why is this cast necessary?

[Graham Fawcett]

Hi Steve,

On Mon, 07 Jun 2010 23:46:40 -0400, Steven Schveighoffer wrote:

> On Mon, 07 Jun 2010 23:02:48 -0400, Graham Fawcett <fawcett at uwindsor.ca>
> wrote:
> 
>> Hi folks,
>>
>> This program works as expected in D2:
>>
>>     import std.stdio;
>>     import std.algorithm;
>>
>>     T largestSubelement(T)(T[][] lol) {
>>       alias reduce!"a>b?a:b" max;
>>       return cast(T) max(map!max(lol));   // the cast matters...
>>     }
>>
>>     void main() {
>>       auto a = [[1,2,3],[4,5,6],[8,9,7]];
>>       assert (largestSubelement(a) == 9);
>>
>>       auto b = ["howdy", "pardner"];
>>       assert (largestSubelement(b) == 'y');
>>
>>       auto c = [[1u, 3u, 45u, 2u], [29u, 1u]]; 
>>       assert (largestSubelement(c) == 45u);
>>     }
>>
>> But if I leave out the 'cast(T)' in line 7, then this program will not
>> compile:
>>
>> lse.d(6): Error: cannot implicitly convert expression
>> 		  (reduce(map(lol))) of type dchar to immutable(char)
>> lse.d(14): Error: template instance
>> 		   lse.largestSubelement!(immutable(char)) error
>> instantiating
>>
>> Where did the 'dchar' came from? And why does the cast resolve the
>> issue?
> 
> In a recent update, Andrei changed char[] and wchar[] to bi-directional
> ranges of dchar instead of straight arrays (at least, I think that was
> the change) in the eyes of the range types.  I think this is where the
> dchar comes from.
> 
> If you had a char[], and the 'max' element was a sequence of 2 code
> points, how do you return a single char for that result?

Thank you. This makes sense.

Regards,
Graham