Why does sum not work in static arrays?

mw mingwu at gmail.com
Sat Oct 10 19:05:56 UTC 2020

On Sunday, 6 December 2015 at 12:27:49 UTC, cym13 wrote:
> On Sunday, 6 December 2015 at 12:23:05 UTC, Tim K. wrote:
>> Hi! I have the following code:
>>     int main(string[] argv)
>>     {
>>         import std.algorithm: sum;
>>         import std.stdio: writeln;
>>         uint[3] a1 = [1, 2, 3];
>>         uint[] a2;
>>         for (int i = 1; i <= 3; ++i)
>>             a2 ~= i;
>>         writeln("a1: ", sum(a1));
>>         writeln("a2: ", sum(a2));
>>         return 0;
>>     }
>> This throws the error:
>> dummy.d(11): Error: template std.algorithm.iteration.sum 
>> cannot deduce function from argument types !()(uint[3]), 
>> candidates are:
>> /usr/include/dmd/phobos/std/algorithm/iteration.d(3916):
>>  std.algorithm.iteration.sum(R)(R r) if (isInputRange!R && 
>> !isInfinite!R && is(typeof(r.front + r.front)))
>> /usr/include/dmd/phobos/std/algorithm/iteration.d(3927):
>>  std.algorithm.iteration.sum(R, E)(R r, E seed) if 
>> (isInputRange!R && !isInfinite!R && is(typeof(seed = seed + 
>> r.front)))
>> So a dynamic array works just fine. In fact, if I uncomment 
>> the line in question the program compiles and outputs the 
>> correct value (for a2). Why does "sum" not work on static 
>> arrays?
>> Regards
> So that you do not shoot yourself in the foot too easily. A 
> static array is a value type so it is passed by value to 
> functions. If you pass a 1M array to a function... well, I 
> guesse you don't want to do that.

Can the template func `sum()` be made to take `ref` of a static 

> The solution is to slice it:   a1[].sum;   That way you avoid 
> the problem.

While I understand the explanation of the current behavior, and 
the work-around, this inconstancy of making func calls means 
either the library, or the language leaves something to be 
desired: i.e

dynamic array and static array cannot be used interchangeably: 
(sure I'm not talking about array decl / allocation, the user 
have to take different actions) I'm talking about a simple 
function call to calc the sum of the array.
   sum(static_array[]);   // v.s.

For example, if the user first decl a static array for fast 
prototyping, and later changed mind to use dynamic array, then 
s/he need to change the call all over the places.

(I hope you are not telling me, every time people should use:
is the correct D-idiom to use array in a func call)

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