I'm trying to test whether a template argument is the type returned by
takeExactly, and I haven't been able to sort out the template voodoo required
yet. It would be a lot easier if I had a variable to work with, but I just
have the type, and the fancy is expression required to pull it off is fancy
enough that I haven't been able to sort it out yet. At present, I have this:
import std.range;
import std.stdio;
template Hello(R)
if(is(R r == U, V, V w, U = typeof(takeExactly(w, 1))))
{
alias R Hello;
}
void main()
{
auto str = "hello";
auto t = takeExactly(str, 3);
writeln(t);
Hello!(typeof(t)) h = t;
writeln(h);
}
I need Hello to instatiate if R is the type returned by takeExactly and fail
to instantiate otherwise. At present, the code gives these compilation errors:
q.d(15): Error: template instance Hello!(Result) Hello!(Result) does not match
template declaration Hello(R) if (is(R r == U,V,V w,U =
typeof(takeExactly(w,1))))
q.d(15): Error: Hello!(Result) is used as a type
q.d(16): Error: template std.stdio.writeln does not match any function
template declaration
q.d(16): Error: template std.stdio.writeln(T...) cannot deduce template
function from argument types !()(_error_)
So, clearly I don't have the is expression right, and this is seriously
pushing the edge of my knowledge of is expressions. So, any help would be
appreciated. Thanks.
- Jonathan M Davis
