• m (33/33) Aug 22 2009 Hello,
• Jarrett Billingsley (10/40) Aug 22 2009 It's because of an (absolutely silly) implicit conversion of char to

m <m m.m> writes:

Hello,

i'm during reading of Learnt to Tango with D. In chapter 5 (about Templates)
there are few samples of using templates. I've got problem with one of them.

1. 
template List( T )
{
pragma( msg, "List( T )" );
}
template List( T : int )
{
pragma( msg, "List( T : int )" );
}
template List( T : T[] )
{
pragma( msg, "List( T : T[] )" );
}
void main()
{
alias List!(char) A;
alias List!(int) B;
alias List!(char[]) C;
}

Should produce messages (according to what is said in book):
 
List( T )
List( T : int )
List( T : T[] )

but i've got: 

List( T : int )
List( T : int )
List( T : T[])

Is there are any error, or what?

best regards
m

Jarrett Billingsley <jarrett.billingsley gmail.com> writes:

On Sat, Aug 22, 2009 at 12:11 PM, m<m m.m> wrote:
 Hello,

 i'm during reading of Learnt to Tango with D. In chapter 5 (about Templates)
there are few samples of using templates. I've got problem with one of them.

 1.
 template List( T )
 {
 pragma( msg, "List( T )" );
 }
 template List( T : int )
 {
 pragma( msg, "List( T : int )" );
 }
 template List( T : T[] )
 {
 pragma( msg, "List( T : T[] )" );
 }
 void main()
 {
 alias List!(char) A;
 alias List!(int) B;
 alias List!(char[]) C;
 }

 Should produce messages (according to what is said in book):

 List( T )
 List( T : int )
 List( T : T[] )

 but i've got:

 List( T : int )
 List( T : int )
 List( T : T[])

 Is there are any error, or what?

It's because of an (absolutely silly) implicit conversion of char to
int, and the way D does template specialization.  Because char is
implicitly convertible to int, D sees that both List!(T) and List!(T:
int) as valid instantiations for 'char'.  Then, it decides that since
List!(T: int) is more specialized, it should use it instead of
List!(T).  You'll also get the same inane List!(T: int) specialization
if you try List!(bool), since bool is implicitly convertible to int
for some reason.

It's extremely frustrating.