Procedure variables and scope in virtual pascal

[bryce]

Hi, having some trouble with procedural variables and their scope.

Heres some example code:

Code: Pascal  [Select][+][-]

1. PROGRAM Test;
2. TYPE
3.    t_Action = PROCEDURE;
4.  
5. PROCEDURE Menu (Action : t_Action);
6.    BEGIN
7.       Action;
8.    END;
9.  
10. PROCEDURE CallMenu;
11.    VAR
12.       Value : LONGINT;
13.       Action : t_Action;
14.    
15.    PROCEDURE DoSomething;
16.       BEGIN
17.          WRITE('Value: ',Value);
18.       END;
19.  
20.    BEGIN
21.       Value := 1;
22.       @Action := @DoSomething;
23.       Menu(Action);
24.    END;
25.  
26. BEGIN
27.    CallMenu;
28.    READLN;
29. END.

I would expect the output to be 1, but instead its a random value, as if I have not initialized Value. How do I initialize value in a way the nested procedure can still see it.


I have tried looking in the sources for a solution, the only thing I could find was some assembly code that 'preserves local variables', but I had no idea how it worked or if it was even relevant to what I want to do.


Any help or resources would be greatly appreciated 

[MarkMLl]

Does this relate to Virtual Pascal or Free Pascal (FPC)?

MarkMLl

[bryce]

I am using virtual pascal, and have compiled this using virtual pascal, but it may work the same in free pascal.

[MarkMLl]

This forum is specifically for Free Pascal and related projects.

[bryce]

This forum is specifically for Free Pascal and related projects.

Free pascal is very similar to virtual pascal. As there are no resources available online for virtual pascal I was hoping my mistake could be identified or some guidance could be given.

[marcov]

I made your example working for Free Pascal, if Virtual Pascal is simular enough you can port it to Virtual Pascal

Code: Pascal  [Select][+][-]

1.  
2.     PROGRAM Test;
3. {$modeswitch nestedprocvars}  
4. // docs see https://www.freepascal.org/docs-html/ref/refse17.html
5.     TYPE
6.        t_Action = PROCEDURE is nested;
7.      
8.     PROCEDURE Menu (Action : t_Action);
9.        BEGIN
10.           Action;
11.        END;
12.      
13.     PROCEDURE CallMenu;
14.        VAR
15.           Value : LONGINT;
16.           Action : t_Action;
17.        
18.        PROCEDURE DoSomething;
19.           BEGIN
20.              WRITE('Value: ',Value);
21.           END;
22.      
23.        BEGIN
24.           Value := 1;
25.           Action := @DoSomething;
26.           Menu(Action);
27.        END;
28.      
29.     BEGIN
30.        CallMenu;
31.        READLN;
32.     END.
33.  
34.  

P.s. afaik this is a feature that VP doesn't have, and it won't work. The fact that you had to add so many @'s was a dead give away that the type safety was violated, and thus unreliable.

The problem is that your procedure is nested, but you use a "global procedure" type declaration when declarating the procedural type (and VP has no other one to my best knowledge, no way to define it as nested) FPC does, the "is nested" in the example.

p.s.2 VP afaik still has a facebook group somewhere that provides support. I suggest you try there or switch to Free Pascal, as you probably should anyway.