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?

Best,
Graham

On Mon, 07 Jun 2010 23:02:48 -0400, Graham Fawcett <fawcett@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?

-Steve

On 06/07/2010 10:02 PM, Graham Fawcett 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?
>
> Best,
> Graham
>

Curious. in std.array, for string types front is defined as

dchar front(A)(A a);

ick. unicode.

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@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