Stop users reassigning '~'

src/compiler.c

@@ -2610,13 +2610,13 @@ CVar CompListTildeExpr (
     list = CompListExpr1( expr );
 
     /* assign the list to '~'                                              */
-    Emit( "AssGVar( Tilde, %c );\n", list );
+    Emit( "AssGVarUnsafe( Tilde, %c );\n", list );
 
     /* evaluate the subexpressions into the list value                     */
     CompListExpr2( list, expr );
 
     /* restore old value of '~'                                            */
-    Emit( "AssGVar( Tilde, %c );\n", tilde );
+    Emit( "AssGVarUnsafe( Tilde, %c );\n", tilde );
     if ( IS_TEMP_CVAR( tilde ) )  FreeTemp( TEMP_CVAR( tilde ) );
 
     /* return the list value                                               */
@@ -2828,13 +2828,13 @@ CVar CompRecTildeExpr (
     rec = CompRecExpr1( expr );
 
     /* assign the record value to the variable '~'                         */
-    Emit( "AssGVar( Tilde, %c );\n", rec );
+    Emit( "AssGVarUnsafe( Tilde, %c );\n", rec );
 
     /* evaluate the subexpressions into the record value                   */
     CompRecExpr2( rec, expr );
 
     /* restore the old value of '~'                                        */
-    Emit( "AssGVar( Tilde, %c );\n", tilde );
+    Emit( "AssGVarUnsafe( Tilde, %c );\n", tilde );
     if ( IS_TEMP_CVAR( tilde ) )  FreeTemp( TEMP_CVAR( tilde ) );
 
     /* return the record value                                             */

src/exprs.c

@@ -1087,13 +1087,13 @@ Obj             EvalListTildeExpr (
     list = ListExpr1( expr );
 
     /* assign the list to '~'                                              */
-    AssGVar( Tilde, list );
+    AssGVarUnsafe( Tilde, list );
 
     /* evaluate the subexpressions into the list value                     */
     ListExpr2( list, expr );
 
     /* restore old value of '~'                                            */
-    AssGVar( Tilde, tilde );
+    AssGVarUnsafe( Tilde, tilde );
 
     /* return the list value                                               */
     return list;
@@ -1441,13 +1441,13 @@ Obj             EvalRecTildeExpr (
     rec = RecExpr1( expr );
 
     /* assign the record value to the variable '~'                         */
-    AssGVar( Tilde, rec );
+    AssGVarUnsafe( Tilde, rec );
 
     /* evaluate the subexpressions into the record value                   */
     RecExpr2( rec, expr );
 
     /* restore the old value of '~'                                        */
-    AssGVar( Tilde, tilde );
+    AssGVarUnsafe( Tilde, tilde );
 
     /* return the record value                                             */
     return rec;

src/gvars.c

@@ -174,6 +174,29 @@ Obj             ErrorMustHaveAssObjHandler (
 
 static Obj REREADING;                   /* Copy of GAP global variable REREADING */
 
+/****************************************************************************
+**
+** AssGVarUnsafe(<gvar>,<val>)
+**
+** Assign to a global variable with no safety checks
+** - Does not check if the variable is readonly
+** - Does not check if it is automatic, has internal copies or fopies
+**
+** The current main use of this function is to handle the `~` variable,
+** both for speed and because it is marked read-only to avoid users editing
+** it.
+*/
+
+void            AssGVarUnsafe (
+    UInt                gvar,
+    Obj                 val )
+{
+    /* assign the value to the global variable                             */
+    VAL_GVAR(gvar) = val;
+    CHANGED_BAG( ValGVars );
+
+}
+
 void            AssGVar (
     UInt                gvar,
     Obj                 val )
@@ -186,10 +209,16 @@ void            AssGVar (
     /* make certain that the variable is not read only                     */
     while ( (REREADING != True) &&
             (ELM_PLIST( WriteGVars, gvar ) == INTOBJ_INT(0)) ) {
-        ErrorReturnVoid(
-            "Variable: '%s' is read only",
-            (Int)CSTR_STRING( ELM_PLIST(NameGVars,gvar) ), 0L,
-            "you can 'return;' after making it writable" );
+
+        if(gvar == Tilde) {
+                ErrorMayQuit("'~' cannot be assigned",0L,0L);
+        }
+        else {
+            ErrorReturnVoid(
+                "Variable: '%s' is read only",
+                (Int)CSTR_STRING( ELM_PLIST(NameGVars,gvar) ), 0L,
+                "you can 'return;' after making it writable" );
+        }
     }
 
     /* assign the value to the global variable                             */
@@ -1134,6 +1163,8 @@ static Int PostRestore (
     /* create the global variable '~'                                      */
     Tilde = GVarName( "~" );
 
+    /* stop unauthorised changes to '~'                                    */
+    MakeReadOnlyGVar(Tilde);
 
     /* update fopies and copies                                            */
     UpdateCopyFopyInfo();

src/gvars.h

@@ -96,6 +96,17 @@ extern  void            AssGVar (
             Obj                 val );
 
 
+/****************************************************************************
+**
+*F  AssGVarUnsafe(<gvar>,<val>) . . assign to a global variable with checks
+**
+**  'AssGVarUnsafe' assigns the value <val> to the global variable <gvar>
+**  without readonly checks or copie/fopie checking.
+*/
+extern  void            AssGVarUnsafe (
+            UInt                gvar,
+            Obj                 val );
+
 /****************************************************************************
 **
 *F  ValAutoGVar(<gvar>) . . . . . . . .  value of a automatic global variable

src/intrprtr.c

@@ -2325,7 +2325,7 @@ void            IntrListExprBegin (
         old = VAL_GVAR( Tilde );
         if ( old != 0 ) { PushObj( old ); }
         else            { PushVoidObj();  }
-        AssGVar( Tilde, list );
+        AssGVarUnsafe( Tilde, list );
     }
 
     /* push the list                                                       */
@@ -2398,7 +2398,7 @@ void            IntrListExprEnd (
     if ( top ) {
         list = PopObj();
         old = PopVoidObj();
-        AssGVar( Tilde, old );
+        AssGVarUnsafe( Tilde, old );
         PushObj( list );
     }
 
@@ -2538,7 +2538,7 @@ void            IntrRecExprBegin (
         old = VAL_GVAR( Tilde );
         if ( old != 0 ) { PushObj( old ); }
         else            { PushVoidObj();  }
-        AssGVar( Tilde, record );
+        AssGVarUnsafe( Tilde, record );
     }
 
     /* push the record                                                     */
@@ -2621,7 +2621,7 @@ void            IntrRecExprEnd (
     if ( top ) {
         record = PopObj();
         old = PopVoidObj();
-        AssGVar( Tilde, old );
+        AssGVarUnsafe( Tilde, old );
         PushObj( record );
     }
 }

tst/testinstall/tilde.tst

@@ -0,0 +1,28 @@
+gap> START_TEST("tilde.tst");
+gap> aqq~ := 1;
+Error, Variable: 'aqq' must have a value
+Syntax error: ; expected in stream:1
+aqq~ := 1;
+   ^
+gap> ~a := 1;
+Error, Variable: '~' must have a value
+Syntax error: ; expected in stream:1
+~a := 1;
+ ^
+gap> ~ := 1;
+Error, '~' cannot be assigned
+gap> l := [2, ~];
+[ 2, ~ ]
+gap> l = l[2];
+true
+gap> r := rec(x := ~, y := [1,2,~]);
+rec( x := ~, y := [ 1, 2, ~ ] )
+gap> r.x;
+rec( x := ~, y := [ 1, 2, ~ ] )
+gap> r.y;
+[ 1, 2, rec( x := ~[3], y := ~ ) ]
+gap> r.x;
+rec( x := ~, y := [ 1, 2, ~ ] )
+gap> r.y[3];
+rec( x := ~, y := [ 1, 2, ~ ] )
+gap> STOP_TEST( "tilde.tst", 1);