Covariance fix (was: Idea: Lazy upcasting)

[Marcin Kuszczak]

Recently I have found that proposed solution is already almost implemented
in D. It is possible to use 

typeof(this) 

in definition of return type of function. The only problem with it is that
when instantiating derived class, 'this' points to base class **NOT** to
derived class, so that type of function is still pointing to base class
although it should point to derived class. I think it is quite unlogical
and unintuitive (in instance of derived class this points to derived
class). Fixing it would decrease number of unnecessary casts.

I hope it could be fixed.

Best Regards
Marcin Kuszczak
(aarti_pl)

/*************************************************************************/

Marcin Kuszczak wrote:

> Hello!
> 
> Any chance to get Walter interested with this? It seems that he has head
> so full of macros and constness that so small improvements will not easily
> catch him :-)
> 
> But anyway, I will try <g>
> 
> ******************************************************
> Subject:
> ---
> Proposition of fixing covariance design in D
> 
> Current problems:
> ---
> 1. It is necessary to make "copy & paste" implementation for base
> functions in derived classes just to return different (covariant) type
> from overriden method
> 2. Syntax is quite unintuitive: In base class you declare method with
> signature containing Base class name, but in derived classes you need to
> use in signature inherited class name.
> 
> Proposed solution:
> ---
> class Base {
>         this foo(int i) { return this; } // function declared as returning
>         "this"
>                                          // in base class
>         this other() { return this; }    // same as above
> }
>  
> class Derived : Base {
>         this foo(int i) { return this; } // function overriding base class
>                                          // returns Derived
>         this bar(int i) { return this; } // another function which will
>         return
>                                          // Derived
> 
>         Base foo1() {} // foo1 returns always Base class instance
>         Derived bar() {} // always returns Derived
>         // in derived class method 'other' from base class is also
>         // available. It returns Derived.
> }
>  
> void main() {
>         Derived d = (new Derived).foo(1).bar(2).other; // no problem here
> }
>  
>  
> Return type 'this' should be just another special return type, like 'void'
> type is currently. This special type shows precisely what will be returned
> from function. It is possible to say exactly what return type will be just
> seeing method (in class) declaration. It's consistent and looks good. It's
> also probably even easier to work with it for compiler.
>  
> Advantages:
> ---
> 1. No problem with loosing information about class type. Currently it is
> lost due to implicit upcasting from derived classes to base classes.
> 2. No need to reimplement same function just to get different return type.
> 3. Clean syntax. You have same signature in base, and in derived classes.
> 4. Possibility to extend syntax also for method parameters and class
> members.
> 
> Use cases:
> ---
> 1. Chaining of method calls from different base and derived classes.
> 2. All interfaces to classes which should return itself - problem for
> library classes which can be overriden by user.
> 
> Counter propositions:
> ---
> 1. Use template mixins to insert implementation into derived classes
> (requires from user of library to remember to mix in parameters to derived
> class)
> 2. Using template magic to achieve proper behaviour (neither easy nor
> intuitive; requires from user of library to use and understand quite
> advanced subjects). Examples in original thread.
> 
> References:
> ---
> 1. Thread:
>
http://www.digitalmars.com/webnews/newsgroups.php?art_group=digitalmars.D&article_id=51201
> 
> ******************************************************
> 
> BTW - some officially blessed by Walter system for tracking enhancement
> requests would be great (bugzilla has its own problems with this, as it
> was already mentioned before on NG).
> 

-- 
Regards
Marcin Kuszczak (Aarti_pl)
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