Interface and delegate D2.0

[Neil Vice]

"Oliver Rübenkönig" <oliver.ruebenkoenigREM at web.de> wrote in message 
news:fr5epu$bos$1 at digitalmars.com...
> Hello Everyone,
>
> the following code segment works. However, I am not sure I understand why 
> it does work. Also, I have some questions regarding way I did it.
>
> I'd appreciate all sorts of comments you might have. Thanks.
>
> ---------------
>
> import std.stdio;
>
> //alias Expr delegate ( ) aFn;
> alias Expr delegate ( Expr ) aFn;
>
> interface Expr {
>    void print();
>    aFn analize();
> }
>
> Expr eval( Expr e, Expr env ) {
>    //return e.analize()();
>    return e.analize()( env );
> }
>
> class String : Expr {
> public:
>    this( const char [] stringData )    { this.itsStringData = 
> stringData; }
>    void print()                        { writef("\"", 
> this.itsStringData,"\""); }
>    //aFn analize()                     { return { return 
> cast(Expr)this; }; }
>    aFn analize()                       { Expr help( Expr env ) {
>                                                this.print();
>                                                return cast(Expr)env; }
>                                            return &help;
>                                         }
> private:
>    const char [] itsStringData;
> }
>
> String makeString( const char [] stringData ) {
>    return new String( stringData );
> }
>
> int main( char[][] arg ) {
>
>    writefln();
>    Expr s = makeString("Wow");
>    Expr s2 = makeString("!");
>    s.print();
>    writefln();
>    //s.analize()().print();
>    s.analize()(s2).print();
>    writefln();
>    eval( s, s2 ).print();
>    writefln();
>
>    return 0;
> }
>
>
> ---------------

Firstly a disclaimer: the following comments are based on D2.012.

> 1) In the String class is it possible to avoid the use of the "help" 
> funcntion?

You can make use of the following syntax:

    aFn analize()
    {
        return delegate Expr (Expr env) { this.print(); return env; }
    }

> 2) In the String class: why do I need to cast this to Expr - String is 
> derived from Expr and should fit - should it not?

I believe it's because the return type of the inline delegate is determined 
from your return statement. The syntax to define the return type is in the 
above example.

> 3) Is it possible to avoid the double brackets in Expr.analize()(Expr)? 
> Something like Expr.analize(Expr)? I assume this could be done with an 
> alias, are still the other ways? - well eval is another way. I think i am 
> asking if you see a way to do the same  thing with less brackets.

I have the same gripe myself and haven't found any work around.

Neil