Lazy variadic not working, any alternatives?

[John Colvin via Digitalmars-d-learn]

On Tuesday, 26 May 2015 at 05:22:26 UTC, Tofu Ninja wrote:
> So I was writing a simple parser and I wanted a functionality 
> that was basically "try list of tokens in order and if any of 
> them fail, rewind input".
>
> I tried using a lazy variadic function:
>
> bool tok_and(lazy bool[] terms ...)
> {
> 	auto backup = getInputLocation();
> 	for(int i = 0; i < terms.length; i++)
> 	{
> 		if(terms[i] == false)
> 		{
> 			rewind(backup);
> 			return false;
> 		}
> 	}
> 	return true;
> }
>
> But this does not work because of BUG9110
> https://issues.dlang.org/show_bug.cgi?id=9110
>
> Any one have an idea how to achieve similar functionality 
> without a bunch of boilerplate at the call site? The lazy 
> version would have been nice because it would have allowed for:
>
> if(tok_and(ident(), equal(), expression())) {...}
> else if(tok_and(some(), other(), grammar())) {...}
> else ...

Something like this appears to work:

import std.typetuple : allSatisfy;

enum implicityConvertibleToBool(T) = is(T : bool);

bool tok_and(Args...)(lazy Args terms)
if(allSatisfy!(implicitlyConvertibleToBool, Args))
{
	auto backup = getInputLocation();
	foreach(term; terms)
	{
		if(term == false)
		{
			rewind(backup);
			return false;
		}
	}
	return true;
}