Testing for template argument being result of takeExactly

[Ali Çehreli]

On 09/23/2012 12:02 PM, monarch_dodra wrote:
 > On Saturday, 22 September 2012 at 23:53:28 UTC, Jonathan M Davis wrote:
 >> I'm trying to test...
 >> [SNIP]
 >> - Jonathan M Davis
 >
 > I *kind of* see what you are doing with U, V, W, but what's wrong with
 > just doing:
 >
 > ----
 > import std.range;
 >
 > template Hello(R)
 > if(is(typeof(R == typeof(takeExactly(R, 1)))))
 > {
 > alias R Hello;
 > }
 >
 > struct S;
 >
 > void main( ) {
 > Hello!(int[]) a; //OK
 > Hello!S b; //FAIL
 > }
 > ----
 > ?
 >
 > It seems to work for me..., but I'm not 100% sure there isn't something
 > illegal/unsafe in there.

The goal is to "test whether a template argument is the type returned by 
takeExactly."

Your solution (I think) is looking at a container and determining 
whether the type of that container can be compared to the return type of 
takeExactly(). (Note that typeof(X == Y) is used for checking whether 
that expression itself is valid.)

Also, I think your code should have passed a range like R.init to 
takeExactly, not R, which is a type:

   takeExactly(R.init, 1)

I don't know why your code compiles.

Ali