null == "" is true?

[Steven Schveighoffer]

On 7/12/22 4:36 PM, Antonio wrote:
> On Tuesday, 12 July 2022 at 18:56:43 UTC, Paul Backus wrote:
>> On Tuesday, 12 July 2022 at 16:40:38 UTC, H. S. Teoh wrote:
>>> Because an empty string is, by default, represented by an empty slice 
>>> of the null pointer.
>>>
>>> Do not rely on this, however; it's possible sometimes to get an empty 
>>> string that isn't null, e.g., if you incrementally shrink a slice 
>>> over a string until it's empty. In that case, .ptr will not be null, 
>>> but the string will still be empty. Always compare strings against "" 
>>> rather than null, because the latter may not do what you think it 
>>> does sometimes.
>>
>> This is actually 100% reliable when comparing with the `==` operator 
>> because two empty strings always compare equal with `==`, regardless 
>> of what they point to.
>>
>>     string s = "hello";
>>     string empty1 = s[0 .. 0];
>>     string empty2 = s[1 .. 1];
>>     assert(empty1 == null);
>>     assert(empty2 == null);
>>     assert(empty1 == empty2);
>>
>> The real problem is that `s == null` looks like it does one thing 
>> (test for a null pointer) while actually doing something slightly 
>> different (test for an empty string).
> 
> 
> Then:
> 
> ```d
> string a = null;
> assert(a is null);
> assert(a == "");
> 
> string b = "");
> assert(b !is null);
> assert(b == null);
> 
> ```
> 
> Honestly, it is difficult to understand for newcomers... there is a 
> reason, but there is a reason in javascript for `0 == ''` too

It's not just for `null`. And it's unrelated to what javascript is doing.

```d
string a = "abcabc";
assert(a[0 .. 3] ==  a[3 .. $])
assert(a[0 .. 3] !is a[3 .. $])
```

The point is, `==` compares *value*, `is` always compares *identity*.

And you cannot override `is`, it must always be predictable. For arrays, 
of course, you can't override `==`, but custom types can.

-Steve