Using __traits(getMember...) in alias statement

[Tobias Pankrath]

It would be cool, if the following would be possible. 

immutable string MemberID = M;

struct A {}
struct B {
alias A M;
}

template Member(T)
{
static if(__traits(hasMember, T, MemberID))
{
alias __traits(getMember, T, MemberID) Member;
}
else
alias TypeTuple!() Member;
}

void main()
{
alias Member!(B) M;
}


Currently dmd will print basic type expected, not __traits.
Why isn't traits allowed here and if we allow __traits there, would this 
introduce any ambiguities?

Tobias

Re: Using __traits(getMember...) in alias statement

[Timon Gehr]

On 11/17/2011 06:12 PM, Tobias Pankrath wrote:

It would be cool, if the following would be possible.

immutable string MemberID = M;

struct A {}
struct B {
 alias A M;
}

template Member(T)
{
 static if(__traits(hasMember, T, MemberID))
 {
 alias __traits(getMember, T, MemberID) Member;
 }
 else
 alias TypeTuple!() Member;
}

void main()
{
 alias Member!(B) M;
}


Currently dmd will print basic type expected, not __traits.
Why isn't traits allowed here and if we allow __traits there, would this
introduce any ambiguities?



This works:

alias TypeTuple!(__traits(getMember, T, MemberID)) Member;

It is a mere grammar issue. The aliased symbol must look like a valid 
type. I'd like that to change too.


This helps a lot with the current state of affairs:

template ID(alias x){alias x ID;}


It will even allow funny things like this:

alias ID!((a,b){return a+b;}) add;

static assert(add(1,2) == 3);

Re: Using __traits(getMember...) in alias statement

[Tobias Pankrath]

 This helps a lot with the current state of affairs:
 
 template ID(alias x){alias x ID;}
 
 It will even allow funny things like this:
 
 alias ID!((a,b){return a+b;}) add;
 
 static assert(add(1,2) == 3);

Nice to know, thanks!

Re: Using __traits(getMember...) in alias statement

[Timon Gehr]

On 11/17/2011 06:41 PM, Timon Gehr wrote:

On 11/17/2011 06:12 PM, Tobias Pankrath wrote:

It would be cool, if the following would be possible.

immutable string MemberID = M;

struct A {}
struct B {
alias A M;
}

template Member(T)
{
static if(__traits(hasMember, T, MemberID))
{
alias __traits(getMember, T, MemberID) Member;
}
else
alias TypeTuple!() Member;
}

void main()
{
alias Member!(B) M;
}


Currently dmd will print basic type expected, not __traits.
Why isn't traits allowed here and if we allow __traits there, would this
introduce any ambiguities?



This works:

alias TypeTuple!(__traits(getMember, T, MemberID)) Member;


This only works if the __traits gets you a TypeTuple back. (It won't 
work in this case, it will create a one-element tuple). Better use ID 
from below.





It is a mere grammar issue. The aliased symbol must look like a valid
type. I'd like that to change too.

This helps a lot with the current state of affairs:

template ID(alias x){alias x ID;}


It will even allow funny things like this:

alias ID!((a,b){return a+b;}) add;

static assert(add(1,2) == 3);