More bugs...

[Timon Gehr]

On 05/01/2012 04:02 AM, Mehrdad wrote:
> Wha..?!
>
> I can't believe delegates work so poorly in D...

It is not the delegates, it is the type deduction algorithm for implicit 
function template instantiation. The issue is that the parameters do not 
cross-talk, which means R is not known during matching: How to detect 
the argument type from bool delegate(ElementType!R) alone if R is not 
known? The obvious solution would be to use an actual inference 
algorithm that uses information that can be gained from other parameters 
as well as information already known from the current parameter.

You might want to file an enhancement request, because this exact thing 
seems to trip up many programmers. (i.e. it would be a lot more 
intuitive and convenient if it worked.)


> they're practically
> unusable unless you're passing them as template parameters (which brings
> on its own set of bugs)...

I haven't encountered those so far.

>
> Seems like every time I try to escape a bug somehow, another one pops up :(
>

The situation is improving. Furthermore, there is a very large subset of 
the language that is already very usable.

Another way to make your code compile:

private import std.range;
void filter(R,S)(R, bool delegate(S)) if(is(S==ElementType!R)){ }
void main() { [1, 2, 3].filter((int x) { return x < 3; }); }