About the in expression, Why can't use with array.

[lili]

On Thursday, 24 October 2019 at 22:40:31 UTC, Jonathan M Davis 
wrote:
> On Thursday, October 24, 2019 7:04:56 AM MDT Paul Backus via 
> Digitalmars-d- learn wrote:
>> On Thursday, 24 October 2019 at 12:58:11 UTC, lili wrote:
>> > Hi:
>> >    In Dlang where is strange design. The in expression can 
>> > only
>> >
>> > use to associative array, why array can not use in 
>> > expression.
>>
>> Checking for the presence of an item in an array requires a 
>> linear search. You can do it with 
>> std.algorithm.searching.canFind:
>>
>> https://dlang.org/phobos/std_algorithm_searching.html#.canFind
>
> In particular, the reason that linear search is considered 
> unacceptable for
> in is so that generic code can rely on its performance. The 
> idea is that
> types shouldn't implement the in operator unless they can do so 
> with
> O(log n) or better (O(log n) being what it costs to get to an 
> item in a
> balanced binary tree like a red-black tree). That way, when you 
> calculate
> the complexity of any algorithm using in, you can assume that 
> it's O(log n)
> at worst. Having it be O(n) instead (like it would be for an 
> array) would
> drastically increase the complexity of the algorithm and make 
> it take much
> longer when processing a large number of items. And the 
> standard library
> provides functions like canFind or find for finding elements in 
> an array, so
> having in work with arrays wouldn't add any functionality. It 
> would
> basically just change the syntax you'd use for finding an 
> element in an
> array.
>
> - Jonathan M Davis

  This reason somewhat farfetched， Think about that if this reason 
is right， the operator overload can not use。 because same 
operator on different type expression same mean but different 
implementation and complexity。 so why operator overload can don't 
care about implementation but 'in' operator need。