challenge: implement the max function

[Bruno Medeiros]

Don Clugston wrote:
> Alain wrote:
>> Don Clugston Wrote:
>>
>>> Andrei Alexandrescu (See Website For Email) wrote:
>>>> Here's a simple challenge: implement the max function. Requirements:
>>>>
>>>> a) generic
>>>> b) efficient
>>>> c) preserve lvalueness when possible such that one can write e.g.
>>>>
>>>> max(arr[0], arr[1]) *= 0.9;
>>>>
>>>> d) should accept two or more arguments
>>>> e) should return the "smartest" type, e.g. max of an unsigned int 
>>>> and unsigned long should return unsigned long
>>>> f) short and easy to understand
>>>>
>>>> I don't think it's possible to implement the function to the spec in 
>>>> current D, so designs are allowed to invent new features, as long as 
>>>> they define them thoroughly.
>>>>
>>>> Looking forward to any takers!
>>>>
>>>>
>>>> Andrei
>>> ----
>>> Here's an implementation I put into Tango a few days ago. Unlike some 
>>> other ones which have been posted, it does *not* require the types to 
>>> support operator '+'. This was only for two arguments, but I believe 
>>> it satisfies the other requirements. Should be easy to generalise.
>>>
>>> ----
>>>
>>> private {
>>>      // Implicitly convert to the smallest type which can store both 
>>> T and U; acts like an 'auto' template return type.
>>> template SharedComparisonType(T, U) {
>>>      static if (is( typeof( (T x, U y){ return y<x? y: x;}) Q == 
>>> return))
>>>          alias Q SharedComparisonType;
>>> }
>>> }
>>>
>>>
>>> /** Return the minimum of x and y.
>>>   *
>>>   * Note: If x and y are floating-point numbers, and either is a NaN,
>>>   * x will be returned.
>>>   */
>>> SharedComparisonType!(T, U) min(T, U)(T x, U y) {
>>>      return y<x? y : x;
>>> }
>>>
>>> /** Return the maximum of x and y.
>>>   *
>>>   * Note: If x and y are floating-point numbers, and either is a NaN,
>>>   * x will be returned.
>>>   */
>>> SharedComparisonType!(T,U) max(T, U)(T x, U y) {
>>>      return y>x? y : x;
>>> }
>>
>> Fine !
>> but it doesn't work for the following common use case:
>> int[] mylist=[1,2,3,4,5];
>> int mymax=max(mylist);
>>
>> Alain
> 
> Yes, as I said, it only works for the two-argument case.
> It was intended as a direct replacement for C's
> #define max(a, b) ((a)>(b)?(a):(b))
> 
> Just (for example) use Xinok's code, and replace typeof(X+Y) with 
> SharedComparisonType!(X, Y).
> 
> Still doesn't solve (c) though; that's a really interesting challenge.
> 

Yes, requirement (c) is the crux of the challenge. Like the ident 
function Alexei mentioned, to be solved it will require the introduction 
of some kind of reference mechanism into the type system, something that 
would has wide repercussions and is complex to design.

-- 
Bruno Medeiros - MSc in CS/E student
http://www.prowiki.org/wiki4d/wiki.cgi?BrunoMedeiros#D