Generics proposals

Why

• Typesafety !
• Speed
• Readability

See "Already existing stuff" for possible but lacking ways right now.

Usage example

(dannym proposal)

 type
 TGenericList<T> = class
 public
   procedure Add(stuff: T);
 end;

 var
   intlist: TGenericList<Integer>;
 begin
   intlist := TGenericList<Integer>.Create;
 end;

<plugwash> tmycollection=template(tobject)[x] <plugwash> tintegercollection=tmycollection[integer]

oliebol:

 template blaat<t:integer>(a:t);
 var x : t;
 begin
   do something with a, and use t with it
 end;

Terms

type user:

• the function in the compiler that defines a variable or return type and thus 'uses a (already defined) type'.

immediate type:

• not generic and not specialized, i.e. 'normal type'

generic type:

• template for a class with some unspecified types, never to be filled in into this class type. Only by specialisation.

specialization:

• copy generic type class to new specialized type class

type parameters:

• T and Q, the unknown types for the generic TSomeGeneric<T,Q>

specialized type:

• a generic type with all type parameters know, written into a real class type (tree) and reused if possible

Changes

Changes in class/record definition representation

Each class definition representation has added fields for:

 - class instantiation mode
    0  immediate type
    1  generic type, no instantiation, just generate specialized type
    2  specialized type

 - when mode 2:
     generic type uplink (XXXX, what, unitname, typename?)
   when mode 1:
     list of specialized types known so far (and where),
     XXXX list items
   when mode 0: 
     nothing

Changes in 'type user' compilation (for class/record types only)

Each class type user will have to check mode of the class type.

For the used class type, If mode is immediate

 - proceed as always till now.

If mode is specialized

 - proceed as always till now 
 - keep in mind the generic type for some checks later (and debugging).

If mode is generic, this:

 - check 'list of specialized types' for the type parameter to use.
   If there is already a specialized type, use that.
   (given that it is compatible with 'compile parameters' XXXX)

 - If not available, clone the generic type in the tree, and fill in the
   type params with the actual types to use.
   Write that out to XXXX ppu file.
   Remember the new specialized type for later in the list by the 
   generic type and type params used.

(the generic type is best written to some ppu as parse tree so that it can be refetched for cloning somewhen. To keep it easy, initially it should be limited to having its .pas file around when wanting to use a generic)

type params

Generics store a list of the type params (names).

 ('T', 'Q')

And Specializations store a list of type mappings from the real types to the type params, in the way of

 type
   T = Integer;
   Q = Boolean;

of course these are local to this class/record specialization.

Other things to note (for *later*)

What if a method implementation depends on the type used to know how the implementation should look like ?

 procedure TGenericList<T is TObject>.Add(value: T);
 procedure TGenericList<T is Integer>.Add(value: T);
 ?

or more cryptic like

 procedure TGenericList<TObject>.Add(value: T);
 procedure TGenericList<Integer>.Add(value: T);
 ?

How to do internal storage classes

 type
 PNode<T> = ^TNode<T>;
 TNode<T> = record
   prev,next: PNode<T>;
   data: T;
 end;

 type
 TGenericList<T> = class
 private
   head, tail: PNode<T>;
   cnt: Integer;
 public
   procedure Add(stuff: T);
 end;
 ?

This implies that generics are best implemented both for records and for classes alike! Also implies that pointer types need to support generic too.

Also one specialization should chain other specializations (probably also when deriving from a generic).

How to derive from the generic type

It would be good if generic types could be derived from.

 type
 TGenericEventyList<T> = class(TGenericList<T>)
 public
   procedure Add(value: T); override;
 end;
 ...

Interfaces should support generics too

type

 IBa<T> = interface
   procedure Add(value: T);
 end;

 TBa<T> = class(...,IBla<T>,IInterface)
 end;

(what to do about the guids of the specializations?)

Of course this has limitations to check, like

• the generic interface cannot be used, just its specializations
• the specialized interface cannot be used in a generic class interface list
• (the generic interface can be used in a generic class interface list)
• the generic interface itself is invalid to use anywhere (just as generic classes are) except for specializing and deriving from
• normal interfaces cannot derive from generic interfaces
• (generic interfaces can derive from generic interfaces) - given the type params are the same
• specialized interfaces cannot be derived from

Limitation of possible specialisations

 type
 TGenericList<T: TObject or Integer> = ...
 end;

Probably useful...

What to do with class functions

I dont know a whole lot of how class functions are stored internally. What happens to class functions in generics?

Implementation details

Sanity checks

• Generic types cannot be instantiated from
• (Generic types can be derived from)
• Specialized types cannot be derived from
• Normal types cannot use generic types in its definition
• (Normal types can use specialized types in its definition)
• There are no half specialized types (one type param specified, the other not)
• prevent loops while 'unpacking' specializations (loop counter field in every specialization)

Things to note and not forget

• A class or record generic can use itself as type within its definition

Example:

 type
 TBla<T> = class
   parent: TBla<T>;
 end;

• A class or record generic can use another generic as type within its definition

 type
 TBla<T> = class
   parent: TBlo<T>;
 end;

Implementation steps

• Extend parser to support '<T>' and '<T: bla or bla or bla>' and '<T,Q>' notation
• Change class/record/object/pointertype/interface parser to mark if it is a generic, a specialization or normal
• Change derivation handlers to do the sanity checks
• Add error messages for it
• Change ppu streaming routines to save the tree of generics to the ppu
• Change type user to instantiate specialized types for generics when used

Already existing stuff

metaclasses

 type
 TObjectClass = class of TObject;
 TTypedList = class
 private
   Fwanttype: TObjectClass;
   ...
 public
   constructor Create(itemtype: TObjectClass);
   procedure Add(item: TObject);
   ...
 end;
 constructor TTypedList.Create(itemtype: TObjectClass);
 begin
   Fwanttype := itemtype;
   ...
 end;
 procedure TTypedList.Add(item: TObject);
 begin
   assert(item.Class = Fwanttype);
   ...
 end;

 ...
 TSomeStuff = class
 end;
 ...
 Flist := TTypedList.Create(TSomeStuff);

Advantages

• already works

Disadvantages

• slow at runtime
• cannot do simple types