[Bug 191] New: Cannot refer to member variable in default value for method parameter

[Bruno Medeiros]

Don Clugston wrote:
> Bruno Medeiros wrote:
>> Don Clugston wrote:
>>> Bruno Medeiros wrote:
>>>> BCS wrote:
>>>>> Deewiant wrote:
>>>>>> All I can find about default parameters in the spec is at the 
>>>>>> "Functions" page,
>>>>>> where they pop out of the blue in the phrase "A function 
>>>>>> parameter's default
>>>>>> value is not inherited", which is all that's said about them. The 
>>> ....
>>>>>
>>>> Hum, quite a catch you got there! Not only should it be better 
>>>> documented, but maybe it shouldn't work this way.
>>>> Perhaps the default values should be set not at the call site, but 
>>>> by the function itself (thus depending on it's run-time type, and 
>>>> not it's compile-time type),
>>>
>>> I'm almost certain that default values should work that way. They 
>>> should just be syntactic sugar for overloaded functions.
>>> Right now, they have the same problem which C++ has -- there's no way 
>>> that a template can find out what the default values are, and the 
>>> default values can't be used by function pointers or delegates.
>>> So for example,
>>>
>>> void foo(int a, int b=2, int c=3) {}
>>>
>>> void goo(int a, int b, int c) { }
>>> void goo(int a, int b) { goo(a, b, 3); }
>>> void goo(int a) { goo(a, 2, 3); }
>>>
>>> void function(int) boo;
>>>
>>> foo(7); // ok
>>> goo(7); // ok
>>>
>>> boo = goo; // ok
>>> boo(5);
>>>
>>> boo = foo; // doesn't compile -- why not?
>>> boo(5);
>>>
>>>
>>
>> You meant:
>>   boo = &goo; // ok
>>   boo(5);
>>
>>   boo = &foo; // doesn't compile -- why not?
>>
>>
>> Anyway, "Why not?" ? How could that compile? Even with that syntatic 
>> sugar the problem would subsist: remember that the expression (&foo) 
>> has to be evaluable by itself, and if there are many overloads of foo 
>> there is no way (with that syntax at least) to choose the correct one. 
>> (DMD currently chooses the lexically first overload)
> 
> But the 'goo' example does compile. Why is there a difference between 
> 'goo' and 'foo'?

What I was trying to say, in the general sense, is that by making 
default parameters be just sugar for function overloads, we become 
susceptible to the problems of ambiguous homonymous function overloads 
(which in fact, is a separate issue, and the one I was more worried about)
I'm not saying default parameters shouldn't be syntactic sugar because 
of that, I'm just pointing it out.


As for the ambiguous overloads issue: in the example above, I said "DMD 
currently chooses the lexically first overload" but that is not entirely 
correct. If that was so, then:

   boo = &goo; // ok

would not work as in the example above, yet it does. This can only be an 
ugly special case hack, and a huge breach of consistency, since the 
following doesn't work:

   (&goo) (1, 2); // Expected func with 2 params

   auto temp = &goo;
   boo = temp; // Type mismatch, expected void function(int)

This worries me.

> AFAIK, default parameters were introduced (in C++) purely as syntactic 
> sugar, and AFAIK they are only ever used in that way. I think they 
> should be just syntactic sugar. As they exist in both D and C++, they 
> are a quirky language primitive which cannot be emulated in any other 
> way. They lead to some horrible corner cases.

Yes, like that problem with overloading default parameters.

-- 
Bruno Medeiros - CS/E student
http://www.prowiki.org/wiki4d/wiki.cgi?BrunoMedeiros#D