What are the differences between these forms of function-like declarations ?

[Anderel Cerol]

On Thursday, 6 September 2012 at 05:18:24 UTC, Anderel Cerol 
wrote:
>
>>
>> a real example is :
>> ....
>> import std.net.curl;
>> auto http=HTTP();
>> ...
>> http.onReceiveStatusLine(...);
>> ...
>>
>> the function http.onReceiveStatusLine(...) is exactly what the 
>> function on(...) is above.
>> The environment is dmd 2.060 Windows.
> plus, what passes to the http.onReceiveStatusLine is just 
> (a){a.writeln();} with different forms above.

by the way ,std.functional.toDelegate doesn't work as UFCS 
style,why ?