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

lili akozhao at tencent.com
Sat Oct 26 04:30:07 UTC 2019


On Friday, 25 October 2019 at 21:06:53 UTC, IGotD- wrote:
> On Friday, 25 October 2019 at 20:44:18 UTC, Dennis wrote:
>> On Friday, 25 October 2019 at 19:49:05 UTC, Ali Çehreli wrote:
>>> I'm still not completely sold on the whole idea though 
>>> because it's not a clear win.
>>>
>>> Do others see other advantages in other places like 
>>> templates? For example, could templates really be written 
>>> generically for arrays and associative arrays?
>>
>> I'm personally not concerned about generic code.
>> - The semantics of `in` aren't very well defined anyways.
>> - Those who write templates like that (hopefully) know what 
>> they're doing, they'll figure it out  ;)
>> - I can't think of situations where you actually want to write 
>> code that generically works on both arrays and associative 
>> arrays like that. (Though if anyone knows one, please share, 
>> I'm interested.)
>>
>> I'm more concerned about the repeated reports of users being 
>> surprised that `in` doesn't work like they expect. In Python, 
>> the expression `3 in [2, 3, 4]` returns a boolean, and in D 
>> you can do `bool b = 15 in iota(10, 20)` because the operator 
>> is overloaded in Phobos for iota; But as far as actual 
>> language support, `in` is only defined for associative arrays:
>>
>> https://dlang.org/spec/expression.html#InExpression
>>
>> It returns a pointer which can be used in an if-statement, but 
>> also to read/modify the value. So should `in` on arrays do the 
>> same? It would be consistent, but usage of raw pointers is 
>> discouraged with the advent of scope and ref etc. Also you 
>> can't implicitly convert a pointer to a boolean.
>> Should it be a boolean then? That means part of the result of 
>> the linear search is discarded, making `in` less flexible.
>> So maybe we should leave it for now, and put a small 
>> explanation in the error message.
>
> I'm also not so fond of that "in" operator returns a pointer 
> which is bad fit for arrays and possibly many other container 
> algorithms as well. I think the best would be that it would 
> return an optional of type T and T would be user defined. For 
> associative arrays, a pointer or a reference to the element. 
> For arrays the index of that element would suitable. Also 
> overloading would be nice so that "in" operator could return 
> several possible optional types, not sure if that would be 
> possible though.
I argee with your talk, lanuage designer need keep consistent 
that the 'in' semantics. or not support 'in' key word. Don't 
assumption that how user use it.



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