Command–query separation principle [re: @mustuse as a function attribute]

[mw]

OK, I saw what they are talking about.


Basically, in Eiffel, all query @mustuse.

Here in D @mustuse is an remedy add-on, which can be specified by 
the programmers in the middle of the inheritance tree, the 
question is what if the pointers got casted either upwards or 
downwards?


(It's too late for me here today.)


On Wednesday, 19 October 2022 at 08:15:32 UTC, Mike Parker wrote:
> On Wednesday, 19 October 2022 at 07:41:33 UTC, mw wrote:
>>
>> Covariance and contravariance is a big topic by itself.
>> But ..., what does it have anything to do here with @mustuse 
>> as a function attribute?
>>
>> @mustuse here only enforces there must be a receiver of the 
>> function call result, and do not throw away the returned 
>> result.
>>
>> I'd really like to see some examples what's this concern is 
>> about.
>>
>
> Following are the examples Walter provided in his email 
> discussions with Paul:
>
> Covariance Ex. 1:
>
> ```d
> class C {
>   T h();
> }
> class D : C {
>   override @mustuse T h();
> }
> ```
>
> Ex. 2
> ```d
> class C {
>   @mustuse T h();
> }
> class D : C {
>   override T h();
> }
> ```
>
> Contravariance Ex. 1:
> ```d
> class C {
>    void t(T function() @mustuse *);
> }
> class D {
>    override void t(T function() *);
> }
> ```
>
> Ex 2:
> ```d
> class C {
>    void t(T function() *);
> }
> class D {
>    override void t(T function() @mustuse *);
> }
> ```
>
> The rules for what happens in each case need to be clearly 
> defined. Walter proposed the following:
>
> ====
> To establish the rule, just resolve this simple case:
>
> ```d
> T f();
> @mustuse T g();
> ```
>
> Can g be implicitly coerced into the type of f? No.
> Can f be implicitly coerced into the type of g? Yes.
>
> I.e. @mustuse can be added in an implicit type conversion, but 
> it cannot be
> subtracted. That's all we need to know, everything else follows 
> inevitably.
> ====
>
> In which case, both Examples 1 would fail, and both Examples 2 
> would pass.
>
> There was more to their discussion beyond this, though, and I'm 
> not the person to summarize it.
>
> So again, if anyone wants to take a stab at fleshing it out, 
> please let me know. I can ask Paul and Walter to provide their 
> thoughts to help get started.