str, std::set<const Value *> & active, const Value & v);
+    friend std::string showType(const Value & v);
+    friend void printValue(std::ostream & str, std::set<const Value *> & seen, const Value & v);
 
 public:
 
diff --git a/src/nix/repl.cc b/src/nix/repl.cc
index 5c0d44c68..3a51a13e6 100644
--- a/src/nix/repl.cc
+++ b/src/nix/repl.cc
@@ -800,7 +800,7 @@ std::ostream & NixRepl::printValue(std::ostream & str, Value & v, unsigned int m
                 else
                     printStringValue(str, i.first.c_str());
                 str << " = ";
-                if (seen.find(i.second) != seen.end())
+                if (seen.count(i.second))
                     str << "«repeated»";
                 else
                     try {

From e9c04c3351b4b3acae2d154b9aba808c3600054f Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Thu, 3 Mar 2022 13:29:14 +0100
Subject: [PATCH 2/2] Be more aggressive in hiding repeated values

We now memoize on Bindings / list element vectors rather than Values,
so that e.g. two Values that point to the same Bindings will be
printed only once.
---
 src/libexpr/eval.cc  | 38 +++++++++++++++++++-------------------
 src/libexpr/value.hh |  2 +-
 2 files changed, 20 insertions(+), 20 deletions(-)

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index d9d36fab2..2d7309738 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -86,7 +86,7 @@ RootValue allocRootValue(Value * v)
 }
 
 
-void printValue(std::ostream & str, std::set<const Value *> & seen, const Value & v)
+void printValue(std::ostream & str, std::set<const void *> & seen, const Value & v)
 {
     checkInterrupt();
 
@@ -115,32 +115,32 @@ void printValue(std::ostream & str, std::set<const Value *> & seen, const Value
         str << "null";
         break;
     case tAttrs: {
-        seen.insert(&v);
-        str << "{ ";
-        for (auto & i : v.attrs->lexicographicOrder()) {
-            str << i->name << " = ";
-            if (seen.count(i->value))
-                str << "<REPEAT>";
-            else
+        if (!v.attrs->empty() && !seen.insert(v.attrs).second)
+            str << "<REPEAT>";
+        else {
+            str << "{ ";
+            for (auto & i : v.attrs->lexicographicOrder()) {
+                str << i->name << " = ";
                 printValue(str, seen, *i->value);
-            str << "; ";
+                str << "; ";
+            }
+            str << "}";
         }
-        str << "}";
         break;
     }
     case tList1:
     case tList2:
     case tListN:
-        seen.insert(&v);
-        str << "[ ";
-        for (auto v2 : v.listItems()) {
-            if (seen.count(v2))
-                str << "<REPEAT>";
-            else
+        if (v.listSize() && !seen.insert(v.listElems()).second)
+            str << "<REPEAT>";
+        else {
+            str << "[ ";
+            for (auto v2 : v.listItems()) {
                 printValue(str, seen, *v2);
-            str << " ";
+                str << " ";
+            }
+            str << "]";
         }
-        str << "]";
         break;
     case tThunk:
     case tApp:
@@ -169,7 +169,7 @@ void printValue(std::ostream & str, std::set<const Value *> & seen, const Value
 
 std::ostream & operator << (std::ostream & str, const Value & v)
 {
-    std::set<const Value *> seen;
+    std::set<const void *> seen;
     printValue(str, seen, v);
     return str;
 }
diff --git a/src/libexpr/value.hh b/src/libexpr/value.hh
index 84c4d09f8..d0fa93e92 100644
--- a/src/libexpr/value.hh
+++ b/src/libexpr/value.hh
@@ -115,7 +115,7 @@ private:
     InternalType internalType;
 
     friend std::string showType(const Value & v);
-    friend void printValue(std::ostream & str, std::set<const Value *> & seen, const Value & v);
+    friend void printValue(std::ostream & str, std::set<const void *> & seen, const Value & v);
 
 public: