From 114824d64eb534c5c7292382ce193b808ca4df4d Mon Sep 17 00:00:00 2001
From: Fabian Zickgraf <f.zickgraf@dashdos.com>
Date: Fri, 14 Oct 2022 17:38:00 +0200
Subject: [PATCH] Print nested sublist extractions correctly

* PrintElmsListLevel preserves the old behaviour of PrintElmsList for
  EXPR_ELMS_LIST_LEV.
* PrintElmsList applies to EXPR_ELMS_LIST and now checks if its first
  argument is of type EXPR_ELMS_LIST or EXPR_ELMS_LIST_LEV. If yes, the
  list is put in brackets.
---
 src/vars.c                   | 28 +++++++++++++++++++++++++++-
 tst/testinstall/function.tst | 23 +++++++++++++++++++++++
 2 files changed, 50 insertions(+), 1 deletion(-)

diff --git a/src/vars.c b/src/vars.c
index 9795f5c0b6..8aecee85d3 100644
--- a/src/vars.c
+++ b/src/vars.c
@@ -1104,6 +1104,32 @@ static void PrintIsbList(Expr expr)
 **  Linebreaks are preferred after the '{'.
 */
 static void PrintElmsList(Expr expr)
+{
+    Expr list = READ_EXPR(expr, 0);
+    Pr("%2>", 0, 0);
+    if (TNUM_EXPR(list) == EXPR_ELMS_LIST || TNUM_EXPR(list) == EXPR_ELMS_LIST_LEV) {
+        Pr("(", 0, 0);
+        PrintExpr(list);
+        Pr(")", 0, 0);
+    } else {
+        PrintExpr(list);
+    }
+    Pr("%<{", 0, 0);
+    PrintExpr(READ_EXPR(expr, 1));
+    Pr("%<}", 0, 0);
+}
+
+
+/****************************************************************************
+**
+*F  PrintElmsListLevel(<expr>) . . print a selection of several elements of a list
+**
+**  'PrintElmsListLevel'  prints the list  elements  expression  <expr> of the   form
+**  '<list>{<positions>}'.
+**
+**  Linebreaks are preferred after the '{'.
+*/
+static void PrintElmsListLevel(Expr expr)
 {
     Pr("%2>", 0, 0);
     PrintExpr(READ_EXPR(expr, 0));
@@ -2242,7 +2268,7 @@ static Int InitKernel (
     InstallPrintExprFunc( EXPR_ELM_LIST       , PrintElmList);
     InstallPrintExprFunc( EXPR_ELMS_LIST      , PrintElmsList);
     InstallPrintExprFunc( EXPR_ELM_LIST_LEV   , PrintElmListLevel);
-    InstallPrintExprFunc( EXPR_ELMS_LIST_LEV  , PrintElmsList);
+    InstallPrintExprFunc( EXPR_ELMS_LIST_LEV  , PrintElmsListLevel);
     InstallPrintExprFunc( EXPR_ISB_LIST       , PrintIsbList);
 
     // install executors, evaluators, and printers for matrix elements
diff --git a/tst/testinstall/function.tst b/tst/testinstall/function.tst
index 97edb76d9a..89391492c8 100644
--- a/tst/testinstall/function.tst
+++ b/tst/testinstall/function.tst
@@ -291,4 +291,27 @@ gap> funcloop(x -> (x in x) and x);
 function ( x ) return x in x and x; end
 gap> funcloop(x -> x in (x and x));
 function ( x ) return x in (x and x); end
+
+# nested sublist extractions
+# a single extraction
+gap> funcloop(x -> [ x ]{[ 1 ]});
+function ( x ) return [ x ]{[ 1 ]}; end
+
+# two extractions
+gap> funcloop(x -> ([ x ]{[ 1 ]}){[ 1 ]});
+function ( x ) return ([ x ]{[ 1 ]}){[ 1 ]}; end
+gap> funcloop(x -> [ [ x ] ]{[ 1 ]}{[ 1 ]});
+function ( x ) return [ [ x ] ]{[ 1 ]}{[ 1 ]}; end
+
+# three extractions
+gap> funcloop(x -> (([ x ]{[ 1 ]}){[ 1 ]}){[ 1 ]});
+function ( x ) return (([ x ]{[ 1 ]}){[ 1 ]}){[ 1 ]}; end
+gap> funcloop(x -> ([ [ x ] ]{[ 1 ]}{[ 1 ]}){[ 1 ]});
+function ( x ) return ([ [ x ] ]{[ 1 ]}{[ 1 ]}){[ 1 ]}; end
+gap> funcloop(x -> ([ [ x ] ]{[ 1 ]}){[ 1 ]}{[ 1 ]});
+function ( x ) return ([ [ x ] ]{[ 1 ]}){[ 1 ]}{[ 1 ]}; end
+
+# four extractions
+gap> funcloop(x -> ([ [ x ] ]{[ 1 ]}{[ 1 ]}){[ 1 ]}{[ 1 ]});
+function ( x ) return ([ [ x ] ]{[ 1 ]}{[ 1 ]}){[ 1 ]}{[ 1 ]}; end
 gap> STOP_TEST("function.tst", 1);