What are the prominent downsides of the D programming language?

[Jean-Louis Leroy]

On Tuesday, 29 September 2020 at 22:36:39 UTC, mw wrote:
> On Tuesday, 29 September 2020 at 22:02:28 UTC, Jean-Louis Leroy 
> wrote:
>> On Tuesday, 29 September 2020 at 21:43:03 UTC, mw wrote:
>>> The key idea is to treat each parent's class attributes 
>>> individually (via rename/undefine/redefine/select) instead of 
>>> as in other multiple inheritance languages, e.g. C++'s 
>>> all-or-none approach, i.e. either all the parents' attributes 
>>> are shared or separated, and even when separated, there is no 
>>> proper renaming mechanism to handle it.
>>
>> If Base has features f and g, and Derived inherits from Base 
>> but undefines g, I assume that a Derived object cannot be 
>> substituted for a Base. Correct?
>
> It still can; but the Derived now becomes a deferred- (in D's 
> term abstract-) class.
>
> because Derived.g becomes a deferred- (in D's term abstract-) 
> method.

Looking at your example, I have the impression that the `addr` 
"feature" is actually a function. If so, `undefine` in this 
example is simply the equivalent of tacking a `= 0` after an 
overridden virtual function in C++ (yes you can inherit an 
implemented virtual function and make it pure again). But, does 
this also work on attributes?

>
>
> This is the demo:
>
> https://github.com/mingwugmail/dlang_tour/tree/master/eiffel/mi
>
> https://github.com/mingwugmail/dlang_tour/blob/master/eiffel/mi/us_resident.e#L3
>   undefine addr
>
> https://github.com/mingwugmail/dlang_tour/blob/master/eiffel/mi/app.e#L17
>   create usr.make  -- ****** Warning: addr is a deferred 
> feature in US_RESIDENT.
>
> https://github.com/mingwugmail/dlang_tour/blob/master/eiffel/mi/app.e#L11
>    print_addr(p: PERSON) is do
>       io.put_string(p.addr + "%N")
>    end
>
>
>
> NOTE: I'm not sure if the SmartEiffel compiler is implemented 
> correctly, it generate warning instead of error, during 
> compilation:
>
> $ make
> compile  app.e -o app
> ****** Warning: addr is a deferred feature in US_RESIDENT.
>
> And if you run it, it error out:
>
> $ ./app
> *** Error at Run Time ***: Deferred routine called.
> 4 frames in current stack.
>
>
> Ideally, this program shouldn't compile; since it detected that 
> we are trying to create an object of a abstract (deferred) 
> class.
>
>
> Hey, just download the SmartEiffel compiler, and try the demo 
> :-) it's all in the same repo:
>
> https://github.com/mingwugmail/dlang_tour/tree/master/eiffel