On Tue, Aug 3, 2010 at 20:59, Andrej Mitrovic <[email protected]>wrote:
> There seem to be some bugs with template constraints. Here's a reduce
> example (from TDPL) which will not compile:
>
>
> V reduce(alias fun, V, R)(V x, R range)
> if (is(typeof(x = fun(x, range.front)))
> && is(typeof(range.empty) == bool)
> && is(typeof(range.popFront())))
>
> I'm filing a bug unless something else is to blame here.
>
I think that's because you cannot directly take the type of a statement. The
assignment in the first typeof() is to blame. To make a statement into an
expression, transform it into an anonymous void delegate(): put it in braces
(with a semicolon at the end) and call it like a function, like this:
V reduce(alias fun, V, R)(V x, R range)
if (is({ typeof(x = fun(x, range.front);}()))
&& is(typeof(range.empty) == bool)
&& is(typeof(range.popFront())))
{...}
Or, more readable, wrap all the code you want to test into curly brackets
and evaluates its global return type:
V reduce(alias fun, V, R)(V x, R range)
if (is(typeof({ // I want to be able to do
that with an R and a V:
x = fun(x, range.front);
if (range.empty) {};
range.popFront();
}())))
{...}
It's a D idiom you'll see in many places in the standard library. I
personally find it a _bit_ heavy on parenthesis, even though I like Lisp.
Philippe
