implementing read-only value parameters using the '-' marker (oakwood guidelines 5.13)

declaring a parameter as 'name-: TYPE' passes it by reference for
efficiency but prevents assignment within the procedure body.
per xds oberon-2 documentation, '-' is a modifier on value parameters
only; combining it with VAR is an error (246).

adding 'readOnlyPar = 3' to the vis field constants on OPT.Object.
setting node^.readonly in OPB.Ident for VarPar nodes whose object
has vis = readOnlyPar, which causes OPB.Assign to emit err(76) on
any attempt to write the parameter or its components.

relaxing OPB.Param to allow passing a read-only variable as an
actual argument to a read-only formal (previously blocked by err(76)
because VarPar checked ap^.readonly unconditionally).

adding Srpar = 42 symbol file tag so that read-only parameters
survive across module boundaries: OutSign writes Srpar and InSign
reconstructs the mode and vis correctly when importing.

code generation in OPV is unchanged; read-only parameters produce
the same C as VAR parameters since the restriction is purely semantic.

src/compiler/OPB.Mod

@@ -24,7 +24,7 @@ MODULE OPB;  (* RC 6.3.89 / 21.2.94 *)  (* object model 17.1.93 *)
     CASE obj^.mode OF
     | OPT.Var:    node := OPT.NewNode(OPT.Nvar);
                   node^.readonly := (obj^.vis = OPT.externalR) & (obj^.mnolev < 0)
-    | OPT.VarPar: node := OPT.NewNode(OPT.Nvarpar)
+    | OPT.VarPar: node := OPT.NewNode(OPT.Nvarpar); node^.readonly := obj^.vis = OPT.readOnlyPar
     | OPT.Con:    node := OPT.NewNode(OPT.Nconst);
                   node^.conval := OPT.NewConst();
                   node^.conval^ := obj^.conval^  (* string is not copied, only its ref *)
@@ -1390,7 +1390,7 @@ MODULE OPB;  (* RC 6.3.89 / 21.2.94 *)  (* object model 17.1.93 *)
         IF NotVar(ap) THEN err(122)
         ELSE CheckLeaf(ap, FALSE)
         END ;
-        IF ap^.readonly THEN err(76) END ;
+        IF ap^.readonly & (fp^.vis # OPT.readOnlyPar) THEN err(76) END ;
         IF fp^.typ^.comp = OPT.DynArr THEN DynArrParCheck(fp^.typ, ap^.typ, TRUE)
         ELSIF (fp^.typ^.comp = OPT.Record) & (ap^.typ^.comp = OPT.Record) THEN
           q := ap^.typ;

src/compiler/OPP.Mod

@@ -206,7 +206,13 @@ MODULE OPP;  (* NW, RC 6.3.89 / 10.2.94 *)  (* object model 4.12.93 *)
         LOOP
           IF sym = OPS.ident THEN
             OPT.Insert(OPS.name, par); OPS.Get(sym);
-            par^.mode := mode; par^.link := NIL;
+            IF sym = OPS.minus THEN
+              IF mode = OPT.VarPar THEN err(246) END;
+              OPS.Get(sym); par^.mode := OPT.VarPar; par^.vis := OPT.readOnlyPar
+            ELSE
+              par^.mode := mode; par^.vis := OPT.internal
+            END;
+            par^.link := NIL;
             IF first = NIL THEN first := par END ;
             IF firstPar = NIL THEN firstPar := par ELSE last^.link := par END ;
             last := par

src/compiler/OPT.Mod

@@ -56,7 +56,7 @@ CONST
   SProc* = 8; CProc*  = 9; IProc* = 10; Mod* = 11; Head* = 12; TProc* = 13;
 
   (* Object.vis - module visibility of objects *)
-  internal* = 0; external* = 1; externalR* = 2;
+  internal* = 0; external* = 1; externalR* = 2; readOnlyPar* = 3;
 
   (* Object.history - History of imported objects *)
   inserted* = 0; same* = 1; pbmodified* = 2; pvmodified* = 3; removed* = 4; inconsistent* = 5;
@@ -179,7 +179,7 @@ CONST
   Shdptr* = 27; Shdpro*  = 28; Stpro*   = 29; Shdtpro* = 30; Sxpro* = 31;
   Sipro*  = 32; Scpro*   = 33; Sstruct* = 34; Ssys*    = 35; Sptr*  = 36;
   Sarr*   = 37; Sdarr*   = 38; Srec*    = 39; Spro*    = 40; Slink* = 37;
-  Scomment* = 41;
+  Scomment* = 41; Srpar*    = 42;
 
 TYPE
   ImpCtxt = RECORD
@@ -835,7 +835,10 @@ BEGIN
     new := NewObj();
     new^.mnolev := -mno;
     IF last = NIL THEN par := new ELSE last^.link := new END;
-    IF tag = Svalpar THEN new^.mode := Var ELSE new^.mode := VarPar END;
+    IF tag = Svalpar THEN new^.mode := Var
+    ELSIF tag = Srpar THEN new^.mode := VarPar; new^.vis := readOnlyPar
+    ELSE new^.mode := VarPar
+    END;
     InStruct(new^.typ);
     new^.adr := OPM.SymRInt(); InName(new^.name);
     last := new;
@@ -1238,7 +1241,10 @@ END Import;
   BEGIN
     OutStr(result);
     WHILE par # NIL DO
-      IF par^.mode = Var THEN OPM.SymWInt(Svalpar) ELSE OPM.SymWInt(Svarpar) END;
+      IF par^.mode = Var THEN OPM.SymWInt(Svalpar)
+      ELSIF par^.vis = readOnlyPar THEN OPM.SymWInt(Srpar)
+      ELSE OPM.SymWInt(Svarpar)
+      END;
       OutStr(par^.typ);
       OPM.SymWInt(par^.adr);
       OutName(par^.name); par := par^.link

src/runtime/Errors.Txt

@@ -178,6 +178,7 @@ Compiler Warnings
 308 SYSTEM.VAL result includes memory past end of source variable; use SYSTEM.GET
 309 you should name this parameter type, or else no actual parameter will match
 310 redefining standard predefined type
+246 read-only parameter '-' cannot be combined with VAR
 
 Run-time Error Messages
  -1 assertion failed, cf. SYSTEM_assert