Re: Testing for template argument being result of takeExactly

[Timon Gehr]

On 09/23/2012 01:54 AM, Jonathan M Davis wrote:

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

import std.range, std.traits;
import std.stdio;
template Hello(R) if(is(typeof(R._input.takeExactly(2)) == R)){
    alias R Hello;
}

void main(){
    auto str = "hello";
    auto t = takeExactly(str, 3);
    writeln(t);
    Hello!(typeof(t)) h = t;
    writeln(h);
}