delegate from lambda expression

[timotheecour]

I'd like to achieve the following:
----
import std.stdio,std.range,std.algorithm,std.array;
void main(){
    auto dg=a=>a*2;
    auto a=iota(0,10);
    writeln(a.map!dg.array);
}
----
but this doesn't compile:
Error: variable [...]dg type void is inferred from initializer 
delegate (__T26 a)
{
return a * 2;
}
, and variables cannot be of type void

However this works:
    writeln(a.map!(a=>a*2).array);
but I want to reuse dg in other expressions (and avoid repeating 
myself)
Also, I want to avoid using string litteral enum dg=`a*2` as in 
my case dg is much more complicated and this is cleaner without a 
string IMHO.

My questions:
1) why can't the compiler infer the type int(int) for dg?
2) how to convert a lambda a=>a*2 to a delegate or function?
3) if 2 is not possible, how to achieve what I'm trying to do 
WITHOUT having to make the type explicit as in "int delegate(int) 
dg=(int a){return a*2;}"?
4) is there a way to have template delegates, and what's the 
syntax in this simple example?