The Expressiveness of D

[Simen kjaeraas]

%u <user at web.news> wrote:

> class A {}
> class B : A {}
> class C : A {}
>
> template T(A...) { alias A T; }
>
> void main() {
>   auto a = true ? new B : new C;
> // these don't work - why?
> //  auto b = [new B, new C];
> //  auto c = { return [1: new B,2: new C]; };

"The type of the first element is taken to be the type of all the
elements, and all elements are implicitly converted to that type."
(http://digitalmars.com/d/2.0/expression.html#ArrayLiteral)

I believe it has been discussed numerous times before that the ?:
test should be used to find the element type - not sure why it
isn't.


>   T!(int,int) e = (1,2);
>   e = T!(3,4);
>
> // ah - so (1,2) syntax on initialization, T!(1,2) when assigning!
>   T!(int,int) d = T!(1,2);
>
>   e = d;
>
> // tuples aren't first class, why?
> //  auto f = { return e; };
> }

Compile-time argument tuples are something of a strange beast. They
behave not very unlike tuples, but due to their ability to hold
types, literals, expressions and aliases to whatever, they are not
a very good match for what you'd expect tuples to be. (e.g, what do
you expect T!(3,T) to be?)

For tuples, you should instead look into std.typecons' Tuple struct
and tuple function:

Tuple!( int, "n", string, "s" ) tup;
tup.n = 4;
tup.s = "A string! My kingdom for a string!";

auto tup2 = tuple( 1, 2 );
assert( is( typeof( tup2 ) == Tuple!( int, int ) ) );


For even better support of tuples, you should have a look-see at
Philippe Sigaud's dranges.tuple and dranges.reftuple
(http://svn.dsource.org/projects/dranges/trunk/dranges/docs/tuple.html
and  
http://svn.dsource.org/projects/dranges/trunk/dranges/docs/reftuple.html)

The latter is absolutely awesome, and should be made part of
phobos ASAP, IMO (though according to the documentation, it
is not in tip-top shape).

Examples from the reftuple page:


int a, b;
_(a,b) = tuple(b,a); // swap
_(a,b) = tuple(b,a+b); // fibonacci

int[] arr = [0,1,2,3,4];
int [] c;
_(a,b,c) = arr; // a = 0, b = 1, c = [2,3,4]



-- 
Simen