How to implement generic TListNode?

[Kyun]

I attempted to learn data structures using Pascal, but soon ran into problems.

Code: Pascal  [Select][+][-]

1. type
2.   generic TListNode<T> = record
3.     Value: T;
4.     Prev: TListNode<T>; // error
5.     Next: TListNode<T>; // error
6.   end;
7.  

I don't know what kind of grammar to use here.

[ALLIGATOR]

Code: Pascal  [Select][+][-]

1.   generic TListNode<T> = record
2.     Value: T;
3.     Prev: Pointer;
4.     Next: Pointer;
5.   end;
6.  

or

Code: Pascal  [Select][+][-]

1. program app;
2. {$modeswitch advancedrecords}
3.  
4. type
5.   generic TListNode<T> = record
6.   type
7.     TSelf = specialize TListNode<T>;
8.     PSelf = ^TSelf;
9.   public
10.     Value: T;
11.     Prev: PSelf;
12.     Next: PSelf;
13.   end;
14.  

[TRon]

but soon ran into problems.

ALLIGATOR showed possible solutions. Reason you got stuck is described here which basically tells that it is not possible to forward types (unless using a pointer to the type).

[Thaddy]

Yes. Prefer the second example over the typeless pointer, though.
The reason that it is the correct way, using a typed pointer,  is that when you would use a record type itself you will get an endless loop if the compiler would not prevent it. And an untyped pointer is almost never a good idea.
Using a record itself would expand into infinity..... (Think!)

[Kyun]

Code: Pascal  [Select][+][-]

1.   generic TListNode<T> = record
2.     Value: T;
3.     Prev: Pointer;
4.     Next: Pointer;
5.   end;
6.  

or

Code: Pascal  [Select][+][-]

1. program app;
2. {$modeswitch advancedrecords}
3.  
4. type
5.   generic TListNode<T> = record
6.   type
7.     TSelf = specialize TListNode<T>;
8.     PSelf = ^TSelf;
9.   public
10.     Value: T;
11.     Prev: PSelf;
12.     Next: PSelf;
13.   end;
14.  

Thanks, Pointer like C++'s void*

[Kyun]

but soon ran into problems.

ALLIGATOR showed possible solutions. Reason you got stuck is described here which basically tells that it is not possible to forward types (unless using a pointer to the type).

Thank you, this document is very useful.