Hi all,

It seems that it is not possible to use minElement on an array of const objects:

class A
{
  int val;
}

const(A) doStuff(const(A)[] v)
{
  import std.algorithm.searching : minElement;
  return v.minElement!"a.val";
}

This gets the following compiler error:

std/algorithm/searching.d(1256,28): Error: cannot implicitly convert expression (front(r)) of type const(A) to app.A
std/algorithm/searching.d(1286,35): Error: cannot implicitly convert expression (r[i]) of type const(A) to app.A
std/algorithm/searching.d(1258,36): Error: template instance std.algorithm.searching.extremum!("a.val", "a < b", const(A)[], A) error instantiating
std/algorithm/searching.d(3345,24):        instantiated from here: extremum!("a.val", "a < b", const(A)[])
source/app.d(11,11):        instantiated from here: minElement!("a.val", const(A)[])

Is there a reason why this is not allowed? Shouldn't minElement be able to return a const(A) in this situation?

Thanks,
Aurelien

On Monday, 13 November 2017 at 10:20:51 UTC, Aurelien Fredouelle wrote:
> Hi all,
>
> It seems that it is not possible to use minElement on an array of const objects:
>
> class A
> {
>   int val;
> }
>
> const(A) doStuff(const(A)[] v)
> {
>   import std.algorithm.searching : minElement;
>   return v.minElement!"a.val";
> }
>
> This gets the following compiler error:
>
> std/algorithm/searching.d(1256,28): Error: cannot implicitly convert expression (front(r)) of type const(A) to app.A
> std/algorithm/searching.d(1286,35): Error: cannot implicitly convert expression (r[i]) of type const(A) to app.A
> std/algorithm/searching.d(1258,36): Error: template instance std.algorithm.searching.extremum!("a.val", "a < b", const(A)[], A) error instantiating
> std/algorithm/searching.d(3345,24):        instantiated from here: extremum!("a.val", "a < b", const(A)[])
> source/app.d(11,11):        instantiated from here: minElement!("a.val", const(A)[])
>
> Is there a reason why this is not allowed? Shouldn't minElement be able to return a const(A) in this situation?
>
> Thanks,
> Aurelien

It should not until it uses recursion to find min element.

const(Class) res = ...;
...
return res;

Algorithm tries to assign to res variable and fails because in D const(Class) means both const object and const reference to it. So we cannot assign another reference to const(Class) variable.

>Unqual!Element seed = r.front;

>alias MapType = Unqual!(typeof(mapFun(CommonElement.init)));

This looks like a job for std.typecons.Rebindable!(const A) instead of Unqual!(const A) which is used currently. I am surprised that this is the first time anyone has run into this.

On Monday, 13 November 2017 at 12:15:26 UTC, Nathan S. wrote:
>>Unqual!Element seed = r.front;
>
>>alias MapType = Unqual!(typeof(mapFun(CommonElement.init)));
>
> This looks like a job for std.typecons.Rebindable!(const A) instead of Unqual!(const A) which is used currently. I am surprised that this is the first time anyone has run into this.

Is Unqual necessary here?:
https://github.com/dlang/phobos/blob/master/std/algorithm/searching.d#L1284
https://github.com/dlang/phobos/blob/master/std/algorithm/searching.d#L1340

And here
https://github.com/dlang/phobos/blob/master/std/algorithm/searching.d#L1301
https://github.com/dlang/phobos/blob/master/std/algorithm/searching.d#L1355
is necessary something like RebindableOrUnqual instead of Unqual:

template RebindableOrUnqual(T){
	static if (is(T == class) || is(T == interface) || isDynamicArray!T || isAssociativeArray!T)alias RebindableOrUnqual = Rebindable!T;
	else alias RebindableOrUnqual = Unqual!T;
}

On Monday, 13 November 2017 at 14:28:27 UTC, vit wrote:
> On Monday, 13 November 2017 at 12:15:26 UTC, Nathan S. wrote:
>> [...]
>
> Is Unqual necessary here?:
> https://github.com/dlang/phobos/blob/master/std/algorithm/searching.d#L1284
> https://github.com/dlang/phobos/blob/master/std/algorithm/searching.d#L1340
>
> And here
> https://github.com/dlang/phobos/blob/master/std/algorithm/searching.d#L1301
> https://github.com/dlang/phobos/blob/master/std/algorithm/searching.d#L1355
> is necessary something like RebindableOrUnqual instead of Unqual:
>
> template RebindableOrUnqual(T){
> 	static if (is(T == class) || is(T == interface) || isDynamicArray!T || isAssociativeArray!T)alias RebindableOrUnqual = Rebindable!T;
> 	else alias RebindableOrUnqual = Unqual!T;
> }

Thanks, I filed https://issues.dlang.org/show_bug.cgi?id=17982 to track the issue.